Q_OBJECT in QT (undefined reference to vtable xxx) [duplicate]

Answer 1

This error is caused of the class extended QObject to be declared in main.cpp, so just add the line #include "main.moc" before (or after) your main method. But in the next time define a separate h/cpp-file for QObject extended classes.

Answer 2

Adding the parenthesis makes it a function declaration. No object is actually created, so the error doesn't happen.

Foo f; //declaration of variable of type Foo

Foo f(); //declaration of function taking no args and returning Foo

The undefined reference to vtable for Foo error is because you added a call to Q_OBJECT without running qmake again. Once it's run, the error should disappear.