[Mingw-users] Address shift when using virtual functions

[John-Philip T. <Joh...@ne...>]

Good day,

I'm using a void* red-black tree to store a bunch of derived classes
and came across a peculiarity (much simplified below):

The code attached yields:

Integer is 12345
Now the integer is 4254256
Shifting the base address yields: 12345

main.cpp:

#include <stdio.h>

class Parent{
 public:
  int i;
};

class Child : public Parent{
 public:
  virtual int MyFunction(){return 0;}
};

int main(){
 Parent* p;
 Child * c;
 void  * v;

 c = new Child;
 c->i = 12345;

 p = c;
 printf("Integer is %d\n", p->i);

 v = c;
 p = (Parent*)v;
 printf("Now the integer is %d\n", p->i);

 p = (Parent*)((char*)v + 4);
 printf("Shifting the base address yields: %d\n", p->i);

 return 0;
}

Is this a bug in MinGW or just a rule in the C++ specification I'm
unaware of? Removing the "virtual" keyword removes the problem.  I'm
using "tdm-gcc-4.5.1"

Also, as long as I cast to the base class when inserting into the tree
and then cast via the base class when taking things out of the tree
there is no problem. Only when I cast directly to void* (when
inserting) or the destination class (when querying) that the address
shift is present.

Regards,
John-Philip