Virtual inheritance error

[IvesRogne]

Hi guys,

I'm using virtual inheritance to solve the "diamond" problem in C++. The following code works fine when compiled with gcc, but does not under Visual C++.

Interface A allows one to print() and clone() the object. Interface B inherits (virtually) from A, and overrides the clone() function to actually return a B*, not merely a A*. Several other classes not included here are assumed to inherit virtually from A.

The C class implements B.

Let c be an instance of class C. c can print and clone itself as expected.
However when c is cast to B*, it can print itself but cloning does NOT work. The problem is when calling C::clone(), the 'this' pointer is 2 bytes away from its correct address.

When using regular inheritance (from B to A), it works. When B does not override clone(), it works as well.

Am I missing something...? Or is my implementation somehow illegal?

Thanks!
Ives

#include <iostream>
using namespace std;

// Base Interface A
class InterfaceA
{
public:
  virtual void print() = 0;

  virtual InterfaceA* clone() = 0;
};

// Interface B inherits virtually from A
class InterfaceB : public virtual InterfaceA
{
public:
  virtual InterfaceB* clone() = 0;
};

// C implements interface B
class ConcreteC : public InterfaceB
{
public:
  ConcreteC(int _c) : c(_c) { }

  virtual void print()
  {
    cout << c << endl;
  }

  virtual ConcreteC* clone()
  {
    return new ConcreteC(c);
  }

private:
  int c;
};

int main(int argc, char *argv[])
{
  ConcreteC *c = new ConcreteC(1);
  c->print();       // prints "1"

  ConcreteC *c_clone = c->clone();
  c_clone->print(); // prints "1"

  InterfaceB *b = c;
  b->print();       // prints "1"

  InterfaceB *b_clone = b->clone(); // doesn't work!
  b_clone->print(); // oups! prints some random number
}