Re: some abstract base class dont need vtbl?
petke wrote:
Some classes are not means to be derivable so they don't have
a any virtual functions or vtable. But even for these classes
it can make sense to have their interface derived from an
abstract base class. (As to enforce that a family of classes
really have the same interface.) But know all the functions
are virtual and we get a vtable we don't need. The abstract
base class is empty as it has no variables or functions, so it
should not take any space. So the vtable is not needed in
construction or deconstruction of the object. Right?
I believe microsoft has the keyword __declspec(novtable) for
this, as often used in COM programming. To bad that's not
portable. I think it would be a useful feature elsewhere also.
Anyone disagree?
Sounds more like an optimization issue for the compiler. At
least one of the compilers I use doesn't generate unused
vtables.
As optimizations go, of course, it's pretty minimal -- you gain
generally less than a hundred bytes in terms of space, and save
one store in the constructor and the destructor in all of the
derived classes. On the other hand, it's cheap for the compiler
to do. And, I suspect, very error prone for the user to
specify.
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