Pointer drift caused by multiple inheritance in C ++

Recently we encountered a strange problem in our work. We finally decided that the pointer drift caused by multi-inheritance was related to the C ++ object model. The diagram is as follows:

Class {...};
Class B {...};
Class AB: Public B, public {...}
...
AB * PAB = new AB ();
A * pA = (A *) PAB;
B * pb = (B *) PAB;

At this time, you find that the values of PA and Pb are different! One of them is equal to PAB, And the other generates an offset. If we change the order of A and B in the AB statement, the pointer that generates the offset will change to another one.

To make sure that this is the reason for the compiler to convert, use the void pointer to fool the compiler:
Void * Pv = (void *) PAB;
Pa = (A *) PV;

At this time, the value of PA is equal to that of PAB, But it points to the wrong place. The transition from PAB to PA depends on the path selection, which is not reassuring. I still don't know if it will make an error if I put the pointer into the container and then retrieve it. Of course, the above uses forced type conversion, in goodProgram. If there is only implicit conversion, the correct result is displayed:

STD: vector <A *> V;
// Implicit type conversion
V. insert (V. Begin (), PAB );
Void * Pv = V [0];
Pa = (A *) PV;

The following program is compiled using cygwin/g ++ B:

# Include <stdio. h>
# Include <vector>

Class
{
Public:
Int;
};
Class B
{
Public:
Int B;
};
Class AB: Public B, public
{
Public:
Int AB;
};

Int main (INT argc, char ** argv)
{
AB * PAB = new AB ();
Pab-> AB = 1;
Pab-> B = 2;
Pab-> A = 3;

A * pA = (A *) PAB;
B * pb = (B *) PAB;

Printf ("AB: % P \ n "\
"A: % P \ n "\
"B: % P \ n ",
PAB, Pa, Pb );

STD: vector <A *> V;
// Implicit type conversion
V. insert (V. Begin (), PAB );
Void * Pv = V [0];
Pa = (A *) PV;
Printf ("PV is % P \ NPA is % P \ npab % s PV \ n", PV, Pa, (PAB = PV )? "= ":"! = ");

Printf ("A. A is % d \ n", pa-> );
// Forced type conversion
Pv = (void *) PAB;
Pa = (A *) PV;
Printf ("Now A. A is % d \ n", pa-> );
}

Running result:
AB: 0x6b01f0
A: 0x6b01f4
B: 0x6b01f0
PV is 0x6b01f4
Pa is 0x6b01f4
Pab! = PV
A. A is 3
Now a. A is 2

Absurd Published on 11:48:00 IP: 211.161.63. * This is a compiler feature. com uses this feature to get the offset of a subclass instance (COM subject: inttable. h ):
# Define base_offset (classname, basename )\
(DWORD (static_cast <basename *> (reinterpret_cast <classname *> (0x10000000)-0x10000000) Reprinted from: http://www.yuanma.org/data/2006/0716/article_1143.htm