When is the v-table created for a class?

Question

I know that, how to implement virtual function call resolution is not part of C++ standrads nor it says anything about vptr or v-table, but let me ask this question here.

I've heard that v-table is a common technique used by compilers to implement virtual function call resolution. My understading about this is that there is only virtual table is needed per class, per process.

What I am wondering is, when is the v-table is created for a class?
Is it When the class of a given type (which needs a v-table) is created in a process space for the first time?
All other subsequently created objects of that type in that process space, refers to the v-table that is already created?
When would this v-table will be deleted?

I am sorry if this is too subjective or discussion type of question, but these questions lingers in my mind for a while and I feel its OK asking it here.

Answer 1

The v-table is statically allocated and is never deleted, nor is it explicitly allocated. The pointers within any given specific object are constants.

Answer 2

The C++ FAQ provides a simplified explanation of the vtable mechanism. You should give it a read, although you will probably have to go through your particular compiler documentation for more details.

The most important ideas from my point of view :

• The vtable for a type is static and built at compile time
• Each of the type instances contains a pointer to this table
• Because this pointer is initialized at construction time, a virtual member function should never be called from the constructor

Answer 3

The vtable is static data so available immediately at load. BTW, it is usually bundled in the compilation unit which contains the definition for the first non-inline virtual function on the class (and that heuristic leads to problem when there is only one virtual function which is inline).

Answer 4

I believe it's all implementation defined, so it's difficultto give a universal answer to this question. I believe the vtable should be a some sort of a static class member.