# std::experimental::ranges::common_reference

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Defined in header `<experimental/ranges/type_traits>` |  |
template< class... T > struct common_reference; |  | (ranges TS)

Determines the common reference type of the types `T...`, that is, the type to which all the types in `T...` can be converted or bound. If such a type exists (as determined according to the rules below), the member `type` names that type. Otherwise, there is no member `type`. The behavior is undefined if any of the types in `T...` is an incomplete type other than (possibly cv-qualified) void.

When given reference types, `common_reference` attempts to find a reference type to which the supplied reference types can all be bound, but may return a non-reference type if it cannot find such a reference type.

- If sizeof...(T) is zero, there is no member `type`.
- If sizeof...(T) is one (i.e., `T...` contains only one type `T0`), the member `type` names the same type as T0.
- If sizeof...(T) is two (i.e., `T...` contains two types `T1` and `T2`):
  - If `T1` and `T2` are both reference types, and the simple common reference type `S` of `T1` and `T2` (as defined below) exists, then the member type `type` names `S`;
  - Otherwise, if basic_common_reference<T1R, T2R, T1Q, T2Q>::type exists, where `TiR` is std::remove_cv_t<std::remove_reference_t<Ti>> and `TiQ` is an alias template such that TiQ<TiR> is Ti, then the member type `type` names that type;
  - Otherwise, if decltype(false? val<T1>() : val<T2>()), where `val` is a function template template<class T> T val();, denotes a valid type, then the member type `type` names that type;
  - Otherwise, if ranges::common_type_t<T1, T2> is a valid type, then the member type `type` names that type;
  - Otherwise, there is no member type.
- If sizeof...(T) is greater than two (i.e., `T...` consists of the types `T1, T2, R...`), then if ranges::common_reference_t<T1, T2> exists, the member `type` denotes ranges::common_reference_t<ranges::common_reference_t<T1, T2>, R...> if such a type exists. In all other cases, there is no member `type`.

The simple common reference type of two reference types `T1` and `T2` is defined as follows:

- If `T1` is `cv1 X &` and `T2` is `cv2 Y &` (i.e., both are lvalue reference types): their simple common reference type is decltype(false? std::declval<cv12 X &>() : std::declval<cv12 Y &>()), where cv12 is the union of cv1 and cv2, if that type exists and is a reference type;
- If `T1` and `T2` are both rvalue reference types: if the simple common reference type of `T1 &` and `T2 &` (determined according to the previous bullet) exists, then let `C` denote that type's corresponding rvalue reference type. If std::is_convertible<T1, C>::value and std::is_convertible<T2, C>::value are both `true`, then the simple common reference type of `T1` and `T2` is `C`.
- Otherwise, one of the two types must be an lvalue reference type `A &` and the other must be an rvalue reference type `B &&` (`A` and `B` might be cv-qualified). Let `D` denote the simple common reference type of A & and B const &, if any. If D exists and std::is_convertible<B &&, D>::value is `true`, then the simple common reference type is `D`.
- Otherwise, there's no simple common reference type.

### Member types

Name | Definition
`type` | the common reference type for all `T...`

### Helper types

template< class... T > using common_reference_t = typename common_reference<T...>::type; |  |
template< class T, class U, template<class> class TQual, template<class> class UQual > struct basic_common_reference { }; |  |

The class template `basic_common_reference` is a customization point that allows users to influence the result of `common_reference` for user-defined types (typically proxy references). The primary template is empty.

### Specializations

A program may specialize `basic_common_reference<T, U, TQual, UQual>` on the first two parameters `T` and `U` if std::is_same<T, std::decay_t<T>> and std::is_same<U, std::decay_t<U>> are both true and at least one of them depends on a program-defined type.

If such a specialization has a member named `type`, it must be a public and unambiguous member type that names a type to which both TQual<T> and UQual<U> are convertible. Additionally, ranges::basic_common_reference<T, U, TQual, UQual>::type and ranges::basic_common_reference<U, T, UQual, TQual>::type must denote the same type.

A program may not specialize `basic_common_reference` on the third or fourth parameters, nor may it specialize `common_reference` itself. A program that adds specializations in violation of these rules has undefined behavior.

### Notes

| This section is incomplete

### Examples

| This section is incomplete Reason: no example

### See also

common_type (C++11) | determines the common type of a group of types (class template)
common_type | determine the common type of a set of types (class template)
CommonReference | specifies that two types share a common reference type (concept)

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