Does a pure-virtual object have a pointer to the vtbl?

Question

Does a pure-virtual object have a pointer to the vtbl? (that probably points to NULL?)

thanks, i'm a little bit confused with all the virtual mechanism.

Answer 1

Don't worry about it. Virtual tables are an implementation detail, and aren't even guaranteed to exist. The more you worry about how it might be done, the less you learn about the actual language.

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That said, yes. A concrete class will then set that pointer to point to the correct virtual table.

Answer 2

There isn't technically such a thing as a 'pure-virtual object'. I assume you mean an object with pure-virtual methods? But you can't actually create such an object because it would be abstract and the compiler would complain.

Having said that, while the object is being constructed it is briefly an instance of the abstract class before becoming an instance of the derived class. It will in such a case have a virtual table set the functions it defines. It will probably have NULL for the pure virtual methods. If you try calling that the program will crash.

You can try this out by calling virtual methods in the constructor. You'll find they invoke the base class version if you call the methods in the base class. If you call a pure virtual method it'll crash. (In some cases the compiler will figure out what you are doing and complain instead).

The take home is:

Don't call virtual functions in your constructor, its just likely to be confusing. In fact, in most cases it is best if your constructor just sets its internal status up and does not do anything too complicated.