Re: Class Inheritance vs Interface Inheritance
Tim Roberts wrote:
"Vladimir Grigoriev" <vlad.moscow@mail.ru> wrote:
"Pure virtual functions have no implementation" and "While pure
virtual functions may, in principle, have implementation, it is
highly unusual." are two different things. So I do not see that I am
wrong.
In terms of semantics, you are technically correct. However,
repeating a speech that I seem to be making a lot these days, it is
vitally important to remember that these newsgroup threads now live
FOREVER. When newbies go searching for answers, they find these
threads, and assume that we are all experts giving the final word.
Newbies should be told that it is a RULE that "pure virtual functions
have no implementation", unconditionally. Later, as their knowledge
grows, they can be gradually exposed to the awful truth that the rule
isn't exactly correct, and there are a very few cases where it is
useful to violate the rule.
I should have written "pure virtual functions CAN have no implementation".
The point I was trying to make is that only a pure virtual function can lack
implementation, all other functions ALWAYS have a behavior which is ALWAYS
inherited by derived classes (although the derived class can override it, it
is still available to the derived class, there is no way to separate
interface from behavior of a non-virtual function).
Especially the constructor of the base class will ALWAYS be called by every
derived class and this can be extended but not overridden. This stands in
contrast to VB6 inheritance which adopts the interface of the inherited
class and none of the behavior, not even the constructors.