[Bug c++/53225] static operator new in multiple inheritance carries incorrect type information for the class

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53225

--- Comment #20 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-05-04 21:29:43 UTC ---
(In reply to comment #18)
> This code compiles:
> 
> #include <cstddef>
> #include <stdlib.h>
> typedef unsigned int uint;
> 
> class C{ // just here to be faithful to the original code
>   int y;
> };
> 
> class A{
> public:
>   typedef A this_type;
> 
>   void* operator new(size_t enfacia_size, uint count){
>       size_t total_size 
>     = enfacia_size
>     + sizeof(int) * count; // the 'tail'
>     ;
>       this_type *new_pt = (this_type *)malloc(total_size);
>       new_pt->count = count;

The bug is here.

>       return new_pt;
>   };
>   uint count;
> };
> 
> class B : public C, public A{
> public:
>     int i;
> };
> 
> int main(){
>   B *b_pt = new(5) B;  
>   uint j=0;
>   j++;
> };
> 
> And this is the debugger output:
> 
> (gdb) r
> Starting program try_offsets 
> Breakpoint 1, main () at try_offsets.cc:32
> (gdb) n
> (gdb) p &(b_pt->count)
> $1 = (uint *) 0x804a00c
> (gdb) x/10 b_pt
> 0x804a008:  5   0   0   0
> 0x804a018:  0   0   0   0
> 0x804a028:  0   135129
> (gdb) p b_pt
> $2 = (B *) 0x804a008
> (gdb) 
> 
> Compare this to the prior code and debugger output of the same form, and you
> will notice that operator new, from a point of view of type, behaves
> differently than other method.  Though it was inherited, its understanding of
> this_type has not changed, as it does in the case for other methods.

No it doesn't!

That never happens for any type or any member function!


#include <iostream>

class C{ // just here to be faithful to the original code
  int y;
};

class A{
public:
  void method(void* p){
    std::cout << "this=" << this << " p=" << p << '\n';
  }
  int count;
};

class B : public C, public A{
};

int main(){

  A a;
  a.method(&a);

  B b;
  b.method(&b);

}

this=0x7fffe08b1640 p=0x7fffe08b1640
this=0x7fffe08b1634 p=0x7fffe08b1630

Note that when calling b.method(&b) the "this" pointer is not the same as &b

The member count is always at the same address within the A object, but the
address of that A object might not be the same as the address of the B object.

A B object looks like this in memory:

-----   <------ C*      <---- B*
| C |
|---|   <------ A*
| A |
-----

When you call A::f() the 'this' pointer is adjusted to point to the A
sub-object, which is not at the same address as the complete B object that
contains it.


> In the least, in a strongly typed system I would expect there to be a warning
> about inheriting operator new when the type was not going to be updated as it
> is for other opertors and methods, though I suspect it is a small matter for
> the compiler to give it the right type info.

The type is never "updated" when a function is inherited!  The base class
function is just visible and accessible in the derived class.  It's not copied
or update, the same, original function is used unaltered.

Please, go and learn C++ somewhere else.  I don't know what language your
mental model corresponds to, but it's not C++.