Re: Innerclass inheriting from an outter class?

From:

"Andrei Alexandrescu (See Website For Email)" <SeeWebsiteForEmail@erdani.org>

Newsgroups:

comp.lang.c++.moderated

Date:

28 Sep 2006 08:58:03 -0400

Message-ID:

<451B388F.4060601@erdani.org>

Thomas Maeder wrote:

Which is correct. A class has to be completely defined for serving as
base class. This

class OuterClass
{
  public:
    ~OuterClass();

    class InnerClass;
};

class OuterClass::InnerClass : public OuterClass
{
};

should work. Unless you convince me otherwise, I don't think this will
result in a good design, though ...

I think there are circumstances in which this might be sensible. For
example, consider defining an interface IWidget. There are two
degenerate possible implementations of this interface: a black hole
(implements all functions to do nothing and return the default value of
the result type); and a white hole (implement all functions to throw an
exception). These degenerate implementations are useful as base classes
for actual implementations.

In such an approach, I'd find it a nice touch to lexically nest the two
degenerate implementations inside the interface, such that people can
use the explanatory IWidget::BlackHole and IWidget::WhiteHole.

Andrei

--
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]