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We always declare a pure virtual function as:
virtual void fun () = 0 ;
I.e., it is always assigned to 0.
What I understand is that this is to initialize the vtable entry for this function to NULL and any other value here results in a compile time error. Is this understanding correct or not?
The reason
=0is used is that Bjarne Stroustrup didn’t think he could get another keyword, such as "pure" past the C++ community at the time the feature was being implemented. This is described in his book, The Design & Evolution of C++, section 13.2.3:He also states explicitly that this need not set the vtable entry to NULL, and that doing so is not the best way of implementing pure virtual functions.