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*).