Question

When in C++ I declare a null pointer to be int* p=0, does that mean the zero is some special constant of integer pointer type, or does it mean that p is pointing to address 0x0?
Of course for that 0x0 would have to be an special address to which C++ never touches during allocation of variables/arrays.

Answer 1

The C++ standard defines that the integer constant 0 converts to a null pointer. This does not mean that null pointers point to address 0x0. It just means that the text ‘0’ turns into a null pointer when converted to a pointer.

Of course, making null pointers have a representation other than 0x0 is rather complicated, so most compilers just let 0x0 be the null pointer address and make sure nothing is ever allocated at zero.

Note that using this zero-conversion is considered bad style. Use NULL (which is a preprocessor macro defined as 0, 0L, or some other zero integral constant), or, if your compiler is new enough to support it, nullptr.