Does C++ virtual function call on derived object go through vtable?

Question

In the following code, it calls a virtual function foo via a pointer to a derived object. Will this call go through the vtable or will it call B::foo directly?<

Answer 1

As usual, the answer to this question is "if it is important to you, take a look at the emitted code". This is what g++ produces with no optimisations selected:

18     b->foo();
0x401375 :  mov    eax,DWORD PTR [esp+28]
0x401379 :  mov    eax,DWORD PTR [eax]
0x40137b :  mov    edx,DWORD PTR [eax]
0x40137d :  mov    eax,DWORD PTR [esp+28]
0x401381 :  mov    DWORD PTR [esp],eax
0x401384 :  call   edx

which is using the vtable. A direct call, produced by code like:

B b;
b.foo();

looks like this:

0x401392 :  lea    eax,[esp+24]
0x401396 :  mov    DWORD PTR [esp],eax
0x401399 :  call   0x40b2d4 <_ZN1B3fooEv>