Null pointer is a stuff which have had changes and advancement through out the development of C and C++. All the implementation had its own draw backs and now a perfect design is about to arrive in C++0x. Let us analyze the NULL pointer in C, C++ and C++0x.
K&R style(C Style)
A NULL pointer is a constant expression which evaluates to either 0 or ((void*) 0). There are machines which use different internal representations for pointers to different type. In such cases by standard it is always guaranteed a 0 cast to void* will be assignable to every pointer. For example if you assign a ((void*) 0) to a pointer of FILE type it is guaranteed to be initialized to a null pointer without any error.
A null pointer constant is an integral constant expression rvalue of an integer type that evaluates to zero. A null pointer constant can be converted to a pointer type; the result is the null pointer value of that type and is distinguishable from every other value of a pointer. So the macro NULL is equivalent to an integer 0 and therefore it is better to avoid NULL macro by directly using a 0.
In all the standards till now the constant 0 had double roles of constant integer 0 and null pointer. This behavior existed throughout the development of C and C++. In C the NULL is a macro which assigns to either 0 or ((void*) 0). But in C++ NULL is always a special case represented as 0. But even using a 0 have its own drawback while overloading. For example we have two declarations,
void foo( char* p );
void foo( int p );
and then call foo(NULL). This will call the integer overload of foo where a programmer may normally intend to call char overload with a null pointer.
To get rid of this issue the new standards will include an additional keyword ‘nullptr’ which would only be assignable to pointer types and comparable to pointer types. The existing 0 will have to suffer the double role again to have backward compatibility. But sooner or later C++ committee would declare deprecated usage of 0 and NULL as null pointers, and eventually avoid this double role.