Does the C++ compiler create VTable for an Abstract Class

[rsodimbakam]

Hi,
Can anyone clarify me whether C++ compiler creates a VTable for an Abstract Class ?

Thanks
Kiran

[Graham]

Yes.

[_uj]

Can anyone clarify me whether C++ compiler creates a VTable for an Abstract Class ?

Most likely so, but it depends on the particular compiler. The vtable is a common implementation technique of virtual function calls, but it isn't required by the C++ language.

[TheCPUWizard]

Additionally the compiler may create it, only to have the linker optimize it out of existance.

[rsodimbakam]

Thanks for the clarification. This question is something which is confusing my mind a lot.

So the compiler creates the vtable for an abstract class, but since there will be a pure virtual function inside it, compiler does not allow to create an object for Abstract class, as one the vtable entry points to NULL pointer.

Is my analysis correct ?

[Graham]

I'm generalising, somewhat, since the standard doesn't explicitly state this, but what do you think the "= 0" part of a pure virtual definition corresponds to?

I always think of it as putting a zero (i.e. an invalid entry) into the vtable for that function. That's why the class has to be inherited from - so that the derived class can put a valid entry in the appropriate slot. If the derived class doesn't do that, then it is abstract itself. The process continues until all the vtable slots have non-zero entries.

[_uj]

Thanks for the clarification. This question is something which is confusing my mind a lot.

So the compiler creates the vtable for an abstract class, but since there will be a pure virtual function inside it, compiler does not allow to create an object for Abstract class, as one the vtable entry points to NULL pointer.

Is my analysis correct ?

No. The reason a compiler won't allow you to create an object of an abstract class is because C++ stipulates that only concrete classes can be instantiated.

C++ works the way it does because it's defined to work that way, not because some compiler happens to have a null in some data structure. If all C++ compilers vanished from the face of the earth, C++ still would work the way it does.

I think this is an important distinction. C++ leads and compilers follow, not the other way around.

[Peter_APIIT]

Nice info.