What C++0x Headers are supposed to define nullptr?

Question

Now that C++0x is almost here, I've been experimenting with it, and in particular using nullptr. I haven't been able to figure out what standard header files one is supposed to include if one needs to use it.

Any help is appreciated.

Answer 1

No headers should be required. It is a built-in keyword (§[lex.nullptr]).

2.14.7 Pointer literals                 [lex.nullptr]

pointer-literal:
        nullptr

The pointer literal is the keyword nullptr. It is a prvalue of type std::nullptr_t. [ Note: std::nullptr_t is a distinct type that is neither a pointer type nor a pointer to member type; rather, a prvalue of this type is a null pointer constant and can be converted to a null pointer value or null member pointer value. See 4.10 and 4.11. —endnote]

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Its type, std::nullptr_t, however, is "defined" in the header <cstddef> (§[support.types]/9).

nullptr_t is defined as follows:

namespace std {
    typedef decltype(nullptr) nullptr_t;
}

The type for which nullptr_t is a synonym has the characteristics described in 3.9.1 and 4.10. [Note: Although nullptr’s address cannot be taken, the address of another nullptr_t object that is an lvalue can be taken. —endnote]