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Defined in header <cstddef> | ||
using nullptr_t= decltype(nullptr); | (since C++11) | |
std::nullptr_t is the type of the null pointer literalnullptr. It is a distinct type that is not itself a pointer type or a pointer to member type. Prvalues of this type arenull pointer constants, and may beimplicitly converted to any pointer and pointer to member type.
sizeof(std::nullptr_t) is equal tosizeof(void*).
The C++ standard requires<stddef.h> to place the contents of<cstddef> in the global namespace, and thereby requiresnullptr_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 ofstd::nullptr_t is available in any other standard library header. An implementation may avoid introducing this name even when the standard requiresstd::nullptr_t to be used, by e.g. spellingdecltype(nullptr) instead.
If two or more overloads accept different pointer types, an overload forstd::nullptr_t is necessary to accept a null pointer argument.
#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)}
Output:
Pointer to integer overloadPointer to double overloadnull pointer overload
| nullptr(C++11) | the pointer literal which specifies a null pointer value[edit] |
| implementation-defined null pointer constant (macro constant)[edit] | |
(C++11)(DR*) | checks if a type isstd::nullptr_t (class template)[edit] |
C documentation fornullptr_t | |
