Qt <- StackOverflow top 100

Question

Is there a QString function which takes an int and outputs it as a QString?

Answer accepted (score 616)

Use QString::number():

int i = 42;
QString s = QString::number(i);

Answer 2 (score 116)

And if you want to put it into string within some text context, forget about + operator. Simply do:

// Qt 5 + C++11
auto i = 13;    
auto printable = QStringLiteral("My magic number is %1. That's all!").arg(i);

// Qt 5
int i = 13;    
QString printable = QStringLiteral("My magic number is %1. That's all!").arg(i);

// Qt 4
int i = 13;    
QString printable = QString::fromLatin1("My magic number is %1. That's all!").arg(i);

Answer 3 (score 24)

Moreover to convert whatever you want, you can use QVariant. For an int to a QString you get:

QVariant(3).toString();

A float to a string or a string to a float:

QVariant(3.2).toString();
QVariant("5.2").toFloat();

Question

How do I set color of text and background of a QLabel ?

Answer accepted (score 247)

The best and recommended way is to use Qt Style Sheet.

To change the text color and background color of a QLabel, here is what I would do :

QLabel* pLabel = new QLabel;
pLabel->setStyleSheet("QLabel { background-color : red; color : blue; }");

You could also avoid using Qt Style Sheets and change the QPalette colors of your QLabel, but you might get different results on different platforms and/or styles.

As Qt documentation states :

Using a QPalette isn’t guaranteed to work for all styles, because style authors are restricted by the different platforms’ guidelines and by the native theme engine.

But you could do something like this :

 QPalette palette = ui->pLabel->palette();
 palette.setColor(ui->pLabel->backgroundRole(), Qt::yellow);
 palette.setColor(ui->pLabel->foregroundRole(), Qt::yellow);
 ui->pLabel->setPalette(palette);

But as I said, I strongly suggest not to use the palette and go for Qt Style Sheet.

Answer 2 (score 33)

You can use QPalette, however you must set setAutoFillBackground(true); to enable background color

QPalette sample_palette;
sample_palette.setColor(QPalette::Window, Qt::white);
sample_palette.setColor(QPalette::WindowText, Qt::blue);

sample_label->setAutoFillBackground(true);
sample_label->setPalette(sample_palette);
sample_label->setText("What ever text");

It works fine on Windows and Ubuntu, I haven’t played with any other OS.

Note: Please see QPalette, color role section for more details

Answer 3 (score 17)

I add this answer because I think it could be useful to anybody.

I step into the problem of setting RGBA colors (that is, RGB color with an Alpha value for transparency) for color display labels in my painting application.

As I came across the first answer, I was unable to set an RGBA color. I have also tried things like:

myLabel.setStyleSheet("QLabel { background-color : %s"%color.name())

where color is an RGBA color.

So, my dirty solution was to extend QLabel and override paintEvent() method filling its bounding rect.

Today, I’ve open up the qt-assistant and read the style reference properties list. Affortunately, it has an example that states the following:

QLineEdit { background-color: rgb(255, 0, 0) }

Thats open up my mind in doing something like the code below, as an example:

myLabel= QLabel()
myLabel.setAutoFillBackground(True) # This is important!!
color  = QtGui.QColor(233, 10, 150)
alpha  = 140
values = "{r}, {g}, {b}, {a}".format(r = color.red(),
                                     g = color.green(),
                                     b = color.blue(),
                                     a = alpha
                                     )
myLabel.setStyleSheet("QLabel { background-color: rgba("+values+"); }")

Note that setAutoFillBackground() set in False will not make it work.

Regards,

Question

I am trying to do something like this:

QString string;
// do things...
std::cout << string << std::endl;

but the code doesn’t compile. How to output the content of qstring into the console (e.g. for debugging purposes or other reasons)? How to convert QString to std::string?

Answer accepted (score 213)

One of the things you should remember when converting QString to std::string is the fact that QString is UTF-16 encoded while std::string… May have any encodings.

So the best would be either:

QString qs;

// Either this if you use UTF-8 anywhere
std::string utf8_text = qs.toUtf8().constData();

// or this if you're on Windows :-)
std::string current_locale_text = qs.toLocal8Bit().constData();

The suggested (accepted) method may work if you specify codec.

See: http://doc.qt.io/qt-5/qstring.html#toLatin1

Answer 2 (score 256)

You can use:

QString qs;
// do things
std::cout << qs.toStdString() << std::endl;

Here’s reference documentation for QString.

Answer 3 (score 35)

If your ultimate aim is to get debugging messages to the console, you can use qDebug().

You can use like,

qDebug()<<string; which will print the contents to the console.

This way is better than converting it into std::string just for the sake of debugging messages.

Question

I’m using Qt4 and C++ for making some programs in computer graphics. I need to be able to print some variables in my console at run-time, not debugging, but cout doesn’t seem to work even if I add the libraries. Is there a way to do this?

Answer accepted (score 185)

If it is good enough to print to stderr, you can use the following streams originally intended for debugging:

#include<QDebug>

//qInfo is qt5.5+ only.
qInfo() << "C++ Style Info Message";
qInfo( "C Style Info Message" );

qDebug() << "C++ Style Debug Message";
qDebug( "C Style Debug Message" );

qWarning() << "C++ Style Warning Message";
qWarning( "C Style Warning Message" );

qCritical() << "C++ Style Critical Error Message";
qCritical( "C Style Critical Error Message" );

// qFatal does not have a C++ style method.
qFatal( "C Style Fatal Error Message" );

Though as pointed out in the comments, bear in mind qDebug messages are removed if QT_NO_DEBUG_OUTPUT is defined

If you need stdout you could try something like this (as Kyle Strand has pointed out):

QTextStream& qStdOut()
{
    static QTextStream ts( stdout );
    return ts;
}

You could then call as follows:

qStdOut() << "std out!";

Answer 2 (score 145)

I found this most useful:

#include <QTextStream>

QTextStream out(stdout);
foreach(QString x, strings)
    out << x << endl;

Answer 3 (score 33)

Writing to stdout

If you want something that, like std::cout, writes to your application’s standard output, you can simply do the following (credit to CapelliC):

QTextStream(stdout) << "string to print" << endl;

If you want to avoid creating a temporary QTextStream object, follow Yakk’s suggestion in the comments below of creating a function to return a static handle for stdout:

inline QTextStream& qStdout()
{
    static QTextStream r{stdout};
    return r;
}

...

foreach(QString x, strings)
    qStdout() << x << endl;

Remember to flush the stream periodically to ensure the output is actually printed.

Writing to stderr

Note that the above technique can also be used for other outputs. However, there are more readable ways to write to stderr (credit to Goz and the comments below his answer):

qDebug() << "Debug Message";    // CAN BE REMOVED AT COMPILE TIME!
qWarning() << "Warning Message";
qCritical() << "Critical Error Message";
qFatal("Fatal Error Message");  // WILL KILL THE PROGRAM!

qDebug() is closed if QT_NO_DEBUG_OUTPUT is turned on at compile-time.

(Goz notes in a comment that for non-console apps, these can print to a different stream than stderr.)

NOTE: All of the Qt print methods assume that const char* arguments are ISO-8859-1 encoded strings with terminating \0 characters.

Question

Edit: Some people started to mark my question as a duplicate. Do not forget that many similar questions existed when I asked this one (see e.g. the list below). However, none of these answers solved my problem. After a long search I found a comment which had been ignored by all users pointing to the missing lib. Now, many months later, the comment has been changed to an answer. However, when I answered this question by msyself I intended to help other people by directly providing the solution. This should not be forgotten and so far my answer helped a lot of people. Therefore my question is definitely not a duplicate. By the way: The accepted answer within the provided link on top does not solve the problem!

Yes, i used the search:

Failed to load platform plugin “windows”. Available platforms are : Error

Deploying Qt C++ Application from Visual Studio qwindows.dll error

failed to load platform plugin “windows” Available platforms are: windows, minimal

However, in my case the problem still persists. I am using Qt 5.1.1 with Visual Studio 2012 and developed my Application on Windows 7 with Qt Creator 2.8.1. Application is compiled in “Release”-mode and can be executed if directly started with Qt Creator.

However, when starting from the “release”-Folder, i get the following message:

This application failed to start because it could not find or load the Qt platform plugin “windows”. Available platform plugins are: minimal, offscreen, windows.

Folder structure looks like this:

release
+ gui.exe
+ icudt51.dll
+ icuin51.dll
+ icuuc51.dll
+ libGLESv2.dll
+ Qt5Core.dll
+ Qt5Gui.dll
+ Qt5Widgets.dll
+ platforms

Platforms is the folder directly copied from Qt.1\5.1.1 including e.g. qwindows.dll. Does not matter if I rename it to “platform” as some other users did. Qt is still not finding the “platform plugin windows”, where is my mistake?

Answer accepted (score 63)

Okay, as posted here https://stackoverflow.com/a/17271172/1458552 without much attention by other users:

The libEGL.dll was missing! Even though this has not been reported when trying to start the application (all other *.dlls such as Qt5Gui.dll had been reported).

Answer 2 (score 35)

I created a platforms directory next to my exe location and put qwindows.dll inside, but I still received the “Failed to load platform plugin”windows“. Available platforms are: windows” error.

I had copied qwindows.dll from C:.1, which is not the right location. I looked at the debug log from running in Qt Creator and found that my app was looking in C:.1\5.1.1_32when it ran in the debugger.

When I copied from C:.1\5.1.1_32, everything worked fine.

Answer 3 (score 21)

The release is likely missing a library/plugin or the library is in the wrong directory and or from the wrong directory.

Create a folder named “platforms” in the same directory as your application.exe file. Copy and paste the qwindows.dll, found in the /bin of whichever compiler you used to release your application, into the “platforms” folder. Like magic it works. If the .dll is not there check plugins/platforms/ ( with plugins/ being in the same directory as bin/ ) : PfunnyGuy’s comment.

It seems like a common issue is that the .dll was taken from the wrong compiler bin. Be sure to copy your the qwindows.dll from the same compiler as the one used to release your app.

Also another helpful tip, Qt comes with platform console applications that will add all dependencies (including ones like qwindows.dll and libEGL.dll) into the folder of your deployed executable. The application for windows is called windeployqt and the documentation can be found here http://doc.qt.io/qt-5/windows-deployment.html.

Question

How can I add external library into a project built by Qt Creator RC1 (version 0.9.2)? For example, the win32 function EnumProcesses() requires Psapi.lib to be added in the project to build.

Answer accepted (score 210)

The proper way to do this is like this:

LIBS += -L/path/to -lpsapi

This way it will work on all platforms supported by Qt. The idea is that you have to separate the directory from the library name (without the extension and without any ‘lib’ prefix). Of course, if you are including a Windows specific lib, this really doesn’t matter.

In case you want to store your lib files in the project directory, you can reference them with the $$_PRO_FILE_PWD_ variable, e.g.:

LIBS += -L"$$_PRO_FILE_PWD_/3rdparty/libs/" -lpsapi

Answer 2 (score 22)

Are you using qmake projects? If so, you can add an external library using the LIBS variable. E.g:

win32:LIBS += path/to/Psapi.lib

Answer 3 (score 11)

LIBS += C:Files

won’t work because you’re using white-spaces in Program Files. In this case you have to add quotes, so the result will look like this: LIBS += “C:Files”. I recommend placing libraries in non white-space locations ;-)

Question

What is the most basic way to do it?

Answer accepted (score 129)

If by string you mean std::string you can do it with this method:

QString QString::fromStdString(const std::string & str)

std::string str = "Hello world";
QString qstr = QString::fromStdString(str);

If by string you mean Ascii encoded const char * then you can use this method:

QString QString::fromAscii(const char * str, int size = -1)

const char* str = "Hello world";
QString qstr = QString::fromAscii(str);

If you have const char * encoded with system encoding that can be read with QTextCodec::codecForLocale() then you should use this method:

QString QString::fromLocal8Bit(const char * str, int size = -1)

const char* str = "zażółć gęślą jaźń";      // latin2 source file and system encoding
QString qstr = QString::fromLocal8Bit(str);

If you have const char * that’s UTF8 encoded then you’ll need to use this method:

QString QString::fromUtf8(const char * str, int size = -1)

const char* str = read_raw("hello.txt"); // assuming hello.txt is UTF8 encoded, and read_raw() reads bytes from file into memory and returns pointer to the first byte as const char*
QString qstr = QString::fromUtf8(str);

There’s also method for const ushort * containing UTF16 encoded string:

QString QString::fromUtf16(const ushort * unicode, int size = -1)

const ushort* str = read_raw("hello.txt"); // assuming hello.txt is UTF16 encoded, and read_raw() reads bytes from file into memory and returns pointer to the first byte as const ushort*
QString qstr = QString::fromUtf16(str);

Answer 2 (score 242)

If compiled with STL compatibility, QString has a static method to convert a std::string to a QString:

std::string str = "abc";
QString qstr = QString::fromStdString(str);

Answer 3 (score 13)

Alternative way:

std::string s = "This is an STL string";
QString qs = QString::fromAscii(s.data(), s.size());

This has the advantage of not using .c_str() which might cause the std::string to copy itself in case there is no place to add the '\0' at the end.

Question

I wrote application for linux which uses Qt5.

But when I am trying to launch it on the linux without Qt SDK installed, the output in console is:

Failed to load platform plugin “xcb”. Available platforms are:

How can I fix this? May be I need to copy some plugin file? When I use ubuntu with Qt5 installed, but I rename Qt directory, the same problem occurs. So, it uses some file from Qt directory…

UPDATE: when I create in the app dir “platforms” folder with the file libqxcb.so, the app still doesnot start, but the error message changes:

Failed to load platform plugin “xcb”. Available platforms are:

xcb

How can this happen? How can platform plugin be available but can’t be loaded?

Answer accepted (score 54)

Use ldd (man ldd) to show shared library dependencies. Running this on libqxcb.so

.../platforms$ ldd libqxcb.so

shows that xcb depends on libQt5DBus.so.5 in addition to libQt5Core.so.5 and libQt5Gui.so.5 (and many other system libs). Add libQt5DBus.so.5 to your collection of shared libs and you should be ready to move on.

Answer 2 (score 50)

As was posted earlier, you need to make sure you install the platform plugins when you deploy your application. Depending on how you want to deploy things, there are two methods to tell your application where the platform plugins (e.g. platforms/plugins/libqxcb.so) are at runtime which may work for you.

The first is to export the path to the directory through the QT_QPA_PLATFORM_PLUGIN_PATH variable.

QT_QPA_PLATFORM_PLUGIN_PATH=path/to/plugins ./my_qt_app

or

export QT_QPA_PLATFORM_PLUGIN_PATH=path/to/plugins
./my_qt_app

The other option, which I prefer is to create a qt.conf file in the same directory as your executable. The contents of which would be:

[Paths]
Plugins=/path/to/plugins

More information regarding this can be found here and at using qt.conf

Answer 3 (score 19)

I tried to start my binary, compiled with Qt 5.7, on Ubuntu 16.04 LTS where Qt 5.5 is preinstalled. Expectedly, it didn’t work as is.

At first, I inspected the binary itself with ldd as was suggested here, and satisfied all “not found” dependencies. Then this notorious This application failed to start because it could not find or load the Qt platform plugin "xcb" error was thrown in.

What should be done on Linux to resolve this

At first, you should create platforms directory where your binary is, because it is the place where Qt looks for XCB library. Copy libqxcb.so there. I wonder why authors of other answers didn’t mention this.

Then you may want to run your binary with QT_DEBUG_PLUGINS=1 environment variable set to check what dependencies of libqxcb.so unsatisfied. (You may also use ldd for this as suggested in the accepted answer).

The command output may look like this:

me@xerus:/media/sf_Qt/Package$ LD_LIBRARY_PATH=. QT_DEBUG_PLUGINS=1 ./Binary
QFactoryLoader::QFactoryLoader() checking directory path "/media/sf_Qt/Package/platforms" ...
QFactoryLoader::QFactoryLoader() looking at "/media/sf_Qt/Package/platforms/libqxcb.so"
Found metadata in lib /media/sf_Qt/Package/platforms/libqxcb.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "xcb"
        ]
    },
    "className": "QXcbIntegrationPlugin",
    "debug": false,
    "version": 329472
}


Got keys from plugin meta data ("xcb")
loaded library "/media/sf_Qt/Package/platforms/libqxcb.so"
QLibraryPrivate::loadPlugin failed on "/media/sf_Qt/Package/platforms/libqxcb.so" : "Cannot load library /media/sf_Qt/Package/platforms/libqxcb.so: (/usr/lib/x86_64-linux-gnu/libQt5DBus.so.5: version `Qt_5' not found (required by ./libQt5XcbQpa.so.5))"
This application failed to start because it could not find or load the Qt platform plugin "xcb"
in "".

Available platform plugins are: xcb.

Reinstalling the application may fix this problem.
Aborted (core dumped)

Note the failing libQt5DBus.so.5 library. Copy it to your libraries path, in my case it was the same directory where my binary is (hence LD_LIBRARY_PATH=.). Repeat this process until all dependencies are satisfied.

P.S. thanks to the author of this answer for QT_DEBUG_PLUGINS=1.

Question

I’m playing around with Qt, and I want to create a simple pause between two commands. However it won’t seem to let me use Sleep(int mili);, and I can’t find any obvious wait functions.

I am basically just making a console application to test some class code which will later be included in a proper Qt GUI, so for now I’m not bothered about breaking the whole event-driven model.

Answer accepted (score 30)

This previous question mentions using qSleep() which is in the QtTest module. To avoid the overhead linking in the QtTest module, looking at the source for that function you could just make your own copy and call it. It uses defines to call either Windows Sleep() or Linux nanosleep().

#ifdef Q_OS_WIN
#include <windows.h> // for Sleep
#endif
void QTest::qSleep(int ms)
{
    QTEST_ASSERT(ms > 0);

#ifdef Q_OS_WIN
    Sleep(uint(ms));
#else
    struct timespec ts = { ms / 1000, (ms % 1000) * 1000 * 1000 };
    nanosleep(&ts, NULL);
#endif
}

Answer 2 (score 128)

I wrote a super simple delay function for an application I developed in Qt.

I would advise you to use this code rather than the sleep function as it won’t let your GUI freeze.

Here is the code:

void delay()
{
    QTime dieTime= QTime::currentTime().addSecs(1);
    while (QTime::currentTime() < dieTime)
        QCoreApplication::processEvents(QEventLoop::AllEvents, 100);
}

To delay an event by n seconds - use addSecs(n).

Answer 3 (score 61)

From Qt5 onwards we can also use

Static Public Members of QThread

void    msleep(unsigned long msecs)
void    sleep(unsigned long secs)
void    usleep(unsigned long usecs)

Question

I have a software in ubuntu that requires me to run qmake to generate the Makefile.

However, running qmake gives back this error,

qmake: could not find a Qt installation of ''

I have installed what I thought to be the required packages using,

sudo apt-get install qt4-qmake
sudo apt-get install qt5-qmake

But the error didn’t go away.

Any help on this would be gladly appreciated!

Answer accepted (score 151)

sudo apt-get install qt5-default works for me.

$ aptitude show qt5-default</pre></code>
tells that</p>

<blockquote>
  <p>This package sets Qt 5 to be the default Qt version to be used when using
   development binaries like qmake. It provides a default configuration for
   qtchooser, but does not prevent alternative Qt installations from being used.</p>
</blockquote>

#### Answer 2 (score 28)
You could check path to qmake using `which qmake`.  

Consider install `qt4-default` or `qt5-default` depends what version of qt you want use.  

You could also use <a href="http://manpages.ubuntu.com/manpages/utopic/man1/qtchooser.1.html">qtchooser</a>  -  a  wrapper  used  to select between Qt development binary versions.  

#### Answer 3 (score 12)
For others in my situation, the solution was:  

```c
qmake -qt=qt5

This was on Ubuntu 14.04 after install qt5-qmake. qmake was a symlink to qtchooser which takes the -qt argument.

Question

I’m using the Qt Designer.

I want to create a QVBoxLayout which will automatically expand to fill the whole window.

The layout of the QVBoxLayout remains fixed.

How can I cause the QVBoxLayout to expand and fill the entire window through the designer?

Answer accepted (score 341)

After creating your QVBoxLayout in Qt Designer, right-click on the background of your widget/dialog/window (not the QVBoxLayout, but the parent widget) and select Lay Out -> Lay Out in a Grid from the bottom of the context-menu. The QVBoxLayout should now stretch to fit the window and will resize automatically when the entire window is resized.

Answer 2 (score 13)

According to the documentation, there needs to be a top level layout set.

A top level layout is necessary to ensure that your widgets will resize correctly when its window is resized. To check if you have set a top level layout, preview your widget and attempt to resize the window by dragging the size grip.

You can set one by clearing the selection and right clicking on the form itself and choosing one of the layouts available in the context menu.

Answer 3 (score 2)

I’ve tried to find a “fit to screen” property but there is no such.

But setting widget’s “maximumSize” to a “some big number” ( like 2000 x 2000 ) will automatically fit the widget to the parent widget space.

Question

I have a QString in my sources. So I need to convert it to integer without “Kb”.

I tried Abcd.toInt() but it does not work.

QString Abcd = "123.5 Kb"

Answer 2 (score 82)

You don’t have all digit characters in your string. So you have to split by space

QString Abcd = "123.5 Kb";
Abcd.split(" ")[0].toInt();    //convert the first part to Int
Abcd.split(" ")[0].toDouble(); //convert the first part to double
Abcd.split(" ")[0].toFloat();  //convert the first part to float

Update: I am updating an old answer. That was a straight forward answer to the specific question, with a strict assumption. However as noted by @DomTomCat in comments and @Mikhail in answer, In general one should always check whether the operation is successful or not. So using a boolean flag is necessary.

bool flag;
double v = Abcd.split(" ")[0].toDouble(&flag); 
if(flag){
  // use v
}

Also if you are taking that string as user input, then you should also be doubtful about whether the string is really splitable with space. If there is a possibility that the assumption may break then a regex verifier is more preferable. A regex like the following will extract the floating point value and the prefix character of ‘b’. Then you can safely convert the captured strings to double.

([0-9]*\.?[0-9]+)\s+(\w[bB])

You can have an utility function like the following

QPair<double, QString> split_size_str(const QString& str){
    QRegExp regex("([0-9]*\\.?[0-9]+)\\s+(\\w[bB])");
    int pos = regex.indexIn(str);
    QStringList captures = regex.capturedTexts();
    if(captures.count() > 1){
        double value = captures[1].toDouble(); // should succeed as regex matched
        QString unit = captures[2]; // should succeed as regex matched
        return qMakePair(value, unit);
    }
    return qMakePair(0.0f, QString());
}

Answer 3 (score 8)

Don’t forget to check if the conversion was successful!

bool ok;
auto str= tr("1337");
str.toDouble(&ok); // returns 1337.0, ok set to true
auto strr= tr("LEET");
strr.toDouble(&ok); // returns 0.0, ok set to false

Question

I am trying to write into a file and if the file doesn’t exist create it. I have searched on the internet and nothing worked for me.

My code looks currently like this:

QString filename="Data.txt";
QFile file( filename );
if ( file.open(QIODevice::ReadWrite) )
{
    QTextStream stream( &file );
    stream << "something" << endl;
}

If I create a text file called Data in the directory, it remains empty. If I don’t create anything it doesn’t create the file either. I don’t know what to do with this, this isn’t the first way in which I tried to create/write into a file and none of the ways worked.

Thanks for your answers.

Answer accepted (score 23)

Are you sure you’re in the right directory?
Opening a file without a full path will open it in the current working directory. In most cases this is not what you want. Try changing the first line to

QString filename="c:\\Data.txt" or
QString filename="c:/Data.txt"

and see if the file is created in c:\

Answer 2 (score 24)

That is weird, everything looks fine, are you sure it does not work for you? Because this main surely works for me, so I would look somewhere else for the source of your problem.

#include <QFile>
#include <QTextStream>


int main()
{
    QString filename = "Data.txt";
    QFile file(filename);
    if (file.open(QIODevice::ReadWrite)) {
        QTextStream stream(&file);
        stream << "something" << endl;
    }
}

The code you provided is also almost the same as the one provided in detailed description of QTextStream so I am pretty sure, that the problem is elsewhere :)

Also note, that the file is not called Data but Data.txt and should be created/located in the directory from which the program was run (not necessarily the one where the executable is located).

Answer 3 (score 9)

#include <QFile>
#include <QCoreApplication>
#include <QTextStream>

int main(int argc, char *argv[])
{
    // Create a new file     
    QFile file("out.txt");
    file.open(QIODevice::WriteOnly | QIODevice::Text);
    QTextStream out(&file);
    out << "This file is generated by Qt\n";

    // optional, as QFile destructor will already do it:
    file.close(); 

    //this would normally start the event loop, but is not needed for this
    //minimal example:
    //return app.exec();

    return 0;
}

Question

I was trying to convert a QString to char* type by the following methods, but they don’t seem to work.

//QLineEdit *line=new QLineEdit();{just to describe what is line here}

QString temp=line->text();
char *str=(char *)malloc(10);
QByteArray ba=temp.toLatin1();
strcpy(str,ba.data());

Can you elaborate the possible flaw with this method, or give an alternative method?

Answer accepted (score 108)

Well, the Qt FAQ says:

int main(int argc, char **argv)
{
 QApplication app(argc, argv);
  QString str1 = "Test";
  QByteArray ba = str1.toLocal8Bit();
  const char *c_str2 = ba.data();
  printf("str2: %s", c_str2);
  return app.exec();
}

So perhaps you’re having other problems. How exactly doesn’t this work?

Answer 2 (score 43)

Maybe

my_qstring.toStdString().c_str();

or safer, as Federico points out:

std::string str = my_qstring.toStdString();
const char* p = str.c_str();

It’s far from optimal, but will do the work.

Answer 3 (score 37)

The easiest way to convert a QString to char* is qPrintable(const QString& str), which is a macro expanding to str.toLocal8Bit().constData().

Question

I want to display .jpg image in an Qt UI. I checked it online and found http://qt-project.org/doc/qt-4.8/widgets-imageviewer.html. I thought Graphics View will do the same, and also it has codec to display video. How to display images using Graphics View? I went through the libraries, but because I am a totally newbie in Qt, I can’t find a clue to start with. Can you direct me to some resources/examples on how to load and display images in Qt?

Thanks.

Answer accepted (score 39)

#include ...

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QGraphicsScene scene;
    QGraphicsView view(&scene);
    QGraphicsPixmapItem item(QPixmap("c:\\test.png"));
    scene.addItem(&item);
    view.show();
    return a.exec();
}

This should work. :) List of supported formats can be found here

Answer 2 (score 42)

You could attach the image (as a pixmap) to a label then add that to your layout…

...

QPixmap image("blah.jpg");

QLabel imageLabel = new QLabel();
imageLabel.setPixmap(image);

mainLayout.addWidget(imageLabel);

...

Apologies, this is using Jambi (Qt for Java) so the syntax is different, but the theory is the same.

Answer 3 (score 12)

If the only thing you want to do is drop in an image onto a widget withouth the complexity of the graphics API, you can also just create a new QWidget and set the background with StyleSheets. Something like this:

MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent)
{
    ...
    QWidget *pic = new QWidget(this);
    pic->setStyleSheet("background-image: url(test.png)");
    pic->setGeometry(QRect(50,50,128,128));
    ...
}

Question

When I try installing the PyQt5 on Windows using the command

python configure.py

I get this error:

Error: Make sure you have a working Qt qmake on your PATH.

I got the pyQt5 from PyQt5 Download.

How can I install PyQt5?

Update:

I installed Qt 5.0.2 for Windows 64-bit (VS 2012, 500 MB) from the Qt Download page and now I have this error:

Querying qmake about your Qt installation...
Determining the details of your Qt installation...
Error: Failed to determine the detail of your Qt installation. Try again using
the --verbose flag to see more detail about the problem.

And when I execute the command python configure.py --verbose:

Querying qmake about your Qt installation...
Determining the details of your Qt installation...
C:\Qt\Qt5.0.2\5.0.2\msvc2012_64\bin\qmake.exe -spec win32-msvc2008 -o qtdetail.m
k qtdetail.pro
nmake -f qtdetail.mk release
'nmake' não é reconhecido como um comando interno
ou externo, um programa operável ou um arquivo em lotes.
Error: Failed to determine the detail of your Qt installation. Try again using
the --verbose flag to see more detail about the problem.

I added C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\bin (contains nmake.exe) to PATH and I got this error:

Querying qmake about your Qt installation...
Determining the details of your Qt installation...
C:\Qt\Qt5.0.2\5.0.2\msvc2012_64\bin\qmake.exe -spec win32-msvc2008 -o qtdetail.mk qtdetail.pro
nmake -f qtdetail.mk release


Microsoft (R) Program Maintenance Utility Version 11.00.50727.1

Copyright (C) Microsoft Corporation.  All rights reserved.


  "C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\bin\nmake.exe" -f qtdetail.mk.Release


Microsoft (R) Program Maintenance Utility Version 11.00.50727.1

Copyright (C) Microsoft Corporation.  All rights reserved.

  cl -c -nologo -Zm200 -Zc:wchar_t -O2 -MD -GR -W3 -w34100 -w34189 -EHsc -DUNICODE -DWIN32 -DQT_NO_DEBUG -DQT_CORE_LIB -I"..\..\..\..\..\..\..\Qt\Qt5.0.2\5.0.2\msvc2012_64\include" -I"..\..\..\..\..\..\..\Qt\Qt5.0.2\5.0.2\msvc2012_64\include\QtCore" -I"release" -I"..\..\..\..\..\..\..\Qt\Qt5.0.2\5.0.2\msvc2012_64\mkspecs\win32-msvc2008" -Forelease\ @C:\Users\Victor\AppData\Local\Temp\nm68EA.tmp

NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\bin\cl.EXE"' : return code '0xc0000135'

Stop.

NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\bin\nmake.exe"' : return code '0x2'

Stop.

Answer accepted (score 12)

The easiest way to install PyQt is to just use the installer (Link in your answer, step #5). If you install python 3.3, the installer will add all of the PyQt5 extras to that python installation automatically. You won’t need to do any compiling (none of: nmake, nmake install, python configure).

All of the build options are available for if you need a custom install (for instance, using a different version of python, where there isn’t an installer provided by riverbank computing).

If you do need to compile your own version of PyQt5, the steps (as you have found) are here, but assume you have python and a compiler installed and in your path. The installed and in your path have been where you have been running into trouble it seems. I’d recommend using the installer version, but you need to install python 3.3 first.

Answer 2 (score 27)

Mainly I use the following command under the cmd

pip install pyqt5

And it works with no problem!

Answer 3 (score 8)

First try this in your Windows cmd window:

pip3 install pyqt5

If that is successful, it will look something like this:

C:\Windows\System32>pip3 install pyqt5
Collecting pyqt5
  Downloading PyQt5-5.9-5.9.1-cp35.cp36.cp37-none-win_amd64.whl (77.2MB)
    100% |################################| 77.2MB 13kB/s
Collecting sip<4.20,>=4.19.3 (from pyqt5)
  Downloading sip-4.19.3-cp35-none-win_amd64.whl (49kB)
    100% |################################| 51kB 984kB/s
Installing collected packages: sip, pyqt5
Successfully installed pyqt5-5.9 sip-4.19.3

If that did not work, you might try this link from SourceForge.

PyQt5 .exe installers for Windows

How to find the installer that’s right for you?

First, determine what version of Python you have and whether you have 32-bit or 64-bit Python. Next, open one of the directories. I’m on Python 3.5 64-bit so I’m looking for a .exe with those specs. When you open a directory on SourceForge, you will see some directories with ONLY .zip or .tar.gz. That’s not what you’re looking for. A good indication of which directory you should click is given by the “Downloads/Week” column. I’ll open the PyQt-5.6 directory in my case.

Here we notice some .exe files:

PyQt-5.6
|_PyQt5-5.6-gpl-Py3.5-Qt5.6.0-x32-2.exe
|_PyQt5-5.6-gpl-Py3.5-Qt5.6.0-x64-2.exe
|_PyQt5_gpl-5.6.zip
|_PyQt5_gpl-5.6.tar.gz

I know these are Python 3.5 by Py3.5 in the file name. I am also looking for the 64-bit version so I’ll download PyQt5-5.6-gpl-Py3.5-Qt5.6.0-x64-2.exe. Final answer!

Note: if you try to install a version that’s not compatible with your system, a dialog box will appear immediately after running the .exe. That’s an indication that you’ve chosen the wrong one. I’m not trying to sound like a dbag… I did that several times!

To test a successful install, in your Python interpreter, try to import:

from PyQt5 import QtCore, QtGui, QtWidgets

Question

How do I show a message box with Yes/No buttons in Qt, and how do I check which of them was pressed?

I.e. a message box that looks like this:

Answer accepted (score 172)

You would use QMessageBox::question for that.

Example in a hypothetical widget’s slot:

#include <QApplication>
#include <QMessageBox>
#include <QDebug>

// ...

void MyWidget::someSlot() {
  QMessageBox::StandardButton reply;
  reply = QMessageBox::question(this, "Test", "Quit?",
                                QMessageBox::Yes|QMessageBox::No);
  if (reply == QMessageBox::Yes) {
    qDebug() << "Yes was clicked";
    QApplication::quit();
  } else {
    qDebug() << "Yes was *not* clicked";
  }
}

_{Should work on Qt 4 and 5, requires QT += widgets on Qt 5, and CONFIG += console on Win32 to see qDebug() output.}

See the StandardButton enum to get a list of buttons you can use; the function returns the button that was clicked. You can set a default button with an extra argument (Qt “chooses a suitable default automatically” if you don’t or specify QMessageBox::NoButton).

Answer 2 (score 38)

You can use the QMessage object to create a Message Box then add buttons :

QMessageBox msgBox;
msgBox.setWindowTitle("title");
msgBox.setText("Question");
msgBox.setStandardButtons(QMessageBox::Yes);
msgBox.addButton(QMessageBox::No);
msgBox.setDefaultButton(QMessageBox::No);
if(msgBox.exec() == QMessageBox::Yes){
  // do something
}else {
  // do something else
}

Answer 3 (score 17)

QT can be as simple as that of Windows. The equivalent code is

if (QMessageBox::Yes == QMessageBox(QMessageBox::Information, "title", "Question", QMessageBox::Yes|QMessageBox::No).exec()) 
{

}

Question

I have a table view with three columns; I have just passed to write into text file using this code

QFile file("/home/hamad/lesson11.txt");
if(!file.open(QIODevice::WriteOnly)) {
    QMessageBox::information(0,"error",file.errorString());
}
QString dd;

for(int row=0; row < model->rowCount(); row++) {
     dd = model->item(row,0)->text() +  ","
                 + model->item(row,1)->text() +  ","
                 + model->item(row,2)->text();

     QTextStream out(&file);
     out << dd << endl;
 }

But I’m not succeed to read the same file again, I tried this code but I don’t know where is the problem in it

QFile file("/home/hamad/lesson11.txt");
QTextStream in(&file);
QString line = in.readLine();
while(!in.atEnd()) {

    QStringList  fields = line.split(",");

    model->appendRow(fields);

}

Any help please ?

Answer 2 (score 87)

You have to replace string line

QString line = in.readLine();

into while:

QFile file("/home/hamad/lesson11.txt");
if(!file.open(QIODevice::ReadOnly)) {
    QMessageBox::information(0, "error", file.errorString());
}

QTextStream in(&file);

while(!in.atEnd()) {
    QString line = in.readLine();    
    QStringList fields = line.split(",");    
    model->appendRow(fields);    
}

file.close();

Question

I’m writing a program in C++ using the Qt library. There is a symbolic link in my home bin directory to the executable. I would like the current working directory of my program to be the directory in which I am with my terminal (ie. the result of the pwd command). I saw the QDir::currentPath() function, but it gives back the directory where the binary is.

How can I find my current working directory?

Answer accepted (score 3)

Thank you RedX and Kaz for your answers. I don’t get why by me it gives the path of the exe. I found an other way to do it :

QString pwd("");
char * PWD;
PWD = getenv ("PWD");
pwd.append(PWD);
cout << "Working directory : " << pwd << flush;

It is less elegant than a single line… but it works for me.

Answer 2 (score 90)

Just tested and QDir::currentPath() does return the path from which I called my executable.

And a symlink does not “exist”. If you are executing an exe from that path you are effectively executing it from the path the symlink points to.

Answer 3 (score 53)

Have you tried QCoreApplication::applicationDirPath()

qDebug() << "App path : " << qApp->applicationDirPath();

Question

The title is pretty self-descriptive. I’ve downloaded Qt Creator 2.7.0, and I am trying to compile some basic C++11 code:

int my_array[5] = {1, 2, 3, 4, 5};
for(int &x : my_array)
{
  x *= 2;
}

I’m receiving the following error:

range based for loops are not allowed in c++ 98 mode

Yet, according to this article this version of Qt Creator supports C++11. So how do I enable it?

Answer accepted (score 259)

According to this site add

CONFIG += c++11

to your .pro file (see at the bottom of that web page). It requires Qt 5.

The other answers, suggesting

QMAKE_CXXFLAGS += -std=c++11 (or QMAKE_CXXFLAGS += -std=c++0x)

also work with Qt 4.8 and gcc / clang.

Answer 2 (score 32)

Add this to your .pro file

QMAKE_CXXFLAGS += -std=c++11

or

CONFIG += c++11

Answer 3 (score 19)

As an alternative for handling both cases addressed in Ali’s excellent answer, I usually add

# With C++11 support
greaterThan(QT_MAJOR_VERSION, 4){    
CONFIG += c++11
} else {
QMAKE_CXXFLAGS += -std=c++0x
}

to my project files. This can be handy when you don’t really care much about which Qt version is people using in your team, but you want them to have C++11 enabled in any case.

Question

I want to get QString from another QString, when I know necessary indexes. For example: Main string: “This is a string”. I want to create new QString from first 5 symbols and get “This”.
input : first and last char number.
output : new QString.

How to create it ?

P.S. Not only first several letters, also from the middle of the line, for example from 5 till 8.

Answer 2 (score 101)

If you do not need to modify the substring, then you can use QStringRef. The QStringRef class is a read only wrapper around an existing QString that references a substring within the existing string. This gives much better performance than creating a new QString object to contain the sub-string. E.g.

QString myString("This is a string");
QStringRef subString(&myString, 5, 2); // subString contains "is"

If you do need to modify the substring, then left(), mid() and right() will do what you need…

QString myString("This is a string");
QString subString = myString.mid(5,2); // subString contains "is"
subString.append("n't"); // subString contains "isn't"

Answer 3 (score 44)

Use the left function:

QString yourString = "This is a string";
QString leftSide = yourString.left(5);
qDebug() << leftSide; // output "This "

Also have a look at mid() if you want more control.

Question

I am trying to compile a hello world program in Qt Using Qt Creator.

I am getting ‘cl’ is not recognized as an internal or external command.

I am using Windows 7 and Both Vs 2008 and 2010 installed in it. When I use Mingw it is compiling fine but if use vs 2008 it is giving this error.

After Setting Env Path =..;..;C:FilesVisual Studio 9.0also it is showing the same error.

Answer accepted (score 44)

I think cl isn’t in your path. You need to add it there. The recommended way to do this is to launch a developer command prompt.

Quoting the article Setting the Path and Environment Variables for Command-Line Builds:

To open a Developer Command Prompt window

1. With the Windows 8 Start screen showing, type Visual Studio Tools. Notice that the search results change as you type; when Visual Studio Tools appears, choose it.

   On earlier versions of Windows, choose Start, and then in the search box, type Visual Studio Tools. When Visual Studio Tools appears in the search results, choose it.

2. In the Visual Studio Tools folder, open the Developer Command Prompt for your version of Visual Studio. (To run as administrator, open the shortcut menu for the Developer Command Prompt and choose Run as Administrator.)

As the article notes, there are several different shortcuts for setting up different toolsets - you need to pick the suitable one.

If you already have a plain Command Prompt window open, you can run the batch file vcvarsall.bat with the appropriate argument to set up the environment variables. Quoting the same article:

To run vcvarsall.bat

1. At the command prompt, change to the Visual C++ installation directory. (The location depends on the system and the Visual Studio installation, but a typical location is C:Files (x86)Visual Studio version.) For example, enter:

   cd "\Program Files (x86)\Microsoft Visual Studio 12.0\VC"
   ```</li>
     <li><p>To configure this Command Prompt window for 32-bit x86 command-line builds, at the command prompt, enter:</p>
   
   ```c
   vcvarsall x86
   ```</li>
     </ol>
   </blockquote>
   
   From the article, the possible arguments are the following:  
   
   <ul>
   <li>`x86` (x86 32-bit native)</li>
   <li>`x86_amd64` (x64 on x86 cross)</li>
   <li>`x86_arm` (ARM on x86 cross)</li>
   <li>`amd64` (x64 64-bit native)</li>
   <li>`amd64_x86` (x86 on x64 cross)</li>
   <li>`amd64_arm` (ARM on x64 cross)</li>
   </ul>
   
   #### Answer 2 (score 6)
   <p>I had the same problem. 
   Try to make a bat-file to start the Qt Creator. Add something like this to the bat-file:</p>
   
   ```c
   call "C:\Program Files\Microsoft Visual Studio 9.0\VC\bin\vcvars32.bat"  
   "C:\QTsdk\qtcreator\bin\qtcreator" 

   Now I can compile and get:

   jom 1.0.8 - empower your cores
   11:10:08: The process "C:\QTsdk\qtcreator\bin\jom.exe" exited normally.

Answer 3 (score 4)

Make sure you restart your computer after you install the Build Tools.

This was what was causing the error for me.

Question

In Qt, I can get the selected text of a QComboBox by using the combobox->currentText() method. How can I get the selected value?

I searched for help but I couldn’t find a method currentData() which I expected to find. I could only find combobox->currentIndex()

Is there a smarter way to do it other than combobox->itemData(combobox->currentIndex())?

Update: This is no longer necessary as of Qt 5. A currentData() method has been added http://doc.qt.io/qt-5/qcombobox.html#currentData-prop

Answer accepted (score 78)

It seems you need to do combobox->itemData(combobox->currentIndex()) if you want to get the current data of the QComboBox.

If you are using your own class derived from QComboBox, you can add a currentData() function.

Answer 2 (score 24)

This one can get the text of current index:

QString cb = cbChoice ->currentText();

Answer 3 (score 20)

you can set QVariant data for all items, then you can get the value when you need it.

there is an example code for this situation:

ui.comboBoxSheetSize->addItem("128 m", QVariant(128));
ui.comboBoxSheetSize->addItem("256 m", QVariant(256));
ui.comboBoxSheetSize->addItem("512 m", QVariant(512));
ui.comboBoxSheetSize->addItem("1024 m", QVariant(1024));

...

void Page::onComboSheetSizeChanged( int index )
{
 int value = ui.comboBoxSheetSize->itemData(index).toInt();
}

by the way, i think i misunderstood your question. i think the way you get data is smart enough?

Question

Installed just the IDE on Windows 7. I want to create a Plain C++ Project (Non-QT Project); however I get an error: No valid kits found. When I click on Options->Kits, I see the Desktop (default) kit, and it shows no errors.

Am I getting the error because I didn’t install a Qt library? If so, is there any way I can bypass downloading/installing that and just use the IDE?

Answer accepted (score 42)

Found the issue. Qt Creator wants you to use a compiler listed under one of their Qt libraries. Use the Maintenance Tool to install this.

To do so:

Go to Tools -> Options…. Select Build & Run on left. Open Kits tab. You should have Manual -> Desktop (default) line in list. Choose it. Now select something like Qt 5.5.1 in PATH (qt5) in Qt version combobox and click Apply button. From now you should be able to create, build and run empty Qt project.

Answer 2 (score 46)

Though OP is asking about Windows, this error also occurs on Ubuntu Linux and Google lists this result first when you search for the error“QtCreator: No valid kits found”.

On Ubuntu this is solved by running:

For Qt5:

sudo apt-get install qt5-default

For Qt4:

sudo apt-get install qt4-dev-tools libqt4-dev libqt4-core libqt4-gui

This question is answered here and here, though those entries are less SEO-friendly…

Answer 3 (score 2)

For QT 5.* if you face error at Kits, like No Valid Kits Found, go to Options->Build&Run-> (Kits tab) then you see a Manual category which should list Desktop as the default.

Just go to your OS Terminal and write sudo apt-get install qt5-default, go back to QT Creator and Start your New Project, and there you see the kit option Desktop included in the list.

Question

How should I quit a Qt Program, e.g when loading a data file, and discovered file corruption, and user need to quit this app or re-initiate data file?

Should I:

1. call exit(EXIT_FAILURE)
2. call QApplication::quit()
3. call QCoreApplication::quit()

And difference between (2) and (3)?

Answer accepted (score 132)

QApplication is derived from QCoreApplication and thereby inherits quit() which is a public slot of QCoreApplication, so there is no difference between QApplication::quit() and QCoreApplication::quit().

As we can read in the documentation of QCoreApplication::quit() it “tells the application to exit with return code 0 (success).”. If you want to exit because you discovered file corruption then you may not want to exit with return code zero which means success, so you should call QCoreApplication::exit() because you can provide a non-zero returnCode which, by convention, indicates an error.

It is important to note that “if the event loop is not running, this function (QCoreApplication::exit()) does nothing”, so in that case you should call exit(EXIT_FAILURE).

Answer 2 (score 29)

You can call qApp.exit();. I always use that and never had a problem with it.

If you application is a command line application, you might indeed want to return an exit code. It’s completely up to you what the code is.

Answer 3 (score 0)

If you’re using Qt Jambi, this should work:

QApplication.closeAllWindows();

Question

I’d like to format a string for Qt label, I’m programming in C++ on Qt.

In ObjC I would write something like:

NSString *format=[NSString stringWithFormat: ... ];

How to do something like that in Qt?

Answer 2 (score 151)

You can use QString.arg like this

QString my_formatted_string = QString("%1/%2-%3.txt").arg("~", "Tom", "Jane");
// You get "~/Tom-Jane.txt"

This method is preferred over sprintf because:

Changing the position of the string without having to change the ordering of substitution, e.g. 

// To get "~/Jane-Tom.txt"
QString my_formatted_string = QString("%1/%3-%2.txt").arg("~", "Tom", "Jane");

Or, changing the type of the arguments doesn’t require changing the format string, e.g. 

// To get "~/Tom-1.txt"
QString my_formatted_string = QString("%1/%2-%3.txt").arg("~", "Tom", QString::number(1));

As you can see, the change is minimal. Of course, you generally do not need to care about the type that is passed into QString::arg() since most types are correctly overloaded.

One drawback though: QString::arg() doesn’t handle std::string. You will need to call: QString::fromStdString() on your std::string to make it into a QString before passing it to QString::arg(). Try to separate the classes that use QString from the classes that use std::string. Or if you can, switch to QString altogether.

UPDATE: Examples are updated thanks to Frank Osterfeld.

UPDATE: Examples are updated thanks to alexisdm.

Answer 3 (score 28)

You can use the sprintf method, however the arg method is preferred as it supports unicode.

QString str;
str.sprintf("%s %d", "string", 213);

Question

I want to set an image on QPushButton, and the size of QPushButton should depend on the size of the image. I am able to do this when using QLabel, but not with QPushButton.

So, if anyone has a solution, then please help me out.

Answer 2 (score 62)

What you can do is use a pixmap as an icon and then put this icon onto the button.

To make sure the size of the button will be correct, you have to reisze the icon according to the pixmap size.

Something like this should work :

QPixmap pixmap("image_path");
QIcon ButtonIcon(pixmap);
button->setIcon(ButtonIcon);
button->setIconSize(pixmap.rect().size());

Answer 3 (score 40)

QPushButton *button = new QPushButton;
button->setIcon(QIcon(":/icons/..."));
button->setIconSize(QSize(65, 65));

Question

I have installed C++SDK that have Qt but when I try compiling a code linking QApplication it gives me the error:

Error QApplication: no such file or directory

How do I link these libraries? I searched into the directories and there is a file named QApplication.h; So I tried to link it with -I (linking the directory) but it was still giving me that error.

Answer accepted (score 57)

To start things off, the error QApplication: no such file or directory means your compiler was not able to find this header. It is not related to the linking process as you mentioned in the question.

The -I flag (uppercase i) is used to specify the include (headers) directory (which is what you need to do), while the -L flag is used to specify the libraries directory. The -l flag (lowercase L) is used to link your application with a specified library.

But you can use Qt to your advantage: Qt has a build system named qmake which makes things easier. For instance, when I want to compile main.cpp I create a main.pro file. For educational purposes, let’s say this source code is a simple project that uses only QApplication and QDeclarativeView. An appropriate .pro file would be:

TEMPLATE += app
QT += gui declarative
SOURCES += main.cpp

Then, execute the qmake inside that directory to create the Makefile that will be used to compile your application, and finally execute make to get the job done.

On my system this make outputs:

g++ -c -pipe -O2 -Wall -W -D_REENTRANT -DQT_NO_DEBUG -DQT_DECLARATIVE_LIB -DQT_GUI_LIB -DQT_CORE_LIB -DQT_SHARED -I/opt/qt_47x/mkspecs/linux-g++ -I. -I/opt/qt_47x/include/QtCore -I/opt/qt_47x/include/QtGui -I/opt/qt_47x/include/QtDeclarative -I/opt/qt_47x/include -I/usr/X11R6/include -I. -o main.o main.cpp
g++ -Wl,-O1 -Wl,-rpath,/opt/qt_47x/lib -o main main.o -L/opt/qt_47x/lib -L/usr/X11R6/lib -lQtDeclarative -L/opt/qt_47x/lib -lQtScript -lQtSvg -L/usr/X11R6/lib -lQtSql -lQtXmlPatterns -lQtNetwork -lQtGui -lQtCore -lpthread

Note: I installed Qt in another directory –> /opt/qt_47x

Edit: Qt 5.x and later

Add QT += widgets to the .pro file and solve this problem.

Answer 2 (score 100)

In Qt 5 you now have to add widgets to the QT qmake variable (in your MyProject.pro file).

 QT += widgets

Answer 3 (score 29)

For QT 5

Step1: .pro (in pro file, add these 2 lines)

QT       += core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

Step2: In main.cpp replace code:

#include <QtGui/QApplication> 

with:

#include <QApplication>

Question

In Qt, how do I check if a given folder exists in the current directory?
If it doesn’t exist, how do I then create an empty folder?

Answer accepted (score 214)

To check if a directory named “Folder” exists use:

QDir("Folder").exists();

To create a new folder named “MyFolder” use:

QDir().mkdir("MyFolder");

Answer 2 (score 137)

To both check if it exists and create if it doesn’t, including intermediaries:

QDir dir("path/to/dir");
if (!dir.exists())
    dir.mkpath(".");

Answer 3 (score 12)

When you use QDir.mkpath() it returns true if the path already exists, in the other hand QDir.mkdir() returns false if the path already exists. So depending on your program you have to choose which fits better.

You can see more on Qt Documentation

Question

I am new to Qt, and I am trying to create a simple GUI Application that displays an image once a button has been clicked on.

I can read the image in a QImage object, but is there any simple way to call a Qt function that takes the QImage as an input, and displays it?

Answer accepted (score 6)

Thanks All, I found how to do it, which is the same as Dave and Sergey:

I am using QT Creator:

In the main GUI window create using the drag drop GUI and create label (e.g. “myLabel”)

In the callback of the button (clicked) do the following using the (*ui) pointer to the user interface window:

void MainWindow::on_pushButton_clicked()
{
     QImage imageObject;
     imageObject.load(imagePath);
     ui->myLabel->setPixmap(QPixmap::fromImage(imageObject));

     //OR use the other way by setting the Pixmap directly

     QPixmap pixmapObject(imagePath");
     ui->myLabel2->setPixmap(pixmapObject);
}

Answer 2 (score 68)

Simple, but complete example showing how to display QImage might look like this:

#include <QtGui/QApplication>
#include <QLabel>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QImage myImage;
    myImage.load("test.png");

    QLabel myLabel;
    myLabel.setPixmap(QPixmap::fromImage(myImage));

    myLabel.show();

    return a.exec();
}

Answer 3 (score 24)

Drawing an image using a QLabel seems like a bit of a kludge to me. With newer versions of Qt you can use a QGraphicsView widget. In Qt Creator, drag a Graphics View widget onto your UI and name it something (it is named mainImage in the code below). In mainwindow.h, add something like the following as private variables to your MainWindow class:

QGraphicsScene *scene;
QPixmap image;

Then just edit mainwindow.cpp and make the constructor something like this:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent), ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    image.load("myimage.png");
    scene = new QGraphicsScene(this);
    scene->addPixmap(image);
    scene->setSceneRect(image.rect());

    ui->mainImage->setScene(scene);
}

Question

What is the correct interpretation of the following segfault messages?

segfault at 10 ip 00007f9bebcca90d sp 00007fffb62705f0 error 4 in libQtWebKit.so.4.5.2[7f9beb83a000+f6f000]
segfault at 10 ip 00007fa44d78890d sp 00007fff43f6b720 error 4 in libQtWebKit.so.4.5.2[7fa44d2f8000+f6f000]
segfault at 11 ip 00007f2b0022acee sp 00007fff368ea610 error 4 in libQtWebKit.so.4.5.2[7f2aff9f7000+f6f000]
segfault at 11 ip 00007f24b21adcee sp 00007fff7379ded0 error 4 in libQtWebKit.so.4.5.2[7f24b197a000+f6f000]

Answer accepted (score 86)

This is a segfault due to following a null pointer trying to find code to run (that is, during an instruction fetch).

If this were a program, not a shared library

Run addr2line -e yourSegfaultingProgram 00007f9bebcca90d (and repeat for the other instruction pointer values given) to see where the error is happening. Better, get a debug-instrumented build, and reproduce the problem under a debugger such as gdb.

Since it’s a shared library

You’re hosed, unfortunately; it’s not possible to know where the libraries were placed in memory by the dynamic linker after-the-fact. Reproduce the problem under gdb.

What the error means

Here’s the breakdown of the fields:

• address (after the at) - the location in memory the code is trying to access (it’s likely that 10 and 11 are offsets from a pointer we expect to be set to a valid value but which is instead pointing to 0)
• ip - instruction pointer, ie. where the code which is trying to do this lives
• sp - stack pointer

• error - An error code for page faults; see below for what this means on x86.

  /*
   * Page fault error code bits:
   *
   *   bit 0 ==    0: no page found       1: protection fault
   *   bit 1 ==    0: read access         1: write access
   *   bit 2 ==    0: kernel-mode access  1: user-mode access
   *   bit 3 ==                           1: use of reserved bit detected
   *   bit 4 ==                           1: fault was an instruction fetch
   */
  ```</li>
  </ul>
  
  #### Answer 2 (score 46)
  Error 4 means "The cause was a user-mode read resulting in no page being found.". There's a tool that decodes it <a href="http://rgeissert.blogspot.com/p/segmentation-fault-error.html" rel="noreferrer">here</a>.  
  
  Here's the definition from the kernel. Keep in mind that 4 means that bit 2 is set and no other bits are set. If you convert it to binary that becomes clear.  
  
  ```c
  /*
   * Page fault error code bits
   *      bit 0 == 0 means no page found, 1 means protection fault
   *      bit 1 == 0 means read, 1 means write
   *      bit 2 == 0 means kernel, 1 means user-mode
   *      bit 3 == 1 means use of reserved bit detected
   *      bit 4 == 1 means fault was an instruction fetch
   */
  #define PF_PROT         (1<<0)
  #define PF_WRITE        (1<<1)
  #define PF_USER         (1<<2)
  #define PF_RSVD         (1<<3)
  #define PF_INSTR        (1<<4)

  Now then, “ip 00007f9bebcca90d” means the instruction pointer was at 0x00007f9bebcca90d when the segfault happened.

  “libQtWebKit.so.4.5.2[7f9beb83a000+f6f000]” tells you:

  • The object the crash was in: “libQtWebKit.so.4.5.2”
  • The base address of that object “7f9beb83a000”
  • How big that object is: “f6f000”

  If you take the base address and subtract it from the ip, you get the offset into that object:

  0x00007f9bebcca90d - 0x7f9beb83a000 = 0x49090D

  Then you can run addr2line on it:

  addr2line -e /usr/lib64/qt45/lib/libQtWebKit.so.4.5.2 -fCi 0x49090D
  ??
  ??:0

  In my case it wasn’t successful, either the copy I installed isn’t identical to yours, or it’s stripped.

Answer 3 (score 10)

Let’s go to the source – 2.6.32, for example. The message is printed by show_signal_msg() function in arch/x86/mm/fault.c if the show_unhandled_signals sysctl is set.

“error” is not an errno nor a signal number, it’s a “page fault error code” – see definition of enum x86_pf_error_code.

“[7fa44d2f8000+f6f000]” is starting address and size of virtual memory area where offending object was mapped at the time of crash. Value of “ip” should fit in this region. With this info in hand, it should be easy to find offending code in gdb.

Question

I’m trying to get the current time as TimeStamp without success.

I have this code:

QDateTime setTime = QDateTime::fromString (QString("1970-07-18T14:15:09"), Qt::ISODate);
QDateTime current = QDateTime::currentDateTime();
uint msecs = setTime.time().msecsTo(current.time());

return  QString::number(msecs);

The output is

Sunday, January 25th 1970, 03:17:35 (GMT)

Answer accepted (score 35)

I think you are looking for this function:

http://doc.qt.io/qt-5/qdatetime.html#toTime_t

uint QDateTime::toTime_t () const

Returns the datetime as the number of seconds that have passed since 1970-01-01T00:00:00, > Coordinated Universal Time (Qt::UTC).

On systems that do not support time zones, this function will behave as if local time were Qt::UTC.

See also setTime_t().

Answer 2 (score 77)

In Qt 4.7, there is the QDateTime::currentMSecsSinceEpoch() static function, which does exactly what you need, without any intermediary steps. Hence I’d recommend that for projects using Qt 4.7 or newer.

Answer 3 (score 4)

Since Qt 5.8, we now have QDateTime::currentSecsSinceEpoch() to deliver the seconds directly, a.k.a. as real Unix timestamp. So, no need to divide the result by 1000 to get seconds anymore.

Credits: also posted as comment to this answer. However, I think it is easier to find if it is a separate answer.

Question

So if I go into QtDesigner and build a UI, it’ll be saved as a .ui file. How can I make this as a python file or use this in python?

Answer accepted (score 58)

Another way to use .ui in your code is:

from PyQt4 import QtCore, QtGui, uic
class MyWidget(QtGui.QWidget)
    ...
    #somewhere in constructor:
    uic.loadUi('MyWidget.ui', self)

both approaches are good. Do not forget, that if you use Qt resource files (extremely useful) for icons and so on, you must compile it too:

pyrcc4.exe -o ui/images_rc.py ui/images/images.qrc

Note, when uic compiles interface, it adds ‘import images_rc’ at the end of .py file, so you must compile resources into the file with this name, or rename it in generated code.

Answer 2 (score 45)

Combining Max’s answer and Shriramana Sharma’s mailing list post, I built a small working example for loading a mywindow.ui file containing a QMainWindow (so just choose to create a Main Window in Qt Designer’s File-New dialog).

This is the code that loads it:

import sys
from PyQt4 import QtGui, uic

class MyWindow(QtGui.QMainWindow):
    def __init__(self):
        super(MyWindow, self).__init__()
        uic.loadUi('mywindow.ui', self)
        self.show()

if __name__ == '__main__':
    app = QtGui.QApplication(sys.argv)
    window = MyWindow()
    sys.exit(app.exec_())

Answer 3 (score 28)

You need to generate a python file from your ui file with the pyuic tool (site-packages)

pyuic form1.ui > form1.py

with pyqt4

pyuic4.bat form1.ui > form1.py

Then you can import the form1 into your script.

Question

I have this function in my program that converts integers to strings:

    QString Stats_Manager::convertInt(int num)
    {
        stringstream ss;
        ss << num;
        return ss.str();
    }

But when ever i run this i get the error:

aggregate 'std::stringstream ss' has incomplete type and cannot be defined

Im not really sure what that means. But if you know how to fix it or need any more code please just comment. Thanks.

Answer accepted (score 137)

You probably have a forward declaration of the class, but haven’t included the header:

#include <sstream>

//...
QString Stats_Manager::convertInt(int num)
{
    std::stringstream ss;   // <-- also note namespace qualification
    ss << num;
    return ss.str();
}

Answer 2 (score 6)

Like it’s written up there, you forget to type #include <sstream>

#include <sstream>
using namespace std;

QString Stats_Manager::convertInt(int num)
{
   stringstream ss;
   ss << num;
   return ss.str();
}

You can also use some other ways to convert int to string, like

char numstr[21]; // enough to hold all numbers up to 64-bits
sprintf(numstr, "%d", age);
result = name + numstr;

check this!

Question

I have a Qt GUI application running on Windows that allows command-line options to be passed and under some circumstances I want to output a message to the console and then quit, for example:

int main(int argc, char *argv[])
{
  QApplication a(argc, argv);

  if (someCommandLineParam)
  {
    std::cout << "Hello, world!";
    return 0;
  }

  MainWindow w;
  w.show();

  return a.exec();
}

However, the console messages do not appear when I run the app from a command-prompt. Does anyone know how I can get this to work?

Answer accepted (score 46)

Windows does not really support dual mode applications.

To see console output you need to create a console application

CONFIG += console

However, if you double click on the program to start the GUI mode version then you will get a console window appearing, which is probably not what you want. To prevent the console window appearing you have to create a GUI mode application in which case you get no output in the console.

One idea may be to create a second small application which is a console application and provides the output. This can call the second one to do the work.

Or you could put all the functionality in a DLL then create two versions of the .exe file which have very simple main functions which call into the DLL. One is for the GUI and one is for the console.

Answer 2 (score 12)

Add:

#ifdef _WIN32
if (AttachConsole(ATTACH_PARENT_PROCESS)) {
    freopen("CONOUT$", "w", stdout);
    freopen("CONOUT$", "w", stderr);
}
#endif

at the top of main(). This will enable output to the console only if the program is started in a console, and won’t pop up a console window in other situations. If you want to create a console window to display messages when you run the app outside a console you can change the condition to:

if (AttachConsole(ATTACH_PARENT_PROCESS) || AllocConsole())

Answer 3 (score 6)

void Console()
{
    AllocConsole();
    FILE *pFileCon = NULL;
    pFileCon = freopen("CONOUT$", "w", stdout);

    COORD coordInfo;
    coordInfo.X = 130;
    coordInfo.Y = 9000;

    SetConsoleScreenBufferSize(GetStdHandle(STD_OUTPUT_HANDLE), coordInfo);
    SetConsoleMode(GetStdHandle(STD_OUTPUT_HANDLE),ENABLE_QUICK_EDIT_MODE| ENABLE_EXTENDED_FLAGS);
}

int main(int argc, char *argv[])
{
    Console();
    std::cout<<"start@@";
    qDebug()<<"start!";

You can’t use std::cout as others have said,my way is perfect even for some code can’t include “qdebug” !

Question

I was trying to create a simple console application to try out Qt’s XML parser. I started a project in VS2008 and got this template:

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    return a.exec();
}

Since I don’t need event processing, I was wondering whether I may get into trouble if I neglect to create a QCoreApplication and running the event loop. The docs state that it’s recommended in most cases.

For the sake of curiosity however, I am wondering how could I make some generic task execute on the event loop and then terminate the application. I was unable to google a relevant example.

Answer accepted (score 99)

Here is one simple way you could structure an application if you want an event loop running.

// main.cpp
#include <QtCore>

class Task : public QObject
{
    Q_OBJECT
public:
    Task(QObject *parent = 0) : QObject(parent) {}

public slots:
    void run()
    {
        // Do processing here

        emit finished();
    }

signals:
    void finished();
};

#include "main.moc"

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    // Task parented to the application so that it
    // will be deleted by the application.
    Task *task = new Task(&a);

    // This will cause the application to exit when
    // the task signals finished.    
    QObject::connect(task, SIGNAL(finished()), &a, SLOT(quit()));

    // This will run the task from the application event loop.
    QTimer::singleShot(0, task, SLOT(run()));

    return a.exec();
}

Answer 2 (score 18)

Don’t forget to add the

CONFIG += console 

flag in the qmake .pro file.

For the rest is just using some of Qt classes. One way I use it is to spawn processes cross-platform.

Answer 3 (score 4)

You don’t need the QCoreApplication at all, just include your Qt objects as you would other objects, for example:

#include <QtCore>

int main()
{
    QVector<int> a; // Qt object

    for (int i=0; i<10; i++)
    {
        a.append(i);
    }

    /* manipulate a here */

    return 0;
}

Question

I have a Qt project and I would like to output compilation files outside the source tree.

I currently have the following directory structure:

/
|_/build
|_/mylib
  |_/include
  |_/src
  |_/resources

Depending on the configuration (debug/release), I will like to output the resulting files inside the build directory under build/debug or build/release directories.

How can I do that using a .pro file?

Answer accepted (score 3)

The short answer is: you don’t.

You should run qmake followed by make in whatever build directory you want to build in. So, run it once in a debug directory, once in a release directory.

That’s how anyone building your project would expect it to work, and that’s how Qt itself is set up to build, that’s also how Qt Creator expects your .pro file to behave: it simply starts qmake and then make in the build folder for your target’s chosen configuration.

If you wish to create these folders and perform the two (or more) builds in them, you’ll need a top-level makefile, possibly created from a top-level project file via qmake.

It’s not uncommon to have more than two build configurations, so you’re unnecessarily committing yourself to only differentiating between a build and a release; you might have builds with different optimization levels, etc. The debug/release dichotomy is best left to rest in peace.

Answer 2 (score 147)

For my Qt project, I use this scheme in *.pro file:

HEADERS += src/dialogs.h
SOURCES += src/main.cpp \
           src/dialogs.cpp

Release:DESTDIR = release
Release:OBJECTS_DIR = release/.obj
Release:MOC_DIR = release/.moc
Release:RCC_DIR = release/.rcc
Release:UI_DIR = release/.ui

Debug:DESTDIR = debug
Debug:OBJECTS_DIR = debug/.obj
Debug:MOC_DIR = debug/.moc
Debug:RCC_DIR = debug/.rcc
Debug:UI_DIR = debug/.ui

It`s simple, but nice! :)

Answer 3 (score 49)

To change the directory for target dll/exe, use this in your pro file:

CONFIG(debug, debug|release) {
    DESTDIR = build/debug
} else {
    DESTDIR = build/release
}

You might also want to change directories for other build targets like object files and moc files (check qmake variable reference for details or qmake CONFIG() function reference).

Question

Platform: QT, Windows XP

I am new to Qt. I want to show another window(what to do to open it as dialog) from mainwindow. I did “add New Item ->Qt Designer Form Class”, named it say MyWindow. But how to show this MyWindow from mainwindow ?

Answer 2 (score 54)

1. Implement a slot in your QMainWindow where you will open your new Window,
2. Place a widget on your QMainWindow,
3. Connect a signal from this widget to a slot from the QMainWindow (for example: if the widget is a QPushButton connect the signal click() to the QMainWindow custom slot you have created).

Code example:

MainWindow.h

// ...
include "newwindow.h"
// ...
public slots:
   void openNewWindow();
// ...
private:
   NewWindow *mMyNewWindow;
// ...
}

MainWindow.cpp

// ...
   MainWindow::MainWindow()
   {
      // ...
      connect(mMyButton, SIGNAL(click()), this, SLOT(openNewWindow()));
      // ...
   }
// ...
void MainWindow::openNewWindow()
{
   mMyNewWindow = new NewWindow(); // Be sure to destroy your window somewhere
   mMyNewWindow->show();
   // ...
}

This is an example on how display a custom new window. There are a lot of ways to do this.

Question

In my application I want that when a loop is being executed, each time the control transfers to the loop, each execution must be delayed by a particular time. How can I do this?

Answer accepted (score 48)

EDIT (removed wrong solution). EDIT (to add this other option):

Another way to use it would be subclass QThread since it has protected *sleep methods.

QThread::usleep(unsigned long microseconds);
QThread::msleep(unsigned long milliseconds);
QThread::sleep(unsigned long second);

Here’s the code to create your own *sleep method.

#include <QThread>    

class Sleeper : public QThread
{
public:
    static void usleep(unsigned long usecs){QThread::usleep(usecs);}
    static void msleep(unsigned long msecs){QThread::msleep(msecs);}
    static void sleep(unsigned long secs){QThread::sleep(secs);}
};

and you call it by doing this:

Sleeper::usleep(10);
Sleeper::msleep(10);
Sleeper::sleep(10);

This would give you a delay of 10 microseconds, 10 milliseconds or 10 seconds, accordingly. If the underlying operating system timers support the resolution.

Answer 2 (score 25)

As an update of @Live’s answer, for Qt ≥ 5.2 there is no more need to subclass QThread, as now the sleep functions are public:

Static Public Members

• QThread * currentThread()
• Qt::HANDLE currentThreadId()
• int idealThreadCount()
• void msleep(unsigned long msecs)
• void sleep(unsigned long secs)
• void usleep(unsigned long usecs)
• void yieldCurrentThread()

cf http://qt-project.org/doc/qt-5/qthread.html#static-public-members

Answer 3 (score 8)

C++11 has some portable timer stuff. Check out sleep_for.

Question

I am developing an application using Qt. I don’t know which slot corresponds to the event of “the user clicking the ‘X’(close) button of the window frame” i.e. this button:

If there isn’t a slot for this, can anyone suggest me some other method by which I can start a function after the user presses that close button.

Answer accepted (score 145)

If you have a QMainWindow you can override closeEvent method.

#include <QCloseEvent>
void MainWindow::closeEvent (QCloseEvent *event)
{
    QMessageBox::StandardButton resBtn = QMessageBox::question( this, APP_NAME,
                                                                tr("Are you sure?\n"),
                                                                QMessageBox::Cancel | QMessageBox::No | QMessageBox::Yes,
                                                                QMessageBox::Yes);
    if (resBtn != QMessageBox::Yes) {
        event->ignore();
    } else {
        event->accept();
    }
}

If you’re subclassing a QDialog, the closeEvent will not be called and so you have to override reject():

void MyDialog::reject()
{
    QMessageBox::StandardButton resBtn = QMessageBox::Yes;
    if (changes) {
        resBtn = QMessageBox::question( this, APP_NAME,
                                        tr("Are you sure?\n"),
                                        QMessageBox::Cancel | QMessageBox::No | QMessageBox::Yes,
                                        QMessageBox::Yes);
    }
    if (resBtn == QMessageBox::Yes) {
        QDialog::reject();
    }
}

Answer 2 (score 15)

Well, I got it. One way is to override the QWidget::closeEvent(QCloseEvent *event) method in your class definition and add your code into that function. Example:

class foo : public QMainWindow
{
    Q_OBJECT
private:
    void closeEvent(QCloseEvent *bar);
    // ...
};


void foo::closeEvent(QCloseEvent *bar)
{
    // Do something
    bar->accept();
}

Answer 3 (score 12)

You can attach a SLOT to the

void aboutToQuit();

signal of your QApplication. This signal should be raised just before app closes.

Question

I am writing a program in QT. I want to convert a double into a Qstring in C++.

Answer accepted (score 82)

Use QString’s number method (docs are here):

double valueAsDouble = 1.2;
QString valueAsString = QString::number(valueAsDouble);

Answer 2 (score 16)

Instead of QString::number() i would use QLocale::toString(), so i can get locale aware group seperatores like german “1.234.567,89”.

Answer 3 (score 8)

You can use arg(), as follow:

double dbl = 0.25874601;
QString str = QString("%1").arg(dbl);

This overcomes the problem of: “Fixed precision” at the other functions like: setNum() and number(), which will generate random numbers to complete the defined precision

Question

How to change the title of the window in Qt? (Both for QDialog and QMainWindow.)

Answer accepted (score 102)

void    QWidget::setWindowTitle ( const QString & )

EDIT: If you are using QtDesigner, on the property tab, there is an editable property called windowTitle which can be found under the QWidget section. The property tab can usually be found on the lower right part of the designer window.

Answer 2 (score 45)

For new Qt users this is a little more confusing than it seems if you are using QT Designer and .ui files.

Initially I tried to use ui->setWindowTitle, but that doesn’t exist. ui is not a QDialog or a QMainWindow.

The owner of the ui is the QDialog or QMainWindow, the .ui just describes how to lay it out. In that case, you would use:

this->setWindowTitle("New Title");

I hope this helps someone else.

Answer 3 (score 2)

I know this is years later but I ran into the same problem. The solution I found was to change the window title in main.cpp. I guess once the w.show(); is called the window title can no longer be changed. In my case I just wanted the title to reflect the current directory and it works.

int main(int argc, char *argv[]) 
{
QApplication a(argc, argv);
MainWindow w;
w.setWindowTitle(QDir::currentPath());
w.show();

return a.exec();
}

Question

Compiling on Fedora 10.

I have just started my first qt GUI application. I used all the default settings.

Its just a simple form. It builds OK without any errors. But when I try and run the application. I get the following message:

Starting /home/rob/projects/qt/test1/test1/test1...
No protocol specified
test1: cannot connect to X server :0.0

Thanks for any advice,

Answer accepted (score 41)

The general causes for this are as follows:

1. DISPLAY not set in the environment.
   Solution:

   export DISPLAY=:0.0
   ./myQtCmdHere

   ( This one doesn’t appear to be the one at fault though, as its saying which X display its trying to connect to. Also, its not always 0.0, but most of the time it is )

2. Non-Authorised User trying to run the X Application
   Solution ( as X owning user, ie: yourself )

   xhost +local:root   # where root is the local user you want to grant access to. 
   ```</li>
   </ol>
   
   #### Answer 2 (score 5)
   Also, if you'd like your X server to be able to receive connection over TCP, these days you must almost always explicitly enable this. To test whether you're server is allowing remote TCP connections try:  
   
   telnet 127.0.0.1 6000  
   
   If telnet is able to connect, then your X server is listening. If it can't, then neither will any remote X application and you need to enable remote TCP connections on your server.  
   
   #### Answer 3 (score 2)
   <p>Adding to above answers. 
   I was in a similar situation while running tests for <a href="https://github.com/tushar-rishav/code2pdf" rel="nofollow">Code2Pdf</a> at travis.
   I solved the problem using <a href="http://manpages.ubuntu.com/manpages/lucid/man1/xvfb-run.1.html" rel="nofollow">xvfb-run</a>. Quoting from the manpage,</p>
   
   <blockquote>
     xvfb-run is a wrapper for the Xvfb(1x)  command  which  simplifies the task of running commands (typically an X client, or a script containing a list of clients to be run) within a virtual X server environment.  
   </blockquote>
   
   The script that I wrote was:  
   
   ```c
   check_install_xvfb() { # check and install xvfb
       if hash xvfb-run 2>/dev/null; then
           :
       else
           sudo apt-get update
           sudo apt-get upgrade
           sudo apt-get install xvfb
       fi
   }
   check_install_xvfb
   export DISPLAY=localhost:1.0
   xvfb-run -a bash .misc/tests.sh
   # .misc/tests.sh is script that runs unit tests. You can replace it with something suitable to you.

   Please bear with my bash code style. I am a noob bash programmer.

   Running the above script helped me. You can see the failing build and passing build.

   Hope it helps

Question

Which is better (or faster), a C++ for loop or the foreach operator provided by Qt? For example, the following condition

QList<QString> listofstrings;

Which is better?

foreach(QString str, listofstrings)
{
    //code
}

or

int count = listofstrings.count();
QString str = QString();
for(int i=0;i<count;i++)
{
    str = listofstrings.at(i);
    //Code
}

Answer accepted (score 148)

It really doesn’t matter in most cases.

The large number of questions on StackOverflow regarding whether this method or that method is faster, belie the fact that, in the vast majority of cases, code spends most of its time sitting around waiting for users to do something.

If you are really concerned, profile it for yourself and act on what you find.

But I think you’ll most likely find that only in the most intense data-processing-heavy work does this question matter. The difference may well be only a couple of seconds and even then, only when processing huge numbers of elements.

Get your code working first. Then get it working fast (and only if you find an actual performance issue).

Time spent optimising before you’ve finished the functionality and can properly profile, is mostly wasted time.

Answer 2 (score 23)

First off, I’d just like to say I agree with Pax, and that the speed probably doesn’t enter into it. foreach wins hands down based on readability, and that’s enough in 98% of cases.

But of course the Qt guys have looked into it and actually done some profiling: http://blog.qt.io/blog/2009/01/23/iterating-efficiently/

The main lesson to take away from that is: use const references in read only loops as it avoids the creation of temporary instances. It also make the purpose of the loop more explicit, regardless of the looping method you use.

Answer 3 (score 14)

It really doesn’t matter. Odds are if your program is slow, this isn’t the problem. However, it should be noted that you aren’t make a completely equal comparison. Qt’s foreach is more similar to this (this example will use QList<QString>):

for(QList<QString>::iterator it = Con.begin(); it != Con.end(); ++it) {
    QString &str = *it;
    // your code here
}

The macro is able to do this by using some compiler extensions (like GCC’s __typeof__) to get the type of the container passed. Also imagine that boost’s BOOST_FOREACH is very similar in concept.

The reason why your example isn’t fair is that your non-Qt version is adding extra work.

You are indexing instead of really iterating. If you are using a type with non-contiguous allocation (I suspect this might be the case with QList<>), then indexing will be more expensive since the code has to calculate “where” the n-th item is.

That being said. It still doesn’t matter. The timing difference between those two pieces of code will be negligible if existent at all. Don’t waste your time worrying about it. Write whichever you find more clear and understandable.

EDIT: As a bonus, currently I strongly favor the C++11 version of container iteration, it is clean, concise and simple:

for(QString &s : Con) {
    // you code here
}

Question

This question must be duplicate many times, but it just doesn’t work and sometimes it still remains unanswered. Sources of information are mainly these
http://www.laganiere.name/opencvCookbook/chap1s1_2.shtml
http://www.youtube.com/watch?v=dgcXYQijV6c

This is the summation of what I think one should/can do. (And now it works for me.) Hopefully I mentioned everything from the very beginning, the aim is to write a very clear tutorial.

Installation of OpenCV for QtCreator

1. I have already MS Visual Studio 2010 Professional installed. (I have a free licence as a student) - I think this is not necessary, just a mention
2. Download: Qt 5.0.1 for Windows 32-bit (MinGW 4.7, 823 MB)
   2.1 Install: Warning, everything that Qt uses (e.g. OpenCV) must be in directories that don’t contain white-spaces in their names. - i.e. “Program Files” is wrong. (But I don’t want different program files to accumulate directly on C, so I’ve only made a folder “Programs” in which everything important is installed)
3. Download: cmake-2.8.10.2-win32-x86.exe - Install for all users (this can be in Program Files)
4. Download: OpenCV-2.4.0.exe, extract to: C: - it’ll create a dir “opencv”; add another folder “opencv_bin”. Now it looks like this:
   C:
   C:_bin
5. Set PATH environment variable, so that there be a link to MinGW compiler. e.g. C:.1;
6. Start cmake-gui.exe
   6.1 source code: set the default dir for OpenCV; C:
   6.2 binaries: set the opencv_bin dir; C:_bin
   6.3 click configure:
   • Choose MinGW Makefiles and Specify native compilers, click next
   • Field C is for gcc.exe; C:/Programs/Qt/Qt5.0.1/Tools/MinGW/bin/gcc.exe
   • Field C++ is for g++.exe; C:/Programs/Qt/Qt5.0.1/Tools/MinGW/bin/g++.exe
   Field fortran can be empty, click finish
   6.4 Many red lines will appear To the search field enter one by one: WITH_QT, WITH_TBB, WITH_IPP, WITH_CUDA, CMAKE_BUILD_TYPE
   • WITH_QT - must be selected.
   • WITH_TBB, WITH_IPP, WITH_CUDA - must be unselected
   • CMAKE_BUILD_TYPE - click and enter a text “Debug” (without quotes).
   Clear the text from the Search field.
   6.5 click configure and keep clicking configure until all red lines are gone, then click generate and close cmake-gui.exe
7. Go to the terminal (~command prompt), cd to the directory where are the builds (opencv_bin) and type mingw32-make
   
8. When the process ends after a long time, type mingw32-make install
   
9. Add into Path variable the path to the QtCreator/bin C:.1

Now I have created a new console app in QtCreator.

//cvHello.pro
QT       += core
QT       -= gui

TARGET = cvHello
CONFIG   += console
CONFIG   -= app_bundle

TEMPLATE = app
INCLUDEPATH += C:/Programs/opencv24/opencv_bin2/install/include
LIBS += "C:/Programs/opencv24/opencv_bin2/bin/*.dll"

SOURCES += main.cpp
OTHER_FILES += \
    img.JPG

And the main file:

//main.cpp
#include <iostream>
#include "opencv2/core/core.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv/cv.h"

using namespace std;

int main()
{
    cout << "Hello World!" << endl;

    cv::Mat mat;
    mat = cv::imread("img.JPG");
    cvNamedWindow("hello");
    cv::imshow("hello",mat);

    cvWaitKey(0);

    return 0;
}

Answer accepted (score 13)

Finally I am starting to be happy. When adjusting this question I had to try all ways, how to define LIBS. Listing them manually helped, at first I wrote them somehow wrongly.

This is how it works finally:

LIBS += -LC:\\Programs\\opencv24\\opencv_bin2\\bin \
    libopencv_core240d \
    libopencv_highgui240d \
    libopencv_imgproc240d \
    libopencv_features2d240d \
    libopencv_calib3d240d \

Btw if I’ve made any grammar mistakes, I am sorry for my english. :)

Answer 2 (score 2)

The originally accepted answer did not work for me, I am running MSVC2013 Professional and QT5.9. I found SIMPLE and SUREFIRE CROSS-PLATFORM solution that should help anyone who is trying to link an external library (like openCV) with QT.

The steps listed below are found in the Qt5 documentation: http://doc.qt.io/qtcreator/creator-project-qmake-libraries.html under the “To Add Library” section.

1. Right click on the project file located in the ‘project pane’ on the left side of the creator… and select “Add Library…”
2. Follow the instructions of the wizard

Let me add some specificity from here:

3. Select “External Library”
4. For the “Library File” navigate to your opencv_worldXXX.lib file (or opencv_worldXXXd.lib file, you will notice that by specifying only one or the other the wizard has a checkbox which includes the other automatically) [ex. …_world310.lib]
5. For the “Include Folder” navigate to the “include” folder within the build. [ex. … ]
6. Select your operating system, dynamic/static library (whichever is appropriate)
7. Hit NEXT, CLEAN UP, and RUN!

Question

I’m having issues with QByteArray and QString.

I’m reading a file and stores its information in a QByteArray. The file is in unicode, so it contains something like: t\0 e\0 s\0 t\0 \0 \0

I’m trying to compare this value to my specified value, but it fails, because in the debugger I see it’s not an unicode string.

The code will explain everything:

QByteArray Data; //contains unicode string "t\0 e\0 s\0 t\0 \0 \0"
QString myValue = "test"; //value to compare.
if(Data.contains(myValue))
    //do some stuff.
else
    //do other stuff.

In the debugger, it shows me that the variable Data has the value "t\0 e\0 s\0 t\0 \0 \0" and myValue has the value "test". How can I fix it?

Answer accepted (score 26)

You can use QTextCodec to convert the bytearray to a string:

QString DataAsString = QTextCodec::codecForMib(1015)->toUnicode(Data);

(1015 is UTF-16, 1014 UTF-16LE, 1013 UTF-16BE, 106 UTF-8)

From your example we can see that the string "test" is encoded as "t\0 e\0 s\0 t\0 \0 \0" in your encoding, i.e. every ascii character is followed by a \0-byte, or resp. every ascii character is encoded as 2 bytes. The only unicode encoding in which ascii letters are encoded in this way, are UTF-16 or UCS-2 (which is a restricted version of UTF-16), so in your case the 1015 mib is needed (assuming your local endianess is the same as the input endianess).

Answer 2 (score 14)

you can use QString::fromAscii()

QByteArray data = entity->getData();
QString s_data = QString::fromAscii(data.data());

with data() returning a char*

for QT5, you should use fromCString() instead, as fromAscii() is deprecated, see https://bugreports.qt-project.org/browse/QTBUG-21872 https://bugreports.qt.io/browse/QTBUG-21872

Answer 3 (score 11)

You can use:

QString::fromStdString(byteArray.toStdString())

Question

How can I read a text file line by line in Qt?

I’m looking for the Qt equivalent to:

std::ifstream infile;
std::string line;
while (std::getline(infile, line))
{
   ...
}

Answer accepted (score 85)

Use this code:

QFile inputFile(fileName);
if (inputFile.open(QIODevice::ReadOnly))
{
   QTextStream in(&inputFile);
   while (!in.atEnd())
   {
      QString line = in.readLine();
      ...
   }
   inputFile.close();
}

Answer 2 (score 3)

QFile inputFile(QString("/path/to/file"));
inputFile.open(QIODevice::ReadOnly);
if (!inputFile.isOpen())
    return;

QTextStream stream(&inputFile);
QString line = stream.readLine();
while (!line.isNull()) {
    /* process information */

    line = stream.readLine();
};

Answer 3 (score 1)

Since Qt 5.5 you can use QTextStream::readLineInto. It behaves similar to std::getline and is maybe faster as QTextStream::readLine, because it reuses the string:

QIODevice* device;
QTextStream in(&device);

QString line;
while (in.readLineInto(&line)) {
  // ...
}

Question

In Qt I’m trying to set a QTimer that calls a function called “update” every second. Here is my .cpp file:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QTimer>
#include "QDebug"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    QTimer *timer = new QTimer(this);
    connect(timer, SIGNAL(timeout()), this, SLOT(update()));
    timer->start(1000);
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::update()
{
    qDebug() << "update";
}

and the main:

#include <QtGui/QApplication>
#include "mainwindow.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

The project is being build, but it doesn’t execute update, since the line “update” is not showing anywhere… Does anybody see what I´m doing wrong?

Answer accepted (score 12)

1. It’s good practice to give a parent to your QTimer to use Qt’s memory management system.

2. update() is a QWidget function - is that what you are trying to call or not? http://qt-project.org/doc/qt-4.8/qwidget.html#update.

3. If number 2 does not apply, make sure that the function you are trying to trigger is declared as a slot in the header.

4. Finally if none of these are your issue, it would be helpful to know if you are getting any run-time connect errors.

Answer 2 (score 15)

Other way is using of built-in method start timer & event TimerEvent.

Header:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private:
    Ui::MainWindow *ui;
    int timerId;

protected:
    void timerEvent(QTimerEvent *event);
};

#endif // MAINWINDOW_H

Source:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QDebug>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    timerId = startTimer(1000);
}

MainWindow::~MainWindow()
{
    killTimer(timerId);
    delete ui;
}

void MainWindow::timerEvent(QTimerEvent *event)
{
    qDebug() << "Update...";
}

Answer 3 (score 0)

mytimer.h:

    #ifndef MYTIMER_H
    #define MYTIMER_H

    #include <QTimer>

    class MyTimer : public QObject
    {
        Q_OBJECT
    public:
        MyTimer();
        QTimer *timer;

    public slots:
        void MyTimerSlot();
    };

    #endif // MYTIME

mytimer.cpp:

#include "mytimer.h"
#include <QDebug>

MyTimer::MyTimer()
{
    // create a timer
    timer = new QTimer(this);

    // setup signal and slot
    connect(timer, SIGNAL(timeout()),
          this, SLOT(MyTimerSlot()));

    // msec
    timer->start(1000);
}

void MyTimer::MyTimerSlot()
{
    qDebug() << "Timer...";
}

main.cpp:

#include <QCoreApplication>
#include "mytimer.h"

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    // Create MyTimer instance
    // QTimer object will be created in the MyTimer constructor
    MyTimer timer;

    return a.exec();
}

If we run the code:

Timer...
Timer...
Timer...
Timer...
Timer...
...

resources

Question

I have a Qt application, and when I run this application, there is a console opening behind it. In development it is nice because i see debug outputs on the console, but when I want to give this executable to the customer there should be no console window. how do I hide it?

(I am using Visual Studio 2008)

Answer accepted (score 49)

It sounds like your linker configuration is incorrect. Right-click the project, Properties, Linker, System, SubSystem setting. Make sure “Windows” is selected, not “Console”.

And, change main() to WinMain().

Answer 2 (score 109)

In the project build linker options set

/SUBSYSTEM:windows
/ENTRY:mainCRTStartup

Or use the following #pragma in the source file with the int main(...)

#pragma comment(linker, "/SUBSYSTEM:windows /ENTRY:mainCRTStartup")

Answer 3 (score 15)

You can get rid of the console by calling:

FreeConsole();

Question

Qt5 has a new JSon parser and I want to use it. The problem is that it isn’t too clear about what the functions do in layman’s terms and how to write code with it. That or I could be reading it wrong.

I want to know the code on creating a Json file in Qt5 and what “encapsulates” mean.

Answer accepted (score 91)

Example: Read json from file

/* test.json */
{
   "appDesc": {
      "description": "SomeDescription",
      "message": "SomeMessage"
   },
   "appName": {
      "description": "Home",
      "message": "Welcome",
      "imp":["awesome","best","good"]
   }
}


void readJson()
   {
      QString val;
      QFile file;
      file.setFileName("test.json");
      file.open(QIODevice::ReadOnly | QIODevice::Text);
      val = file.readAll();
      file.close();
      qWarning() << val;
      QJsonDocument d = QJsonDocument::fromJson(val.toUtf8());
      QJsonObject sett2 = d.object();
      QJsonValue value = sett2.value(QString("appName"));
      qWarning() << value;
      QJsonObject item = value.toObject();
      qWarning() << tr("QJsonObject of description: ") << item;

      /* in case of string value get value and convert into string*/
      qWarning() << tr("QJsonObject[appName] of description: ") << item["description"];
      QJsonValue subobj = item["description"];
      qWarning() << subobj.toString();

      /* in case of array get array and convert into string*/
      qWarning() << tr("QJsonObject[appName] of value: ") << item["imp"];
      QJsonArray test = item["imp"].toArray();
      qWarning() << test[1].toString();
   }

OUTPUT

QJsonValue(object, QJsonObject({"description": "Home","imp": ["awesome","best","good"],"message": "YouTube"}) ) 
"QJsonObject of description: " QJsonObject({"description": "Home","imp": ["awesome","best","good"],"message": "YouTube"}) 
"QJsonObject[appName] of description: " QJsonValue(string, "Home") 
"Home" 
"QJsonObject[appName] of value: " QJsonValue(array, QJsonArray(["awesome","best","good"]) ) 
"best" 

Example: Read json from string

Assign json to string as below and use the readJson() function shown before:

val =   
'  {
       "appDesc": {
          "description": "SomeDescription",
          "message": "SomeMessage"
       },
       "appName": {
          "description": "Home",
          "message": "Welcome",
          "imp":["awesome","best","good"]
       }
    }';

OUTPUT

QJsonValue(object, QJsonObject({"description": "Home","imp": ["awesome","best","good"],"message": "YouTube"}) ) 
"QJsonObject of description: " QJsonObject({"description": "Home","imp": ["awesome","best","good"],"message": "YouTube"}) 
"QJsonObject[appName] of description: " QJsonValue(string, "Home") 
"Home" 
"QJsonObject[appName] of value: " QJsonValue(array, QJsonArray(["awesome","best","good"]) ) 
"best" 

Answer 2 (score 2)

Sadly, many JSON C++ libraries have APIs that are non trivial to use, while JSON was intended to be easy to use.

So I tried jsoncpp from the gSOAP tools on the JSON doc shown in one of the answers above and this is the code generated with jsoncpp to construct a JSON object in C++ which is then written in JSON format to std::cout:

value x(ctx);
x["appDesc"]["description"] = "SomeDescription";
x["appDesc"]["message"] = "SomeMessage";
x["appName"]["description"] = "Home";
x["appName"]["message"] = "Welcome";
x["appName"]["imp"][0] = "awesome";
x["appName"]["imp"][1] = "best";
x["appName"]["imp"][2] = "good";
std::cout << x << std::endl;

and this is the code generated by jsoncpp to parse JSON from std::cin and extract its values (replace USE_VAL as needed):

value x(ctx);
std::cin >> x;
if (x.soap->error)
  exit(EXIT_FAILURE); // error parsing JSON
#define USE_VAL(path, val) std::cout << path << " = " << val << std::endl
if (x.has("appDesc"))
{
  if (x["appDesc"].has("description"))
    USE_VAL("$.appDesc.description", x["appDesc"]["description"]);
  if (x["appDesc"].has("message"))
    USE_VAL("$.appDesc.message", x["appDesc"]["message"]);
}
if (x.has("appName"))
{
  if (x["appName"].has("description"))
    USE_VAL("$.appName.description", x["appName"]["description"]);
  if (x["appName"].has("message"))
    USE_VAL("$.appName.message", x["appName"]["message"]);
  if (x["appName"].has("imp"))
  {
    for (int i2 = 0; i2 < x["appName"]["imp"].size(); i2++)
      USE_VAL("$.appName.imp[]", x["appName"]["imp"][i2]);
  }
}

This code uses the JSON C++ API of gSOAP 2.8.28. I don’t expect people to change libraries, but I think this comparison helps to put JSON C++ libraries in perspective.

Answer 3 (score 0)

An example on how to use that would be great. There is a couple of examples at the Qt forum, but you’re right that the official documentation should be expanded.

QJsonDocument on its own indeed doesn’t produce anything, you will have to add the data to it. That’s done through the QJsonObject, QJsonArray and QJsonValue classes. The top-level item needs to be either an array or an object (because 1 is not a valid json document, while {foo: 1} is.)

Question

I’m looking for the equivalent in Qt to GetTickCount()

Something that will allow me to measure the time it takes for a segment of code to run as in:

uint start = GetTickCount();
// do something..
uint timeItTook = GetTickCount() - start;

any suggestions?

Answer accepted (score 90)

How about QTime? Depending on your platform it should have 1 millisecond accuracy. Code would look something like this:

QTime myTimer;
myTimer.start();
// do something..
int nMilliseconds = myTimer.elapsed();

Answer 2 (score 115)

I think it’s probably better to use QElapsedTimer since that is why the class exists in the first place. It was introduced with Qt 4.7. Note that it is also immuned to system’s clock time change.

Example usage:

#include <QDebug>
#include <QElapsedTimer>
...
...
QElapsedTimer timer;
timer.start();
slowOperation();  // we want to measure the time of this slowOperation()
qDebug() << timer.elapsed();

Answer 3 (score 37)

Even if the first answer was accepted, the rest of the people who read the answers should consider sivabudh’s suggestion.
QElapsedTimer can also be used to calculate the time in nanoseconds.
Code example:

QElapsedTimer timer;
qint64 nanoSec;
timer.start();
//something happens here
nanoSec = timer.nsecsElapsed();
//printing the result(nanoSec)
//something else happening here
timer.restart();
//some other operation
nanoSec = timer.nsecsElapsed();

Question

What would be the best way of selecting an item in a QT combo box out of a predefined list of enum based unique values.

In the past I have become accustomed to .NET’s style of selection where the item can be selected by setting the selected property to the item’s value you wish to select:

cboExample.SelectedValue = 2;

Is there anyway to do this with QT based on the item’s data, if the data is a C++ enumeration?

Answer accepted (score 97)

You lookup the value of the data with findData() and then use setCurrentIndex()

QComboBox* combo = new QComboBox;
combo->addItem("100",100.0);    // 2nd parameter can be any Qt type
combo->addItem .....

float value=100.0;
int index = combo->findData(value);
if ( index != -1 ) { // -1 for not found
   combo->setCurrentIndex(index);
}

Answer 2 (score 22)

You can also have a look at the method findText(const QString & text) from QComboBox; it returns the index of the element which contains the given text, (-1 if not found). The advantage of using this method is that you don’t need to set the second parameter when you add an item.

Here is a little example :

/* Create the comboBox */
QComboBox   *_comboBox = new QComboBox;

/* Create the ComboBox elements list (here we use QString) */
QList<QString> stringsList;
stringsList.append("Text1");
stringsList.append("Text3");
stringsList.append("Text4");
stringsList.append("Text2");
stringsList.append("Text5");

/* Populate the comboBox */
_comboBox->addItems(stringsList);

/* Create the label */
QLabel *label = new QLabel;

/* Search for "Text2" text */
int index = _comboBox->findText("Text2");
if( index == -1 )
    label->setText("Text2 not found !");
else
    label->setText(QString("Text2's index is ")
                   .append(QString::number(_comboBox->findText("Text2"))));

/* setup layout */
QVBoxLayout *layout = new QVBoxLayout(this);
layout->addWidget(_comboBox);
layout->addWidget(label);

Answer 3 (score 3)

If you know the text in the combo box that you want to select, just use the setCurrentText() method to select that item.

ui->comboBox->setCurrentText("choice 2");

From the Qt 5.7 documentation

The setter setCurrentText() simply calls setEditText() if the combo box is editable. Otherwise, if there is a matching text in the list, currentIndex is set to the corresponding index.

So as long as the combo box is not editable, the text specified in the function call will be selected in the combo box.

Reference: http://doc.qt.io/qt-5/qcombobox.html#currentText-prop

Question

I use a QLabel to display the content of a bigger, dynamically changing QPixmap to the user. It would be nice to make this label smaller/larger depending on the space available. The screen size is not always as big as the QPixmap.

How can I modify the QSizePolicy and sizeHint() of the QLabel to resize the QPixmap while keeping the aspect ratio of the original QPixmap?

I can’t modify sizeHint() of the QLabel, setting the minimumSize() to zero does not help. Setting hasScaledContents() on the QLabel allows growing, but breaks the aspect ratio thingy…

Subclassing QLabel did help, but this solution adds too much code for just a simple problem…

Any smart hints how to accomplish this without subclassing?

Answer accepted (score 86)

In order to change the label size you can select an appropriate size policy for the label like expanding or minimum expanding.

You can scale the pixmap by keeping its aspect ratio every time it changes:

QPixmap p; // load pixmap
// get label dimensions
int w = label->width();
int h = label->height();

// set a scaled pixmap to a w x h window keeping its aspect ratio 
label->setPixmap(p.scaled(w,h,Qt::KeepAspectRatio));

There are two places where you should add this code:

• When the pixmap is updated
• In the resizeEvent of the widget that contains the label

Answer 2 (score 29)

I have polished this missing subclass of QLabel. It is awesome and works well.

aspectratiopixmaplabel.h

#ifndef ASPECTRATIOPIXMAPLABEL_H
#define ASPECTRATIOPIXMAPLABEL_H

#include <QLabel>
#include <QPixmap>
#include <QResizeEvent>

class AspectRatioPixmapLabel : public QLabel
{
    Q_OBJECT
public:
    explicit AspectRatioPixmapLabel(QWidget *parent = 0);
    virtual int heightForWidth( int width ) const;
    virtual QSize sizeHint() const;
    QPixmap scaledPixmap() const;
public slots:
    void setPixmap ( const QPixmap & );
    void resizeEvent(QResizeEvent *);
private:
    QPixmap pix;
};

#endif // ASPECTRATIOPIXMAPLABEL_H

aspectratiopixmaplabel.cpp

#include "aspectratiopixmaplabel.h"
//#include <QDebug>

AspectRatioPixmapLabel::AspectRatioPixmapLabel(QWidget *parent) :
    QLabel(parent)
{
    this->setMinimumSize(1,1);
    setScaledContents(false);
}

void AspectRatioPixmapLabel::setPixmap ( const QPixmap & p)
{
    pix = p;
    QLabel::setPixmap(scaledPixmap());
}

int AspectRatioPixmapLabel::heightForWidth( int width ) const
{
    return pix.isNull() ? this->height() : ((qreal)pix.height()*width)/pix.width();
}

QSize AspectRatioPixmapLabel::sizeHint() const
{
    int w = this->width();
    return QSize( w, heightForWidth(w) );
}

QPixmap AspectRatioPixmapLabel::scaledPixmap() const
{
    return pix.scaled(this->size(), Qt::KeepAspectRatio, Qt::SmoothTransformation);
}

void AspectRatioPixmapLabel::resizeEvent(QResizeEvent * e)
{
    if(!pix.isNull())
        QLabel::setPixmap(scaledPixmap());
}

Hope that helps! (Updated resizeEvent, per @dmzl’s answer)

Answer 3 (score 12)

I just use contentsMargin to fix the aspect ratio.

#pragma once

#include <QLabel>

class AspectRatioLabel : public QLabel
{
public:
    explicit AspectRatioLabel(QWidget* parent = nullptr, Qt::WindowFlags f = Qt::WindowFlags());
    ~AspectRatioLabel();

public slots:
    void setPixmap(const QPixmap& pm);

protected:
    void resizeEvent(QResizeEvent* event) override;

private:
    void updateMargins();

    int pixmapWidth = 0;
    int pixmapHeight = 0;
};

#include "AspectRatioLabel.h"

AspectRatioLabel::AspectRatioLabel(QWidget* parent, Qt::WindowFlags f) : QLabel(parent, f)
{
}

AspectRatioLabel::~AspectRatioLabel()
{
}

void AspectRatioLabel::setPixmap(const QPixmap& pm)
{
    pixmapWidth = pm.width();
    pixmapHeight = pm.height();

    updateMargins();
    QLabel::setPixmap(pm);
}

void AspectRatioLabel::resizeEvent(QResizeEvent* event)
{
    updateMargins();
    QLabel::resizeEvent(event);
}

void AspectRatioLabel::updateMargins()
{
    if (pixmapWidth <= 0 || pixmapHeight <= 0)
        return;

    int w = this->width();
    int h = this->height();

    if (w <= 0 || h <= 0)
        return;

    if (w * pixmapHeight > h * pixmapWidth)
    {
        int m = (w - (pixmapWidth * h / pixmapHeight)) / 2;
        setContentsMargins(m, 0, m, 0);
    }
    else
    {
        int m = (h - (pixmapHeight * w / pixmapWidth)) / 2;
        setContentsMargins(0, m, 0, m);
    }
}

Works perfectly for me so far. You’re welcome.

Question

I am trying to set the background color for a double spin box, and I am not sure what function I should use.

I saw some function called SetBackgroundRole which accepts a Qt::ColorRole, but I am not sure how to use this one as well.

Kindly let me know, what’s the simple way to change the background color of a QComboBox or QDoubleSpinBox?

Answer accepted (score 37)

Using a QPalette isn’t guaranteed to work for all styles, because style authors are restricted by the different platforms’ guidelines and by the native theme engine.

To make sure your background color will be correct, I would suggest to use the Qt Style Sheet. Here is what I did to change the background color of a QComboBox:

myComboBox->setStyleSheet("QComboBox { background-color: blue; }");

I haven’t specifically tried for a QSpinBox, but I guess it’ll work the same !

Answer 2 (score 42)

fhe is generally correct, but doesn’t account for the widgets (like spin boxes and buttons/comboboxes) that use a different background role in the palette. A more general solution would be something like this:

QPalette pal = widget.palette();
pal.setColor(widget.backgroundRole(), Qt::blue);
widget.setPalette(pal);

Alternatively, you could look into the descriptions of the various palette roles and figure out the one you want, then apply it to the widget containing the others you want changed. The palette changes should propagate to the children widgets.

Answer 3 (score 12)

Actually, if you look at the Qt docs for QPalette in the case of a QComboBox the background role is probably not what you want. What you want is:

QPalette::Base Used mostly as the background color for text entry widgets, but can also be used for other painting - such as the background of combobox drop down lists and toolbar handles. It is usually white or another light color.

So here is the code I am using to set the background color of a combo box I am using to match the color of the widget it is on:

QPalette pal = myComboBox->palette();
pal.setColor(QPalette::Base, pal.color(QPalette::Window));
myComboBox->setPalette(pal);

Question

I am new to Android Studio and I am having problems while using the emulator. When I try to run it keeps crashing saying:

“Cannot launch AVD in emulator” [6816]:ERROR:./android/qt/qt_setup.cpp:28:Qt library not found at C:

Could not launch ‘C:/qemu/windows-x86_64/qemu-system-i386.exe’: No such file or directory

I have enabled VT-x from BIOS settings yet I am having the problem. I have searched a lot and cannot find an answer. Any help will be much appreciated. Can anyone give me a solution?

Screenshot of error

Answer 2 (score 145)

This seems to be an issue relating to the recent update. A temporary solution is to launch emulator from within the /path/to/android-sdk/tools directory in the commandline.

See https://code.google.com/p/android/issues/detail?id=235461 to follow the issue.

Answer 3 (score 100)

All other answers did not work for me as “Android Emulator” was not installed with a standard installation of Android Studio. Make sure you have installed it and then try other answers if required.

Question

I have a QLineEdit where the user should input only numbers.

So is there a numbers-only setting for QLineEdit?

Answer accepted (score 113)

QLineEdit::setValidator(), for example:

myLineEdit->setValidator( new QIntValidator(0, 100, this) );

or

myLineEdit->setValidator( new QDoubleValidator(0, 100, 2, this) );

See: QIntValidator, QDoubleValidator, QLineEdit::setValidator

Answer 2 (score 22)

The best is QSpinBox.

And for a double value use QDoubleSpinBox.

QSpinBox myInt;
myInt.setMinimum(-5);
myInt.setMaximum(5);
myInt.setSingleStep(1);// Will increment the current value with 1 (if you use up arrow key) (if you use down arrow key => -1)
myInt.setValue(2);// Default/begining value
myInt.value();// Get the current value
//connect(&myInt, SIGNAL(valueChanged(int)), this, SLOT(myValueChanged(int)));

Answer 3 (score 7)

Why don’t you use a QSpinBox for this purpose ? You can set the up/down buttons invisible with the following line of codes:

// ...
QSpinBox* spinBox = new QSpinBox( this );
spinBox->setButtonSymbols( QAbstractSpinBox::NoButtons ); // After this it looks just like a QLineEdit.
//...

Question

I Have Installed Qt Creator5 with latest binaries. but when i am running any program it is giving me this error.

:-1: error: Qt Creator needs a compiler set up to build. Configure a compiler in the kit options.

i have searched the whole internet for answer but its inadequate and not helping. kindly reply the way to get rid of this error.

Answer accepted (score 43)

i was also having the same problem so what i did is

For linux

sudo apt-get install g++ 
sudo apt-get install libgl1-mesa-dev libglu1-mesa-dev

and then closed the qt creator and restarted it and it worked

i do not know which of the above two made it work but it worked !!

For Windows

Below given solution have worked for few people. so it is worth a try.

Delete this file C:\Users\AppData\Roaming\QtProject\qtcreator\toolchains.xml to solve the problem.

Answer 2 (score 13)

For me on Windows 7 this was solved by clicking on the arrow icon next to the computer icon on the bottom left of the Qt Creator screen above green arrows and then double clicked a compiler on the list and rebuild the project.

Answer 3 (score 8)

* for windows users only *

Before continuing to next steps make sure u have downloaded latest version of Qt.

1. Download and install minGW-64-bit from link: https://sourceforge.net/projects/mingw-w64/files/latest/download
2. Remember The directory to which you install minGW.
3. Open Qt creator and go to tools -> options -> Build and Run

4. In Compilers tab Select add -> MinGW ->
   Name: MinGW
   path: Browse for mingw-w64.bat (you will find this file in the directory in which you have installed MinGW).
   ABI: x86 Windows msvc2015 pe 64bit
   
   Don’t forget to apply your edits.

5. Go to Kit -> add
   Name: GCC
   Device Type: Desktop
   Device: Local PC (Default for desktop)
   Compiler: minGW
   Qt Version: select newest version
   
   Apply changes and restart qt

While creating new project make sure you use GCC as kit

If it still doesn’t works Click computer icon on bottom left above play button -> GCC -> Build

OR

Go to projects (It is in the menu on left hand side) -> Build Steps -> make -> Override minGw31-make.exe -> browse to path in which you have installed minGW/bin-make.exe
Do the same for clean steps too

If u Still get any error after these steps try setting Environment Variables

1. Press Win+Q
2. Type " Environment variables "
3. click on " Edit the system environment variables "
4. System Properties -> Advance -> Environment Variables
5. Click on path (NOTE : Use These steps very carefully and do not mess with other options)
6. Click edit
7. press right arrow to go to end of text
8. type a semi-colon (if it isn’t at the end of it because all the system paths are distinguished by a semi-colon)
9. paste path “MinGW_installation_directory” (In my case it was “G:”). Make sure you copy and paste CORRECT path
10. Click OK and apply the changes.

That should do it !!!

Question

Qt documentation states that signals and slots can be direct, queued and auto.

It also stated that if object that owns slot ‘lives’ in a thread different from object that owns signal, emitting such signal will be like posting message - signal emit will return instantly and slot method will be called in target thread’s event loop.

Unfortunately, documentation do not specify that ‘lives’ stands for and no examples is available. I have tried the following code:

main.h:

class CThread1 : public QThread
{
Q_OBJECT
public:
    void run( void )
    {
        msleep( 200 );
        std::cout << "thread 1 started" << std::endl;
        MySignal();
        exec();
    }
signals:
    void MySignal( void );
};

class CThread2 : public QThread
{
Q_OBJECT
public:
    void run( void )
    {
        std::cout << "thread 2 started" << std::endl;
        exec();
    }
public slots:
    void MySlot( void )
    {
        std::cout << "slot called" << std::endl;
    }
};

main.cpp:

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    CThread1 oThread1;
    CThread2 oThread2;
    QObject::connect( & oThread1, SIGNAL( MySignal() ),
        & oThread2, SLOT( MySlot() ) );
    oThread1.start();
    oThread2.start();
    oThread1.wait();
    oThread2.wait();
    return a.exec();
}

Output is:

thread 2 started
thread 1 started

MySlot() is never called :(. What I’m doing wrong?

Answer accepted (score 46)

There are quite a few problems with your code :

• like said by Evan the emit keyword is missing
• all your objects live in the main thread, only the code in the run methods live in other threads, which means that the MySlot slot would be called in the main thread and I’m not sure that’s what you want
• your slot will never be called since the main event loop will never been launched : your two calls to wait() will only timeout after a very long time (and you’ll probably kill your application before that happens) and I don’t think that’s what you want either, anyway they really have no use in your code.

This code would most likely work (though I have not tested it) and I think it does what you want it to do :

class MyObject : public QObject
{
    Q_OBJECT
public slots:
    void MySlot( void )
    {
        std::cout << "slot called" << std::endl;
    }
};

class CThread1 : public QThread
{
    Q_OBJECT
public:
    void run( void )
    {
        std::cout << "thread 1 started" << std::endl;
        int i = 0;
        while(1)
        {
           msleep( 200 );
           i++;
           if(i==1000)
              emit MySignal();
        }
    }
signals:
    void MySignal( void );
};

class CThread2 : public QThread
{
    Q_OBJECT
public:
    void run( void )
    {
        std::cout << "thread 2 started" << std::endl;
        exec();
    }
};

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    CThread1 oThread1;
    CThread2 oThread2;
    MyObject myObject;
    QObject::connect( & oThread1, SIGNAL( MySignal() ),
        & myObject, SLOT( MySlot() ) );
    oThread2.start();
    myObject.moveToThread(&oThread2)
    oThread1.start();
    return a.exec();
}

Now MyObject will live in thread2 (thanks to moveToThread).

MySignal should be sent from thread1 (thought I’m not sure on that one, it might be sent from main thread, it doesn’t really matter).

No event loop is needed in thread1 since emitting a signal doesn’t need an event loop. An event loop is needed in thread2 (lanched by exec()) to receive the signal.

MySlot will be called in thread2.

Answer 2 (score 36)

Do not subclass QThread for Qt 4.4+

While Aiua’s answer is good, I want to point out some issues with QThread and Qt 4.6 or 4.7.

This article sums it up: http://blog.qt.io/blog/2010/06/17/youre-doing-it-wrong/

Lack of Documentation on Qt’s part

Unfortunately the problem stems from a lack of updates to documentation. Prior to Qt 4.4 QThread had no default run() implementation, which meant that you had to subclass QThread in order to use it.

If you’re using Qt 4.6 or 4.7 then you almost certainly should not subclass QThread.

Use moveToThread

The key to getting slots to execute in a worker thread is to use the moveToThread method as Aiua pointed out.

Answer 3 (score 0)

you should emit the signal to start your thread function like

emit operateCut(examId,examName_examTemplate[examName].studentIdRec,examName_examTemplate[examName].choiceRecA,examName_examTemplate[examName].choiceRecB,examName_examTemplate[examName].objectRecA,examName_examTemplate[examName].objectRecB);

you can add more than one argument in this signal

Question

I am trying to deploy(release to public) a simple qt application I made recently, but got stuck at static linking qt libs.

I followed the guide on qt docs to re-build qt and my app statically. But the release build still require qtgui / qtcore dll for no apparent reasons, I wonder if anyone has seen this kind of problems before ? Or even better, has successfully resolved it ?

http://doc.qtsoftware.com/4.5/deployment-windows.html

Answer 2 (score 42)

I wrote a guide to static linking

and How to build Qt static with multiple compilers and keep it small

(because it can get pretty big, especially for simple programs). You may also want to check out the BitRock installer, which is free for open source projects.

In short, it turns out to be a little more complex if you are using anything Qt thinks of as a plugin, such as support for most image types (JPEG, GIF) or databases. For example, if you want to include support for Oracle DBMS and GIF images for your icons, you add the following to your .PRO file:

QTPLUGIN += qsqloci qgif
CONFIG += static

You will then need to:

#include <QtPlugin>

in your project, and import any plugins used. You need to change these settings back order to get it to compile with dynamic linking again (like when debugging or adding features), though this can be easily automated. There are also considerations when building the Qt libraries for use with static linking, though the Qt instructions will at least get you started.

Answer 3 (score 16)

With Qt 5.5, things are quite easy. There are following orthogonal settings that you pass to configure when building Qt:

1. Do you want a static Qt library?

   -static option should be passed to configure

2. Do you want the build of Qt, and of your application, to use a static C++ runtime?

   -static-runtime option should be passed to configure

3. Do you want XP targeting?

   -target xp option should be passed to configure

   Additionally, follow the instructions from this blog post.

   Qt Creator didn’t support XP targeting automagically at least until v.3.5.0 since it doesn’t set up the environment for the build tools properly. You have to modify the build environment manually per the blog post.

Question

The “Browse For Folder” Windows dialog is very inconvenient because:

• it has no Path box where I can paste the path I want (eg from Total Commander)
• it always starts from the Desktop with everything closed

Is there a way to use the “Open File” dialog (which is much better) to select a Folder? Some flag or option or something?

Context: the calibre eLibrary manager which is written in Python and Qt.

It currently displays as on the left. I’d like it to display as on the right

or even better, as the Open File dialog:

Answer 2 (score 90)

You can try this one:

QString QFileDialog::getExistingDirectory ( QWidget * parent = 0, const QString & caption = QString(), const QString & dir = QString(), Options options = ShowDirsOnly ) [static]

This one is used to choose a directory, and will popup a dialog like you show at last.

Demo:

 QString dir = QFileDialog::getExistingDirectory(this, tr("Open Directory"),
                                             "/home",
                                             QFileDialog::ShowDirsOnly
                                             | QFileDialog::DontResolveSymlinks);

Answer 3 (score 22)

You can set the file mode in QFileDialog to QFileDialog::Directory

see http://qt-project.org/doc/qt-5.0/qtwidgets/qfiledialog.html#FileMode-enum

Or You can use QFileDialog::setOption with value QFileDialog::ShowDirsOnly

Question

The question here is the same as mine but the solution there is not working for me. I am using QtCreator 2.4.1 on Windows 7. When i try to debug a simple hello world app the message displayed is: “The preferred debugger engine for debugging binaries of type ‘x86-windows-msvc2010-pe-32bit’ is not available….Details: There is no CDB binary available for binaries in format ’x86-windows-msvc2010-pe-32bit”

When i found that cdb.exe was not on my system i had Windows SDK installed with debugger tools and the current location of cdb.exe is “C:Files (x86)Kits\8.0.exe” and “C:Files (x86)Kits\8.0.exe”,

In the CDB tab i configured symbol server as well as the path of CDB.exe, but in vain!

QtCreator->Tools->options->Debugger->CDB Tab->has the following fields:
Additional arguments
use CDB console
Symbol Paths
Source Paths
Correct breakpoint location etc…

Can somebody kindly explain in detail what these mean and what values are expected here. Has anybody successfully debugged any applicaiton using Qt Creator.

Please refer this link:
http://qt-project.org/forums/viewthread/16018/

Answer accepted (score 58)

I was having the same problems too, and finally figured out how to solve this. Styne666 gave me a hint. Do the following:

1. Tools
2. Options…
3. Build & Run
4. Tool Chains
5. Use the “Clone” button to make a copy of each of the auto-detected MSVC items
6. In each of the cloned items, you can specify the debugger in the “Debugger” field (e.g. C:FilesKits\8.0.exe)
7. After this everything seems to work…

I should further mention that if you are running a 64-bit version of Windows and a 32-bit version of Qt Creator like I am, you need to make sure that you are pointing to the 32-bit versions of cdb (of which there are x86 and x64 target versions). So for the 32-bit target builds I run this:

C:\Program Files (x86)\Windows Kits\8.0\Debuggers\x86\cdb.exe

And for x64 builds I run this:

C:\Program Files (x86)\Windows Kits\8.0\Debuggers\x64\cdb.exe

The tricky part is that it is somewhat difficult to get Microsoft to install the 32-bit version of the debugging tools on your 64-bit machine. A co-worker tried to do so recently and said that he needed to install the Windows 8 tools in order to have the option to do that - the Windows 7 versions would not let you.

Answer 2 (score 4)

Took me some time to figure out this with Qt Creator 2.6.0 (on Windows 7), so here it is:

• Tools, Options…
• Build & Run, Kits tab, select “Desktop (default)” (under Manual)
• Click the Manage button and select Edit on the right side of “Debugger:”
• Engine: CDB, Binary: C:FilesTools for Windows (x64).exe

Answer 3 (score 2)

Depending on the version of Visual Studio you have installed you can find the CDB.exe in different locations. Check if it is available in your system by searching in C: drive.

In my case I have Windows 7 (x64) with Visual Studio 2012 Express and Visual Studio 2013 installed and I had to install Microsoft Windows SDK for Windows 7 available here in order to have CDB.exe in my system. After the install, QT Creator was able to detect CDB.exe in this location: C:FilesTools for Windows (x64).exe.

The only thing you need from the SDK is the Common Utilities | Debugging Tools For Windows.

Question

I want to ask about resources for learning Qt for beginners. What resources do you recommend for that?

Answer 2 (score 26)

Have also a look at the Qt programming videos on VoidRealms’ Youtube channel.

Answer 3 (score 9)

The transition from C++ to QT may seem bit awkward at first. But don’t be afraid its somewhat easier than c++ when you get more familiar with it.

To start of I would like to recommend the first few videos of voidRealms youvideos on c++ QT to get a feel on QT framework. All of them are really good. Try to do the examples by your own. You will be quite comfortable with the framework after watching these videos.

Next if possible try to refer Foundations of Qt Development (Expert’s Voice in Open Source) which is also a very good reference for QT. After going through few chapters you will be very accustomed to the Framework.

Reading C++ GUI Programming with Qt 4 which is a usually recommended book will look good if you have this basic background on QT.

I would also like to recommend the On-line Reference Documentation which will give a great depth in to QT when going forward.

Question

I’m using Code::Blocks 8.02 and the mingw 5.1.6 compiler. I’m getting this error when I compile my Qt project:

C:and Settings Fuzz_interface.cpp|33|undefined reference to `vtable for AddressBook’

File AddressBook.h:

 #ifndef ADDRESSBOOK_H
 #define ADDRESSBOOK_H

 #include <QWidget>

 class QLabel;
 class QLineEdit;
 class QTextEdit;

 class AddressBook : public QWidget
 {
     Q_OBJECT

 public:
     AddressBook(QWidget *parent = 0);

 private:
     QLineEdit *nameLine;
     QTextEdit *addressText;
 };

 #endif

File AddressBook.cpp:

#include <QtGui>
#include "addressbook.h"

AddressBook::AddressBook(QWidget *parent)
     : QWidget(parent)
{
    QLabel *nameLabel = new QLabel(tr("Name:"));
    nameLine = new QLineEdit;

    QLabel *addressLabel = new QLabel(tr("Address:"));
    addressText = new QTextEdit;

    QGridLayout *mainLayout = new QGridLayout;
    mainLayout->addWidget(nameLabel, 0, 0);
    mainLayout->addWidget(nameLine, 0, 1);
    mainLayout->addWidget(addressLabel, 1, 0, Qt::AlignTop);
    mainLayout->addWidget(addressText, 1, 1);

    setLayout(mainLayout);
    setWindowTitle(tr("Simple Address Book"));
}

Answer accepted (score 43)

Warning: Do not do this if you already have a .pro file - you’ll lose it!

In order to automatically ensure that all moc cpp files are generated, you can get qmake to automatically generate a .pro file for you instead of writing one yourself.

Run

qmake -project

in the project directory, and qmake will scan your directory for all C++ headers and source files to generate moc cpp files for.

Answer 2 (score 47)

When using Qt Creator:

1. Build → Run qmake
2. Build → Rebuild All

Answer 3 (score 16)

The problem is almost certainly that you are not compiling or not linking in the generated moc_AddressBook.cpp file. (It should have been generated for you – you are running Qt’s moc on your code before compiling, right?)

To answer a little more thoroughly, the Q_OBJECT macro signals Qt’s moc tool to create an extra implementation file that contains the code necessary to support QObject’s meta-information system. If you had any signals or slots, it would do a few things for those as well.

An alternative solution might be to remove the Q_OBJECT macro. You probably don’t want to do this, but it would help the immediate problem, and it isn’t strictly necessary with the code that you’ve presented.

Also, I would note that your line:

#include "addressbook.h"

Should probably be:

#include "AddressBook.h"

based on how you presented the filenames in the question.

Question

I want to “stringify” a number and add zero-padding, like how printf("%05d") would add leading zeros if the number is less than 5 digits.

Answer accepted (score 163)

Use this:

QString number = QString("%1").arg(yourNumber, 5, 10, QChar('0'));

5 here corresponds to 5 in printf("%05d"). 10 is the radix, you can put 16 to print the number in hex.

Answer 2 (score 31)

QString QString::rightJustified ( int width, QChar fill = QLatin1Char( ’ ’ ), bool truncate = false ) const

int myNumber = 99;
QString result;
result = QString::number(myNumber).rightJustified(5, '0');

result is now 00099

Answer 3 (score 17)

The Short Example:

int myNumber = 9;

//Arg1: the number
//Arg2: how many 0 you want?
//Arg3: The base (10 - decimal, 16 hexadecimal - if you don't understand, choose 10)
//      It seems like only decimal can support negative numbers.
QString number = QString("%1").arg(myNumber, 2, 10, QChar('0')); 

Output will be: 09

Question

I just reinstalled QtCreator, created new project (Qt Application) an got this after compilation:

/usr/bin/ld: **cannot find -lGL**
collect2: error: ld returned 1 exit status
make: *** [untitled1] Error 1
18:07:41: The process "/usr/bin/make" exited with code 2.
Error while building/deploying project untitled1 (kit: Desktop Qt 5.1.0 GCC 32bit)
When executing step 'Make'

(Project is empty, I did’n commit any changes)

Qt Creator 2.7.2
Based on Qt 5.1.0 (32 bit)
Ubuntu 13.04

How do I solve this problem?

Answer accepted (score 142)

You should install package “libgl1-mesa-dev”:

sudo apt install libgl1-mesa-dev

Answer 2 (score 62)

you don’t need to install anything. libGL is already installed with Ubuntu, you just need to soft link it. (tested for ubuntu 14.x and 15.x, might work for later versions)

1. First locate the GL library
2. Then link it under /usr/lib
3. If the library is missing, it can be installed via libgl1-mesa-dev package

Here is how you could do this:

$ locate libGL
/usr/lib/i386-linux-gnu/mesa/libGL.so.1
/usr/lib/i386-linux-gnu/mesa/libGL.so.1.2.0
/usr/lib/x86_64-linux-gnu/libGLEW.so.1.10
/usr/lib/x86_64-linux-gnu/libGLEW.so.1.10.0
/usr/lib/x86_64-linux-gnu/libGLEWmx.so.1.10
/usr/lib/x86_64-linux-gnu/libGLEWmx.so.1.10.0
/usr/lib/x86_64-linux-gnu/libGLU.so.1
/usr/lib/x86_64-linux-gnu/libGLU.so.1.3.1
/usr/lib/x86_64-linux-gnu/mesa/libGL.so.1
/usr/lib/x86_64-linux-gnu/mesa/libGL.so.1.2.0
/usr/lib/x86_64-linux-gnu/mesa-egl/libGLESv2.so.2
/usr/lib/x86_64-linux-gnu/mesa-egl/libGLESv2.so.2.0.0
$ sudo ln -s /usr/lib/x86_64-linux-gnu/mesa/libGL.so.1 /usr/lib/libGL.so

Answer 3 (score 2)

write:

yum provides */libGL.so 

after providing:

yum install mesa-libGL-devel mesa-libGLU-devel

Question

How do you set application icon for application made using Qt? Is there some easy way? It’s a qmake-based project.

Answer accepted (score 88)

For Qt 5, this process is automated by qmake. Just add the following to the project file:

win32:RC_ICONS += your_icon.ico

The automated resource file generation also uses the values of the following qmake variables: VERSION, QMAKE_TARGET_COMPANY, QMAKE_TARGET_DESCRIPTION, QMAKE_TARGET_COPYRIGHT, QMAKE_TARGET_PRODUCT, RC_LANG, RC_CODEPAGE.

For Qt 4, you need to do it manually. On Windows, you need to create a .rc file and add it to your project (.pro). The RC file should look like this:

IDI_ICON1 ICON DISCARDABLE "path_to_you_icon.ico"

The .pro entry should also be win32 specific, e.g.:

win32:RC_FILE += MyApplication.rc

Answer 2 (score 3)

One more way to do that. Verified in Linux (Qt 4.8.6) and Windows (Qt 5.6):

1. Add the icon file to your project folder;

2. In the main function call setWindowIcon() method. For example:

QApplication a(argc, argv);
a.setWindowIcon(QIcon("./images/icon.png"));

Answer 3 (score 2)

To extend Rob’s answer, you can set an application icon for macOS by adding and modifying the following line onto your .pro file.

macx: ICON = <app_icon>.icns

Note that the ICON qmake variable is only meant to target macOS.

For Windows, use

• RC_ICONS = <app_icon>.ico if you’re attaching a .ico file
• or RC_FILE = <app_icon>.rc if you want to attach your icon through a .rc file. (Be sure to add IDI_ICON1 ICON DISCARDABLE "myappico.ico" into the rc file. Indentation not mine.)

For further reading, see Setting the Application Icon.

Question

I know some similar questions may have been asked already, but the answers to those I found covered very specific problems and I still haven’t figured it out.

In my program I’m creating a QObject (called QPeer) that uses a QTcpSocket to communicate with another such object over a network. QPeer has a slot that accepts a QByteArray with data (sendData(QByteArray)). The entire contents of that array are considered to be one ‘message’ and they are written to the socket. I want to do the following: every time a message is written, I want the receiving QPeer to emit its signal dataReceived(QByteArray) exactly once, that QByteArray containing the entire message. (NOTE: all signals/slots, both private ones connecting the QPeer with its socket and the public ones such as sendData(QByteArray) are serialized by using Qt::QueuedConnection whenever necessary.)

I use the signal QTcpSocket::readyRead() for asynchronous reading from the socket. Now I know I can’t just call QTcpSocket::write() once in sendData and then assume that for every write I do, the QTcpSocket on the other side produces exactly one readyRead signal. So what should I do?

This is my idea, please tell me if this will work:

WRITING:

void QPeer::sendData(QByteArray data)
{
    // TODO: write data.size() as raw int of exactly 4 bytes to socket
    const char *bytes = data.constData();
    int bytesWritten = 0;
    while (bytesWritten < data.size())
        bytesWritten += _socket->write(bytes + bytesWritten);
}

READING:

now I want the read function (connected to QTcpSocket::readyRead()) to use the header (the 4 byte int specifying the length of the message) and then read that amount of bytes; next emit dataReceived with exactly those bytes. I’m having serious trouble trying to do this. For example: what to do if readyRead is emitted and I can read the header of a message, but not the amount of bytes specified? Or what if a header has only been received partially?

1. How do I correctly write the header (4 byte int) to the socket?

2. How do I correctly implement the read function so that it does what I want?

Any tips are welcome. Thanks!

Answer accepted (score 37)

I worked on a project that does what you expect, see here the solution that I developed to our problems, simplified to be easier to understand:

Edited, added support to the server deal with multiple clients.

Client.h:

#include <QtCore>
#include <QtNetwork>

class Client : public QObject
{
    Q_OBJECT
public:
    explicit Client(QObject *parent = 0);

public slots:
    bool connectToHost(QString host);
    bool writeData(QByteArray data);

private:
    QTcpSocket *socket;
};

Client.cpp:

#include "client.h"

static inline QByteArray IntToArray(qint32 source);

Client::Client(QObject *parent) : QObject(parent)
{
    socket = new QTcpSocket(this);
}

bool Client::connectToHost(QString host)
{
    socket->connectToHost(host, 1024);
    return socket->waitForConnected();
}

bool Client::writeData(QByteArray data)
{
    if(socket->state() == QAbstractSocket::ConnectedState)
    {
        socket->write(IntToArray(data.size())); //write size of data
        socket->write(data); //write the data itself
        return socket->waitForBytesWritten();
    }
    else
        return false;
}

QByteArray IntToArray(qint32 source) //Use qint32 to ensure that the number have 4 bytes
{
    //Avoid use of cast, this is the Qt way to serialize objects
    QByteArray temp;
    QDataStream data(&temp, QIODevice::ReadWrite);
    data << source;
    return temp;
}

Server.h:

#include <QtCore>
#include <QtNetwork>

class Server : public QObject
{
    Q_OBJECT
public:
    explicit Server(QObject *parent = 0);

signals:
    void dataReceived(QByteArray);

private slots:
    void newConnection();
    void disconnected();
    void readyRead();

private:
    QTcpServer *server;
    QHash<QTcpSocket*, QByteArray*> buffers; //We need a buffer to store data until block has completely received
    QHash<QTcpSocket*, qint32*> sizes; //We need to store the size to verify if a block has received completely
};

Server.cpp:

#include "server.h"

static inline qint32 ArrayToInt(QByteArray source);

Server::Server(QObject *parent) : QObject(parent)
{
    server = new QTcpServer(this);
    connect(server, SIGNAL(newConnection()), SLOT(newConnection()));
    qDebug() << "Listening:" << server->listen(QHostAddress::Any, 1024);
}

void Server::newConnection()
{
    while (server->hasPendingConnections())
    {
        QTcpSocket *socket = server->nextPendingConnection();
        connect(socket, SIGNAL(readyRead()), SLOT(readyRead()));
        connect(socket, SIGNAL(disconnected()), SLOT(disconnected()));
        QByteArray *buffer = new QByteArray();
        qint32 *s = new qint32(0);
        buffers.insert(socket, buffer);
        sizes.insert(socket, s);
    }
}

void Server::disconnected()
{
    QTcpSocket *socket = static_cast<QTcpSocket*>(sender());
    QByteArray *buffer = buffers.value(socket);
    qint32 *s = sizes.value(socket);
    socket->deleteLater();
    delete buffer;
    delete s;
}

void Server::readyRead()
{
    QTcpSocket *socket = static_cast<QTcpSocket*>(sender());
    QByteArray *buffer = buffers.value(socket);
    qint32 *s = sizes.value(socket);
    qint32 size = *s;
    while (socket->bytesAvailable() > 0)
    {
        buffer->append(socket->readAll());
        while ((size == 0 && buffer->size() >= 4) || (size > 0 && buffer->size() >= size)) //While can process data, process it
        {
            if (size == 0 && buffer->size() >= 4) //if size of data has received completely, then store it on our global variable
            {
                size = ArrayToInt(buffer->mid(0, 4));
                *s = size;
                buffer->remove(0, 4);
            }
            if (size > 0 && buffer->size() >= size) // If data has received completely, then emit our SIGNAL with the data
            {
                QByteArray data = buffer->mid(0, size);
                buffer->remove(0, size);
                size = 0;
                *s = size;
                emit dataReceived(data);
            }
        }
    }
}

qint32 ArrayToInt(QByteArray source)
{
    qint32 temp;
    QDataStream data(&source, QIODevice::ReadWrite);
    data >> temp;
    return temp;
}