error: ‘NULL’ was not declared in this scope

Question

I get this message when compiling C++ on gcc 4.3

error: ‘NULL’ was not declared in this scope

It appears and disappears and I don't know why. Why?

Thanks.

Answer 1

NULL isn't a keyword; it's a macro substitution for 0, and comes in stddef.h or cstddef, I believe. You haven't #included an appropriate header file, so g++ sees NULL as a regular variable name, and you haven't declared it.

Answer 2

NULL is not a keyword. It's an identifier defined in some standard headers. You can include

#include <cstddef>

To have it in scope, including some other basics, like std::size_t.