Why is `= 0` Used to Declare Pure Virtual Functions in C ?

Why is a pure virtual function initialized by 0?

Pure virtual functions are an essential part of C 's object-oriented programming model. They allow classes to define methods that must be overridden by derived classes, effectively forcing derived classes to provide implementations for those methods.

When declaring a pure virtual function, it is common practice to initialize it with 0. This has led many to believe that this initialization is necessary to set the virtual table entry for the function to NULL. However, this understanding is incorrect.

The =0 at the end of a pure virtual function declaration is not an initialization. Instead, it is a syntax indicator that this function is pure virtual. It is a consequence of the limitations during the language's design process, as explained by Bjarne Stroustrup in his book "The Design & Evolution of C ".

Implementing pure virtual functions does not necessarily involve setting the virtual table entry to NULL. In fact, Stroustrup explicitly states that this approach is not the best way to implement pure virtual functions.

So, why is 0 used for pure virtual functions? The answer lies in language design rather than technical implementation needs.

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