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 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";
    }
}

NULL implemented using an integral type

Defect reports

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

See also