Karsten Karsten - 7 months ago 25
C++ Question

Why does this Simple Qt Application not link

I tried to write a simple Qt application like this:

main.cpp:

#include <QApplication>

class MyApp : public QApplication {
Q_OBJECT
public:
MyApp(int argc, char* argv[]);
};

MyApp::MyApp(int argc, char* argv[]) :
QApplication(argc,argv) {
}

int main(int argc, char* argv[]) {
MyApp app(argc,argv);
return app.exec();
}


But when I tried to compile and link it with Qt Creator 2.3.1 (Qt 4.7.4) I get 3 "unresolved external symbol" errors:



  • main.obj:-1: error:
    LNK2001: unresolved external symbol

    ""public: virtual struct QMetaObject const * __thiscall MyApp::metaObject(void)const "

    (?metaObject@MyApp@@UBEPBUQMetaObject@@XZ)".


  • main.obj:-1: error:
    LNK2001: unresolved external symbol

    ""public: virtual void * __thiscall MyApp::qt_metacast(char const*)"

    (?qt_metacast@MyApp@@UAEPAXPBD@Z)".



  • main.obj:-1: error:
    LNK2001: unresolved external symbol

    ""public: virtual int __thiscall MyApp::qt_metacall(enum QMetaObject::Call,int,void * *)"

    (?qt_metacall@MyApp@@UAEHW4Call@QMetaObject@@HPAPAX@Z)".



I think they are somehow related to the MetaObjectCompiler of Qt, but I can't figure out a solution.
I know it's not considered good programming style in c++ to put declarations and definitions in one file, but that's not the point here. In my opinion it should be possible since there is nothing syntactically wrong here.

Answer Source

Use the code below, and make sure to run qmake (Build > Run qmake) before building.

#include <QApplication>

class MyApp : public QApplication {
  Q_OBJECT
public:
  MyApp(int argc, char* argv[]);
};

MyApp::MyApp(int argc, char* argv[]) :
  QApplication(argc,argv) {
}

int main(int argc, char* argv[]) {
  MyApp app(argc,argv);
  return app.exec();
}

#include "main.moc"

Explanation: When you include the Q_OBJECT macro, this signals Qt to do a bunch of stuff that is not standard C++, such as signals and slots. It does this by running moc, which in large part is a code generator. Running qmake creates the metadata so that when your project is built, it knows which files to moc, etc.