g++ and virtual table

[Lada 'Ray' Lostak]

Hi !

    Please, can any1 help me with following problem please ? I need to call
C++ object based on pure virtual classes (defined by structures) from
'assembler' basically. It is pretty easy (all parameter conversions, stack,
and similar - it is done and pretty well working)... But:

Let's say I have following project:

#include <new.h>
#include <stdio.h>

// this should be SHARED interface - acros BINARY
// files compiled in MORE compilers
typedef struct base {

    virtual void uch()=0;

} base;

// some 'end' object supporting 'base'
class object:public base {

public:

    object();
    ~object();

    virtual void uch();

protected:

    int x,y;   // just something
};

object::object() {

    printf("object\n");
}

object::~object() {

    printf("~object\n");
}

void object::uch() {

    printf("uch\n");
}

void main() {

    object* o;
    base*   b;

    o=new object();

    b=o;

    // let's call it - this is my interest
    b->uch();

    delete o;
}

Nothing special. Now let's look on generated code of line 'b=0' and
'b->uch()'
latest g++ release, without optimalizations (disasm, I do not like listing):

b=0
000009EC:i8B45FC                         mov       eax,[ebp-04]
000009EF:i8945F8                         mov       [ebp-08],eax

Yes, normal...

b->uch()
000009F7:i8B55F8                         mov       edx,[ebp-08]
000009FA:i8B02                           mov       eax,[edx]
000009FC:i83C008                         add (d)   eax,+08
000009FF:i8B55F8                         mov       edx,[ebp-08]
00000A02:i52                             push      edx
00000A03:i8B30                           mov       esi,[eax]
00000A05:iFFD6                           call (d)  esi
00000A07:i83C404                         add (d)   esp,+04


[ebp-08] - instance of b...
eax,[edx] - loads virtual table base...
esi,[eax] - loads adress of method 'uch'...

With optimalizations it looks like:

[ebx is instance from previous code]
0000095A:i8B03                           mov       eax,[ebx]
0000095C:i53                             push      ebx
0000095D:i8B4008                         mov       eax,[eax+08]
00000960:iFFD0                           call (d)  eax

But I do not understand why compiler "add (d)   eax,+08" ? What this mean ?
Why compiler skips 2 methods/pointers ? It is problem for me when compiler
adds to the start own things. Adding '2' VT items makes hard to MIX more
compiler binaries together.

Can anyone help how g++ handles 'Virtual Table' internally ? What are 2
initial items ? Can I tell compiler 'do not put these items here - use
something more clever for own things' :) ?

We are assuming 'our' virtual methods starts at offset 0. Works for all
major compilers we're supporting.

Thanx in advance for any coment, hint, or where to get these infos.

Best regards
Lada 'Ray' Lostak
DSPaudio inc.
http://www.dspaudio.com