Why C++ has given pure virtual methods?

[a2z]

From design point of view, I have other work around to get following
behavior of pure virtual methods:

1. To make a class abstract I can make constructor/destructor private.

What is the thing I'm missing because of which C++ has supported
pure virtual mechanism?
Kindly let me know?

~Thanks,
Ramesh

 

[Juha Nieminen]

From design point of view, I have other work around to get following
behavior of pure virtual methods:

1. To make a class abstract I can make constructor/destructor private.

Don't you mean protected?

You can make a whole class abstract that way, but you can't make an
individual member function abstract that way.

In some cases it makes sense design-wise that the inherited class must
implement a given virtual function because it has no sensible default
implementation.