c++ - How much space does a Derived class cost?If itself and its base class both have virtual methods? -

i'm using vs2013(win32) testing following program:

#include <iostream>  class {     virtual void funa(); };  class b {     virtual void funb(); };  class c :public a{     int i;     virtual void func(); };  class d :public b, c{     virtual void fund(); };  int main(){     std::cout << "size " << sizeof(a) << std::endl;     std::cout << "size b " << sizeof(b) << std::endl;     std::cout << "size c " << sizeof(c) << std::endl;     std::cout << "size d " << sizeof(d) << std::endl;     return 0; } 

and result

size 4
size b 4
size c 8
size d 12

why sizeof(c) != 8 + sizeof(a), , sizeof(d) != 4 + sizeof(b) + sizeof(c)?

a single base class virtual methods, hence single vtable pointer, hence 4 bytes on 32-bit platform yours.

b a.

c a plus 1 4-byte integer. note still has single base class (a) means still 1 vtable pointer.

d inherits 2 base classes virtual methods not related each other, hence d gets 2 vtable pointers (one each base class), plus 4-byte integer c. hence 12 bytes total.