When is a vtable created in C++?

Question

When exactly does the compiler create a virtual function table?

1) when the class contains at least one virtual function.

OR

2) when the immediate

Answer 1

The answer is, 'it depends'. It depends on what you mean by 'contain a vtbl' and it depends on the decisions made by the implementor of the particular compiler.

Strictly speaking, no 'class' ever contains a virtual function table. Some instances of some classes contain pointers to virtual function tables. However, that's just one possible implementation of the semantics.

In the extreme, a compiler could hypothetically put a unique number into the instance that indexed into a data structure used for selecting the appropriate virtual function instance.

If you ask, 'What does GCC do?' or 'What does Visual C++ do?' then you could get a concrete answer.

@Hassan Syed's answer is probably closer to what you were asking about, but it is really important to keep the concepts straight here.

There is behavior (dynamic dispatch based on what class was new'ed) and there's implementation. Your question used implementation terminology, though I suspect you were looking for a behavioral answer.

The behavioral answer is this: any class that declares or inherits a virtual function will exhibit dynamic behavior on calls to that function. Any class that does not, will not.

Implementation-wise, the compiler is allowed to do whatever it wants to accomplish that result.