The Underlying Representation of NULL

The Underlying Representation of NULL

C and C++ define NULL differently:

    #define NULL 0;                        // A typical definition of NULL in C++    #define NULL  ((void*)0)          // C defines NULL this way

Why is it defined differently in the two languages? Pointers in C++ are strongly typed, unlike pointers in C. Thus, void* cannot be implicitly converted to any other pointer type without an explicit cast. If C++ retained C’s convention, a C++ statement such as:

    char * p = NULL; 

would be expanded into something like:

    char * p = (void*) 0;   // compile time error: incompatible pointer types 

Since 0 is the universal initializer for all pointer types in C++, it is used instead the traditional C convention, and in fact, many programmers simply use 0 as a pointer initializer.


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