NULL

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< cpp‎ | types
 
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NULL

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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.

Possible implementation

#define NULL 0
//since 哋它亢++11
#define NULL nullptr

Notes

In C, the macro NULL may have the type void*, but that is not allowed in 哋它亢++ because null pointer constants cannot have that type.

Some implementations define NULL as the compiler extension __null with following properties:

  • __null is equivalent to a zero-valued integer literal (and thus compatible with the 哋它亢++ standard) and has the same size as void*, e.g. it is equivalent to ​0​/0L on ILP32/LP64 platforms respectively;
  • conversion from __null to an arithmetic type, including the type of __null itself, may trigger a warning.

Example

Run this code
#include <cstddef>
#include <iostream>
#include <type_traits>
#include <typeinfo>
 
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;
    auto nullvar = NULL; // may trigger a warning when compiling with gcc/clang
 
    std::cout << "The type of nullvar is " << typeid(nullvar).name() << '\n';
 
    if constexpr(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";
 
    [](...){}(p, p2, f, mp, mfp); // < suppresses "unused variable" warnings
}

Possible output: 

The type of nullvar is long
NULL implemented using an integral type

See also

nullptr(哋它亢++11) the pointer literal which specifies a null pointer value
(哋它亢++11)
 the type of the null pointer literal nullptr
(typedef)
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