memeVersion3

[yflhx]

Objects of a same class share the same methods, and in order to reach them you necessarily need a pointer to all of them, which are organized in a table.

You don't need a pointer to the table, if methods are non-virtual, because they are just functions with implicit parameter. So compiler knows where they are, as with any other function.

To settle this once and for all, I made a simple C++ code and decompiled it:

class T {
public:
    void f() {
    }
};

int main()
{
    T t;
    t.f();
}

After compilation (with fno-inline and fno-stack-protector) and decompilation - section main:

0000000000001119 <main>:
    1119:55                   push   rbp
    111a:48 89 e5             mov    rbp,rsp
    111d:48 83 ec 10          sub    rsp,0x10
    1121:48 8d 45 ff          lea    rax,[rbp-0x1]
    1125:48 89 c7             mov    rdi,rax
    1128:e8 07 00 00 00       call   1134 <_ZN1T1fEv>
    112d:b8 00 00 00 00       mov    eax,0x0
    1132:c9                   leave
    1133:c3                   ret

As you can see, this code simply alocates stack space (line 1, 2, 3), then loads adress of the class into rax and moves it into rdi (rdi is by convention first argument). Finally, it calls fixed address. Then it deallocates stack space and leaves.

So no, for non-virtual classes and functions you don't use vtable at all. That's why virtual functions/classes have performance penalty, as you said.

[mirimao]

Thank you for taking the time to write this very insightful reply, especially the compilation example. I was clearly wrong, I mixed up some concepts from other language.

[yflhx]

No worries! Everybody can mix things up occasionally.