Concept of vptr and vtable in C++

Question

Why only default constructor only able to create vptr(Virtual Table Pointer) and vtable(Virtual Table)? Why parameter constructor not able to

*"Why only default constructor only able to create vptr(Virtual Table Pointer) and vtable(Virtual Table)?"* Where did you get that idea? That does not make any sense. — Baum mit Augen, Oct 05 '16 at 08:33
constructors don't make virtual anythings. having a virtual function in a type makes the type polymorphic, but that's a compile-time thing and has nothing to do with what constructors do. — xaxxon, Oct 05 '16 at 08:42

score 5 · Accepted Answer · answered Oct 05 '16 at 08:33

First, vtables and vptrs are not specified by the C++ language standard.

They're an implementation detail, although as far as I know all extant C++ implementation use that technique to implement virtual function dispatch.

With such an implementation, all constructors for a class with virtual member functions, necessarily establish the object's vptr. Things wouldn't work without it. So …

” Why parameter constructor not able to

… is simply an incorrect assumption.

Also, *no* constructor creates any vtables. – Baum mit Augen Oct 05 '16 at 08:37 — Baum mit Augen, Oct 05 '16 at 08:37

score 0 · Answer 2 · answered Oct 05 '16 at 09:42

Assuming the implementation uses vtables (a pretty common implementation choice, as noted by "Cheers and hth - Alf"), the creation/population of vtables and invoking constructors are distinct operations.

However, all instances of a given (non-abstract) class will - once constructed fully - have the same set of vtables (a class that inherits from multiple bases may have more than one vtable). There will certainly not be a different vtable depending on how the object is constructed (e.g. what parameters are passed to constructors, which constructor is invoked, etc).

Concept of vptr and vtable in C++

2 Answers2