Re: pure virttual function

"=?iso-8859-1?q?Kirit_S=E6lensminde?=" <>
5 Jul 2006 08:22:40 -0700
sks wrote:

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

The easiest to understand is the case for serialization frameworks. If
all classes inherit from a single class, for argument's sake Document
then you will have:

class Document {
    void save( ofstream & ) = 0;
    void load( ifstream & ) = 0;

    std::string author;

Derived classes must implement these members and those members must
call the Document member in order that it can save and load the
author's name. Document needs to implement both as well in order to
save and load the author's name.

This is basically an example of the occasion where the sub-class
overrides the virtual to augment the bahaviour of the super class. In
these cases the sub-class must always call the super-class
implementation as they are adding to its behaviour. Even if the upper
level implementations are empty they should still be provided so that
all sub-class implementors know that there is no reason to leave it out
and there will be no link errors when sub-classes call ones that didn't
get implement (probably 'implemented yet').


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