Question

I created a simple program that demonstrates the runtime error I’m getting with my Qt application that uses multiple inheritance. The inheritance tree looks like:

QGraphicsItem (abstract)
      \
     QGraphicsLineItem      MyInterface (abstract)
                 \          /
                  \        /
                  MySubclass

And here is the code:

/* main.cpp */
#include <QApplication>
#include <QGraphicsScene>
#include <QGraphicsLineItem>

//simple interface with one pure virtual method
class MyInterface
{
public:
  virtual void myVirtualMethod() = 0;
};

//Multiple inheritance subclass, simply overrides the interface method
class MySubclass: public QGraphicsLineItem, public MyInterface
{
public:
  virtual void myVirtualMethod() { }
};

int main(int argc, char** argv)
{
  QApplication app(argc, argv); //init QApplication
  QGraphicsScene *scene = new QGraphicsScene(); //create scene

  scene->addItem(new MySubclass()); // add my subclass to the scene

  Q_FOREACH(QGraphicsItem *item, scene->items()) // should only have one item
  {
    MyInterface *mInterface = (MyInterface*)item; // cast as MyInterface
    mInterface->myVirtualMethod(); // <-- this causes the error
  }
  return 0;
}

Debugging in visual studio results in a runtime error when my interface method is called:

    Run-Time Check Failure #0 - The value of ESP was not properly 
    saved across a function call.  This is usually a result of 
    calling a function declared with one calling convention with 
    a function pointer declared with a different calling convention.

Any idea what the problem is?

Answer 1

Because you’re using multiple inheritance, the vftable pointer to what is expected to be a MyInterface* is actually a pointer to a QGraphicsLineItem vftable.

A dynamic_cast would solve the issue because it will return the correct vftable

MyInterface* mInterface = dynamic_cast<MyInterface*>(item);

A simple example:

class A
{
public:
    virtual void foo() = 0;
};

class B
{
public:
    virtual void goo() {};
};

class C : public B, public A
{
public:
    virtual void foo() {}; 
};

//....

B* c = new C;                  // c  is at 0x00a97c78 
A* a = (A*)c;                  // a  is at 0x00a97c78 (vftable pointer of B) 
A* a1 = dynamic_cast<A*>(c);   // a1 is at 0x00a97c7c (vftable pointer of A)