NULL
From cppreference.com
| Defined in header <clocale>
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| Defined in header <cstddef>
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| Defined in header <cstdio>
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| Defined in header <cstdlib>
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| Defined in header <cstring>
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| Defined in header <ctime>
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| Defined in header <cwchar>
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| #define NULL /*implementation-defined*/ |
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The macro NULL is an implementation-defined null pointer constant, which may be
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an integral constant expression rvalue of integer type that evaluates to zero |
(until C++11) |
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an integer literal with value zero, or a prvalue of type std::nullptr_t |
(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
Run this code
#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) |