NULL
Defined in header <clocale> | ||
---|---|---|
Defined in header <cstddef> | ||
Defined in header <cstdio> | ||
Defined in header <cstdlib> | ||
Defined in header <cstring> | ||
Defined in header <ctime> | ||
Defined in header <cwchar> | ||
#define NULL /*implementation-defined*/
|
The macro NULL
is an implementation-defined null pointer constant, which may be.
an integral constant expression rvalue of integer type that evaluates to zero. | (until C++11) |
an integer literal with value zero, or a prvalue of type | (since C++11) |
A null pointer constant may be implicitly converted to any pointer type; such conversion results in the null pointer value of that type. If a null pointer constant has integer type, it may be converted to a prvalue of type std::nullptr_t
.
Possible implementation
#define NULL 0
//since C++11
#define NULL nullptr |
Notes
In C, the macro NULL
may have the type void*
, but that is not allowed in C++.
Example
#include <cstddef>
#include <type_traits>
#include <iostream>
class S;
int main()
{
int* p = NULL;
int* p2 = static_cast<std::nullptr_t>(NULL);
void(*f)(int) = NULL;
int S::*mp = NULL;
void(S::*mfp)(int) = NULL;
if (std::is_same_v<decltype(NULL), std::nullptr_t>) {
std::cout << "NULL implemented with type std::nullptr_t\n";
} else {
std::cout << "NULL implemented using an integral type\n";
}
}
Possible output:
NULL implemented using an integral type
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
---|---|---|---|
CWG 903 | C++11 | constant expressions with zero value such as 1-1 were allowed | only the literal zero is allowed |
See also
nullptr | the pointer literal which specifies a null pointer value (C++11) |
(C++11) | the type of the null pointer literal nullptr (typedef) |
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