From cppreference.com
usingnullptr_t=decltype(nullptr);(since C++11)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
, and may be
to any pointer and pointer to member type.
sizeof(std::nullptr_t) is equal to sizeof(void*).
Notes
The C++ standard requires
to place the contents of
in the global namespace, and thereby requires nullptr_t to be available in the global namespace when
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
If two or more overloads accept different pointer types, an overload for std::nullptr_t is necessary to accept a null pointer argument.
Run this code
#include<cstddef>#include<iostream>voidf(int*){std::cout<<"Pointer to integer overload\n";}voidf(double*){std::cout<<"Pointer to double overload\n";}voidf(std::nullptr_t){std::cout<<"null pointer overload\n";}intmain(){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)}Output:
Pointer to integer overload Pointer to double overload null pointer overload See also