Re: pure virttual function

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Wed, 05 Jul 2006 16:53:32 +0200

Message-ID:

<4h223fF1pufsqU2@individual.net>

* Rolf Magnus:

Alf P. Steinbach wrote:

* Murali Krishna:

*sks:

could anyone explain me why definition to a pure virtual function
is allowed ?

May be you are asking why it is not allowed.

Sorry, the OP is correct that you can provide a definition for a pure
virtual function. But that definition can't be provided in the class
definition. As to the why of that, I don't know any good reason, and
that's better asked in [comp.std.c++].

One use for a defined pure virtual function is a "marker interface" like

   struct Serializable
   {
       inline virtual ~Serializable() = 0;
   };

   inline Serializable::~Serializable() {}

Here a definition is necessary because the destructor will be called
(although it's never called virtually), and the destructor is the only
member function that for this class can be used to make it abstract.

Well, if no polymorphism is needed, but the class shouldn't be
instantiatable, one can always make the destructor protected.

Yes, it seems that the practical advantage reduces to the single case of
a polymorphic marker interface through which objects can be deleted, but
heck, there must be more to it for having it as a langugage feature.

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