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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]) |
   > `…`