std::nullptr_t - cppreference.com (original) (raw)
| | | | | -------------------------------------- | | ------------- | | 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 *).
[edit] 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.
[edit] 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 #include 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