Why is NULL undeclared?

Solution 1:

NULL is not a built-in constant in the C or C++ languages. In fact, in C++ it's more or less obsolete, just use a plain literal 0 instead, the compiler will do the right thing depending on the context.

In newer C++ (C++11 and higher), use nullptr (as pointed out in a comment, thanks).

Otherwise, add

#include <stddef.h>

to get the NULL definition.

Solution 2:

Do use NULL. It is just #defined as 0 anyway and it is very useful to semantically distinguish it from the integer 0.

There are problems with using 0 (and hence NULL). For example:

void f(int);
void f(void*);

f(0); // Ambiguous. Calls f(int).

The next version of C++ (C++0x) includes nullptr to fix this.

f(nullptr); // Calls f(void*).