Why does this Simple Qt Application not link

Question

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 1

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.

Answer 2

I think you need to moc the file and include the resulting main.moc at the bottom.

Answer 3

I think this has something to do with QMake. It's not that the executable app can't see the exported DLL class. It's that the obj file for the class doesn't exist. Running QMake from the QT Creator Build menu and then building seems to work.

Why does this Simple Qt Application not link

Answer 4

I've just met the same problem, and it has been solved by changing the Character set of my header from Unicode to ANSI.