< cpp‎ | types
Utilities library
General utilities
Date and time
Function objects
Formatting library (C++20)
Relational operators (deprecated in C++20)
Integer comparison functions
Swap and type operations
Common vocabulary types
Elementary string conversions
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 std::nullptr_t

(since C++11)

A null pointer constant may be implicitly converted to any pointer and pointer to member 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.


[edit] Possible implementation

#define NULL 0
//since C++11
#define NULL nullptr

[edit] Notes

In C, the macro NULL may have the type void*, but that is not allowed in C++.

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 C++ 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.

[edit] Example

#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";
        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

[edit] 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

[edit] See also

nullptr(C++11) the pointer literal which specifies a null pointer value[edit]
the type of the null pointer literal nullptr
(typedef) [edit]