问题
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.
回答1:
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.
回答2:
GCC is taking steps towards C++11, which is probably why you now need to include cstddef in order to use the NULL constant. The preferred way in C++11 is to use the new nullptr keyword, which is implemented in GCC since version 4.6. nullptr is not implicitly convertible to integral types, so it can be used to disambiguate a call to a function which has been overloaded for both pointer and integral types:
void f(int x);
void f(void * ptr);
f(0); // Passes int 0.
f(nullptr); // Passes void * 0.
回答3:
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.
回答4:
To complete the other answers: If you are using C++11, use nullptr, which is a keyword that means a void pointer pointing to null. (instead of NULL, which is not a pointer type)
回答5:
NULL can also be found in:
#include <string.h>
String.h will pull in the NULL from somewhere else.
回答6:
You can declare the macro NULL. Add that after your #includes:
#define NULL 0
or
#ifndef NULL
#define NULL 0
#endif
No ";" at the end of the instructions...
来源：https://stackoverflow.com/questions/462165/error-null-was-not-declared-in-this-scope