why does nullptr not require header while nullptr_t does

Question

nullptr_t is the type for nullptr. When I use nullptr_t, I have to use the header file <cstddef> but why isn't it required when using the nullptr keyword alone?

Answer 1

As per [lex.key]/1, nullptr is a keyword:

The identifiers shown in Table 5 are reserved for use as keywords (that is, they are unconditionally treated as keywords in phase 7) [...]

[tab:lex.key]: ... nullptr

Whereas std::nullptr_t is an alias declaration defined by the standard. Particularly, as of [cstddef.syn], is is defined as the type of the a nullptr expression:

Header <cstddef> synopsis

...

using nullptr_t = decltype(nullptr);

as specified in [support.types.nullptr]/1:

The type nullptr_­t is a synonym for the type of a nullptr expression, and it has the characteristics described in [basic.fundamental] and [conv.ptr].

Thus, to make use of std::nullptr_t, you need to include the <cstddef> header, but you can likewise use decltype(nullptr) if you simply want the same underlying type but without using the std::nullptr_t alias.

Answer 2

nullptr is a keyword, std::nullptr_t is a type defined by the C++ Stadnard library. Keywords are not introduced by header files, they are "wired" in the compiler itself.