为什么nullptr是核心语言的一部分，但是nullptr_t是STL的一部分？

Question

As far as I'm aware nullptr is a part of the core language.

Quoting C++11: (18.2/9)

nullptr_t is defined as follows:

namespace std { typedef decltype(nullptr) nullptr_t; }

and is defined in the header <cstddef> .

Answer 1

Because it can. A central aim in the C++ standardization process is to alter the core language as little as possible when adding to the language.

nullptr usurps the use of 0 to mean both a null pointer and, er, zero. Using 0 for both caused problems for obvious reasons, does f(0) call f(int) or f(int*) ? So a brand new literal was added to the core language: nullptr . Its type is simply decltype(nullptr) so nullptr_t was added as a short cut:

namespace std {
    using nullptr_t = decltype(nullptr);
}

Answer 2

The proposal that introduced nullptr , N2431 , indicates in section 1.1 that it was desirable to not force users to include a header in order to use nullptr .

It also remarks, "We do not expect to see much direct use of nullptr_t in real programs". Thus, it was considered preferable to add nullptr_t to the library rather than create a new keyword only to be used for this obscure purpose. In addition, if you don't want to include the header, you can always just write decltype(nullptr) yourself.

Answer 3

From cppreference.com:

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.

If two or more overloads accept different pointer types, an overload for std::nullptr_t is necessary to accept a null pointer argument .

You can then solve overloaded function call ambiguity with std::nullptr_t .

For example:

void Foo(int* ptr) {}
void Foo(double* ptr) {}
void Foo(std::nullptr_t ptr) {} // This overload is called if Foo(nullptr) is invoked

Read more about std::nullptr_t here: https://en.cppreference.com/w/cpp/types/nullptr_t