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Thread: QWidget: Must construct a QApplication before a QPaintDevice

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    Default QWidget: Must construct a QApplication before a QPaintDevice

    Hi all,

    I am getting this error on my application, even before main() is executed:
    Qt Code:
    1. QWidget: Must construct a QApplication before a QPaintDevice
    To copy to clipboard, switch view to plain text mode 

    The funny (?) thing is that I only get it when choosing "debug and build" in QDevelop's menus. When compiling for release it works flawlessly. Any ideas?

    Here is my main, not even the first line (line number 9 in this code) is printed:
    Qt Code:
    1. #include <cstdlib>
    2. #include "mainwindowimpl.h"
    3. #include <QApplication>
    4.  
    5. using namespace std;
    6.  
    7. int main(int argc, char ** argv)
    8. {
    9. printf("111111111111");
    10. QApplication app( argc, argv );
    11. MainWindowImpl win;
    12. win.show();
    13. app.connect( &app, SIGNAL( lastWindowClosed() ), &app, SLOT( quit() ) );
    14. return app.exec();
    15. }
    To copy to clipboard, switch view to plain text mode 

    Here is the generated faulty pro file
    Qt Code:
    1. TEMPLATE = app
    2. QT = gui core
    3. CONFIG += qt warn_on console debug_and_release
    4. OBJECTS_DIR = build
    5. MOC_DIR = build
    6. UI_DIR = build
    7. FORMS = ui/mainwindow.ui
    8. HEADERS = src/mainwindowimpl.h
    9. SOURCES = src/mainwindowimpl.cpp src/main.cpp
    10. INCLUDEPATH = d:\source\qwt-5.0.2\src
    11. LIBS = d:\source\qwt-5.0.2\lib\libqwt5.a
    To copy to clipboard, switch view to plain text mode 

    Here is the working pro file:
    Qt Code:
    1. TEMPLATE = app
    2. QT = gui core
    3. CONFIG += qt warn_on console release
    4. OBJECTS_DIR = build
    5. MOC_DIR = build
    6. UI_DIR = build
    7. FORMS = ui/mainwindow.ui
    8. HEADERS = src/mainwindowimpl.h
    9. SOURCES = src/mainwindowimpl.cpp src/main.cpp
    10. INCLUDEPATH = d:\source\qwt-5.0.2\src
    11. LIBS = d:\source\qwt-5.0.2\lib\libqwt5.a
    To copy to clipboard, switch view to plain text mode 

    System is MingW, Xp, Qt 4.3.2, QDevelop 0.25.

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    Default Re: QWidget: Must construct a QApplication before a QPaintDevice

    Do you have any static QWidgets in your application? Static objects are instantiated before main gets control.

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    Default Re: QWidget: Must construct a QApplication before a QPaintDevice

    Alternatively, debug and release object files might be getting mixed. Try cleaning and then rebuilding the project.
    J-P Nurmi

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    Default Re: QWidget: Must construct a QApplication before a QPaintDevice

    Cleaning the project and re-building does not help. This means that (probably) the problem is a static widgets.

    I am using QWT, can anyone tell me if it contains a static widget? (I am linking statically).

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    Default Re: QWidget: Must construct a QApplication before a QPaintDevice

    Qwt itself does not contain static widgets.

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    Default Re: QWidget: Must construct a QApplication before a QPaintDevice

    Perhaps you compiled Qwt in debug mode and application in release mode?
    J-P Nurmi

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    Default Re: QWidget: Must construct a QApplication before a QPaintDevice

    Quote Originally Posted by elcuco View Post
    I am linking statically
    Maybe Qwt uses a different version of Qt libraries?

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    Default Re: QWidget: Must construct a QApplication before a QPaintDevice

    Quote Originally Posted by elcuco View Post
    I am linking statically
    In qwtconfig.pri

    CONFIG += QwtDll this line must be ->
    #CONFIG += QwtDll

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    Default Re: QWidget: Must construct a QApplication before a QPaintDevice

    Quote Originally Posted by jpn View Post
    Perhaps you compiled Qwt in debug mode and application in release mode?
    it is the answer!!

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    Default Re: QWidget: Must construct a QApplication before a QPaintDevice

    hi

    i have the same problem. i use qt-4.5.2 and qwt 5.2 on winXP. i tried all my code in linux and all works fine.

    can anybody of you tell me the procedure on how to compile first qt and then qwt , in the right way?? i don't know why i get these problem! please!

    thank you in advance

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    Default Re: QWidget: Must construct a QApplication before a QPaintDevice

    Hi... i also have a similar question.. what if my application doesnot have any main function.. how would i construct QApplication???? Any idea????
    I am using firebreath integrated with QT library it doesnt have any main function and i am using QMacCocoaViewContainer during the compilation its giving me same error "QWidget Must construct a QApplication before a QPaintDevice" how should i declare QApplication

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    Default Re: QWidget: Must construct a QApplication before a QPaintDevice

    Quote Originally Posted by Saurabh Singh View Post
    what if my application doesnot have any main function..
    Then it is not an application.
    Your biological and technological distinctiveness will be added to our own. Resistance is futile.

    Please ask Qt related questions on the forum and not using private messages or visitor messages.


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    Default Re: QWidget: Must construct a QApplication before a QPaintDevice

    Quote Originally Posted by jacek View Post
    Maybe Qwt uses a different version of Qt libraries?
    Often, this is the cause for my problems.

    In release mode, I must add -lqwt to LIBS, where in debug mode, -lqwtd should be added. Here a snippet of my .pro file:

    Qt Code:
    1. #LIBS+= -lqwt #Release
    2. LIBS+= -lqwtd #Debug
    To copy to clipboard, switch view to plain text mode 

    Cheers, Bilderbikkel

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    Default Re: QWidget: Must construct a QApplication before a QPaintDevice

    If you are using a static Widget and not on purpose there is an easy solution. All you have to do is remove the parent class parenthesis.

    Qt Code:
    1. class MyQtWidget(QtGui.QWidget())
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    change to

    Qt Code:
    1. class MyQtWidget(QtGui.QWidget)
    To copy to clipboard, switch view to plain text mode 

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    Default Re: QWidget: Must construct a QApplication before a QPaintDevice

    Perhaps you compiled "xyz" in debug mode and application in release mode?
    Man, this stupid thing just nailed me for two days. When I set up my release mode build in Visual Studio, I copied the list of libraries from the debug settings, then carefully edited all of the "d" suffixes off. On all but two libraries...

    For the first day, I couldn't figure out what was happening - the app wouldn't even get into main(). You could see it start in Task Manager, then it immediately exited. No error message, no crash, nothing. Finally thought to link it with debug turned on, and saw the "QPixmap: Must construct a QGuiApplication first" message. Spent a wasted hour looking for QPixmap, and of course there aren't any defined as static in my code. Finally came across this thread and thought, "I wonder if I've been stupid..."

    So, for anyone else using Visual Studio or the MSVC compilers to build your Qt projects, you can't mix debug and release libraries or debug libraries and a release executable. If you get the incomprehensible "QPixmap: Must construct..." error, it means that's what you've done.

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