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2247. Type traits and std::nullptr_t

Section: 21.3.5.2 [meta.unary.cat] Status: C++14 Submitter: Joe Gottman Opened: 2013-03-15 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

According to 21.3.5.2 [meta.unary.cat], for every type T, exactly one of the primary type traits is true. So which is true for the type std::nullptr_t? By 5.13.7 [lex.nullptr] std::nullptr_t is not a pointer type or a pointer-to-member type, so is_pointer, is_member_object_pointer and is_member_function_pointer can't be true for std::nullptr_t, and none of the other primary type traits seem to apply.

[2013-04-20, Bristol]

Rename to is_null_pointer, move to Ready

Previous wording:

This wording is relative to N3485.

  1. Edit 21.3.3 [meta.type.synop], header <type_traits> synopsis: 

    namespace std {
  […]
  // 20.9.4.1, primary type categories:
  template <class T> struct is_void;
  template <class T> struct is_nullptr;
  template <class T> struct is_integral;
  template <class T> struct is_floating_point;
  […]
}

  2. Edit Table 47 — "Primary type category predicates" as indicated:

    Table 47 — Primary type category predicates
    Template Condition Comments
    …
    template <class T>
    struct is_nullptr;         		T is std::nullptr_t ([basic.fundamental])  
    …

[2013-09-29, Chicago]

Apply to the Working Paper

Proposed resolution:

This wording is relative to N3485.

  1. Edit 21.3.3 [meta.type.synop], header <type_traits> synopsis: 

    namespace std {
  […]
  // 20.9.4.1, primary type categories:
  template <class T> struct is_void;
  template <class T> struct is_null_pointer;
  template <class T> struct is_integral;
  template <class T> struct is_floating_point;
  […]
}

  2. Edit Table 47 — "Primary type category predicates" as indicated:

    Table 47 — Primary type category predicates
    Template Condition Comments
    …
    template <class T>
    struct is_null_pointer;         		T is std::nullptr_t ([basic.fundamental])  
    …