QOBJECT MURT HERURNITANTE Y OPERADOR NUEVO

Question

atascado con esta extraña pregunta

¿Por qué seguir el código está bien para G ++

#include <QObject>

class B {
public:
  B(){}
  ~B(){}
};

class A : public QObject, public B {
  Q_OBJECT
public:
  A(QObject * parent = 0 ) : QObject( parent ), B() {}
  ~A(){}
};

int main(int argc, char *argv[])
{
  A a1();
  //A * a = new A();
  //delete a;
  return 0;
}

y esto no puede ser compilado

/*... the same class definitions as above */    

int main(int argc, char *argv[])
{
  //A a1();
  A * a = new A();
  delete a;
  return 0;
}

//error: undefined reference to `vtable for A'

Me refiero a qué hacer para hacer el segundo bien también?

PS Bueno, pongo todo en archivos separados, y funciona bien.Así que es una cuestión de Macros Q_Object, creo.

Answer 1

If you define a QObject-derived class, build an application, and realize you forgot to add the Q_OBJECT macro, and you add it later, it is important that you qmake to explicitly update the Makefile. Furthermore, to be safe, I recommend a make clean to get rid of old files. make is not smart enough to clean up all of its generated files under such circumstances, and this is an issue that often causes headaches to new Qt developers.

For more information about this error message, see

http://cartan.cas.suffolk.edu/oopdocbook/html/commonlinkererrors.html#undefinedreftovtable

Answer 2

Why does the First example compile & Link cleanly while Second doesn't?

The first example compiles and links because:
It does not create an object of A,

A a1();

Declares a function a1() which takes no parameter and returns a A type.

While the Second example creates an object when new is called.

Note that the *undefined reference to vtable for A'* is a linking error and will only be emitted when a object ofclass A` is created. Hence only the Second example shows the error.

How to resolve the problem?
You need to provide definition for all virtual functions which you derive from QObject.

Answer 3

The code works in Vis. Studio. Your problem may be that B is not a polymorphic class - I don't know why that would give you an error - but you could try making something in B virtual: virtual ~B(){} for example.