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NULL

From cppreference.net
C++
Utilities library
Type support
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 .

Contents

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++ because null pointer constants cannot have that type.

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 (C++11) the pointer literal which specifies a null pointer value
(C++11)
the type of the null pointer literal nullptr
(typedef)
C documentation for NULL