std::nullptr_t
Defined in header <cstddef> | ||
|---|---|---|
using nullptr_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 null pointer constants, and may be implicitly converted to any pointer and pointer to member type.
sizeof(std::nullptr_t) is equal to sizeof(void *).
Notes
The C++ standard requires <stddef.h> to place the contents of <cstddef> 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
If two or more overloads accept different pointer types, an overload for std::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 overload Pointer to double overload null pointer overload
See also
nullptr (C++11) | the pointer literal which specifies a null pointer value |
| implementation-defined null pointer constant (macro constant) |
|
|
(C++11)(DR*) | checks if a type is std::nullptr_t (class template) |
C documentation for nullptr_t |
|
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