NULL

#define NULL /*implementation-defined*/

The macro NULL is an implementation-defined null pointer constant, which may be.

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.

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++.

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.

Example

#include <cstddef>
#include <type_traits>
#include <iostream>
#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; // a warning may be triggered 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
}

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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also