std::nullptr_t

Since C++11

Header: <cstddef>

std::nullptr_t is the type of the null pointer literal nullptr. It is a distinct type that is not itself a pointer type or a pointer to member type. Prvalues of this type are null pointer constants, and may be implicitly converted to any pointer and pointer to member type.

# Declarations

using nullptr_t = decltype(nullptr);

(since C++11)

# Notes

The C++ standard requires <stddef.h> to place the contents of in the global namespace, and thereby requires nullptr_t to be available in the global namespace when <stddef.h> is included.

nullptr_t is not a part of C until C23.

It is unspecified whether the declaration of std::nullptr_t is available in any other standard library header. An implementation may avoid introducing this name even when the standard requires std::nullptr_t to be used, by e.g. spelling decltype(nullptr) instead.

# Example

#include <cstddef>
#include <iostream>
 
void f(int*)
{
    std::cout << "Pointer to integer overload\n";
}
 
void f(double*)
{
    std::cout << "Pointer to double overload\n";
}
 
void f(std::nullptr_t)
{
    std::cout << "null pointer overload\n";
}
 
int main()
{
    int* pi{};
    double* pd{};
 
    f(pi);
    f(pd);
    f(nullptr); // would be ambiguous without void f(nullptr_t)
    // f(0);    // ambiguous call: all three functions are candidates
    // f(NULL); // ambiguous if NULL is an integral null pointer constant 
                // (as is the case in most implementations)
}

# See also