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?
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)