c++ - Is vftable[0] stores the first virtual function or RTTI Complete Object Locator? -


it's known c++ use vftable dynamicly decide virtual function should called. , want find out mechanism behind when call virtual function. have compiled following code assembly.

using namespace std;  class animal {     int age; public:     virtual void speak() {}     virtual void wash() {} };  class cat : public animal { public:     virtual void speak() {}     virtual void wash() {} };  void main() {     animal* animal = new cat;     animal->speak();     animal->wash(); }

the assembly code massive. don't quite understand following part.

const   segment ??_7cat@@6b@ dd flat:??_r4cat@@6b@          ; cat::`vftable'     dd  flat:?speak@cat@@uaexxz     dd  flat:?wash@cat@@uaexxz const   ends

this part defines vftable of cat. have 3 entries. first entry rtti complete object locator. second cat::speak. third cat::wash. think vftable[0] should imply rtti complete object locator. when checking assembly code in main proc , cat::cat proc, invoke animal->speak() implemented calling vftable[0], , invoke animal->wash() implemented calling vftable[4]. why not vftable[4] , vftable[8]?

the assembly code of proc main , cat::cat shows below.

_text   segment tv75 = -12                      ; size = 4 $t1 = -8                        ; size = 4 _animal$ = -4                       ; size = 4 _main   proc  ; 23   : {      push    ebp     mov ebp, esp     sub esp, 12                 ; 0000000ch  ; 24   :    animal* animal = new cat;      push    8     call    ??2@yapaxi@z                ; operator new     add esp, 4     mov dword ptr $t1[ebp], eax     cmp dword ptr $t1[ebp], 0     je  short $ln3@main     mov ecx, dword ptr $t1[ebp]     call    ??0cat@@qae@xz     mov dword ptr tv75[ebp], eax     jmp short $ln4@main $ln3@main:     mov dword ptr tv75[ebp], 0 $ln4@main:     mov eax, dword ptr tv75[ebp]     mov dword ptr _animal$[ebp], eax  ; 25   :    animal->speak();      mov ecx, dword ptr _animal$[ebp]     mov edx, dword ptr [ecx]     mov ecx, dword ptr _animal$[ebp]     mov eax, dword ptr [edx]     call    eax  ; 26   :    animal->wash();      mov ecx, dword ptr _animal$[ebp]     mov edx, dword ptr [ecx]     mov ecx, dword ptr _animal$[ebp]     mov eax, dword ptr [edx+4]     call    eax  ; 27   : }      xor eax, eax     mov esp, ebp     pop ebp     ret 0 _main   endp _text   ends  ; function compile flags: /odtp ;   comdat ??0cat@@qae@xz _text   segment _this$ = -4                     ; size = 4 ??0cat@@qae@xz proc                 ; cat::cat, comdat ; _this$ = ecx     push    ebp     mov ebp, esp     push    ecx     mov dword ptr _this$[ebp], ecx     mov ecx, dword ptr _this$[ebp]     call    ??0animal@@qae@xz     mov eax, dword ptr _this$[ebp]     mov dword ptr [eax], offset ??_7cat@@6b@     mov eax, dword ptr _this$[ebp]     mov esp, ebp     pop ebp     ret 0 ??0cat@@qae@xz endp                 ; cat::cat _text   ends

supplement: msvc compiler x86 19.00.23026

the layout of vtables implementation dependent. in particular case, when compiling example code, microsoft c++ compiler generates vtable cat speak virtual function @ offset 0, , wash function @ offset 4. rtti information located before these functions @ offset -4.

the problem here microsoft's assembly output lying. generated assembly code puts rtti information @ offset 0 , speak , wash functions @ offset 4 , 8. not how it's laid out in object file compiler generates. disassembling object file reveals layout:

                .new_section .rdata, "dr2" 0000    00 00 00 00                                     .long   ??_r4cat@@6b@ 0004                          ??_7cat@@6b@: 0004    00 00 00 00                                     .long   ?speak@cat@@uaexxz 0008    00 00 00 00                                     .long   ?wash@cat@@uaexxz

unfortunately assembly output of microsoft's c/c++ compiler meant informational. it's not accurate , complete representation of actual code compiler generates. in particular can't reliably assembled working object file.


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