c++ - What are those two long in vtable assembly code emitted by GCC? -

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• what first (int (*)(…))0 vtable entry in output of g++ -fdump-class-hierarchy? 2 answers

i checked assembly code generated gcc following c++ code snippet.

class interface { public:  virtual int f() = 0;  virtual int g() = 0; };  class concrete : public interface { public:  virtual int f();  virtual int g(); }; 

seams there 2 long in each vtable, , assembly code seams doesn't use it, purpose of 2 viable?

          .globl __ztv8concrete           .const_data           .align 3   __ztv8concrete:           .long   0   <<<< thos 2 long doesn"t used.           .long   0            .long   __zn8concrete1gev           .long   __zn8concrete1fev           .globl __ztv9interface           .weak_definition __ztv9interface           .section __data,__const_coal,coalesced           .align 3   __ztv9interface:           .long   0    <<<< thos 2 long doesn"t used also.           .long   0            .long   ___cxa_pure_virtual           .long   ___cxa_pure_virtual           .section __text,__textcoal_nt,coalesced,pure_instructions           .weak_definition        ___x86.get_pc_thunk.cx           .private_extern ___x86.get_pc_thunk. 

the constructor:

  __zn8concretec1ev:   lfb8:           pushl   %ebp   lcfi9:           movl    %esp, %ebp   lcfi10:           pushl   %ebx           subl    $20, %esp   lcfi11:           call    ___x86.get_pc_thunk.bx   l2$pb:           movl    8(%ebp), %eax           movl    %eax, (%esp)           call    __zn9interfacec2ev           movl    8(%ebp), %edx 2>        leal    __ztv8concrete-l2$pb(%ebx), %eax           leal    8(%eax), %eax    <<<< skip 2 two long vtable           movl    %eax, (%edx)           addl    $20, %esp           popl    %ebx 

one of 2 "spaces" should rtti (the second one), other multiple inheritance (the first one). see example http://tinydrblog.appspot.com/?p=89001, or better https://stackoverflow.com/a/5712953/613130