# std::experimental::ranges::Common

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< cpp‎ | experimental‎ | ranges

Defined in header `<experimental/ranges/concepts>` |  |
template <class T, class U> concept bool Common = Same<ranges::common_type_t<T, U>, ranges::common_type_t<U, T>> && ConvertibleTo<T, ranges::common_type_t<T, U>> && ConvertibleTo<U, ranges::common_type_t<T, U>> && CommonReference< std::add_lvalue_reference_t<const T>, std::add_lvalue_reference_t<const U>> && CommonReference< std::add_lvalue_reference_t<ranges::common_type_t<T, U>>, ranges::common_reference_t< std::add_lvalue_reference_t<const T>, std::add_lvalue_reference_t<const U>>>; |  | (ranges TS)

The concept `Common<T, U>` specifies that two types `T` and `U` share a common type (as computed by ranges::common_type_t) to which both can be converted.

`Common<T, U>` is satisfied only if, given expressions `t` and `u` such that decltype((t)) is `T` and decltype((u)) is `U`,

- ranges::common_type_t<T, U>(t) is equal to ranges::common_type_t<T, U>(t) if and only if `t` is an equality-preserving expression; and
- ranges::common_type_t<T, U>(u) is equal to ranges::common_type_t<T, U>(u) if and only if `u` is an equality-preserving expression.

In other words, the conversion to the common type must not alter the equality-preservation property of the original expression.

### Equality preservation

An expression is equality preserving if it results in equal outputs given equal inputs.

- The inputs to an expression consist of its operands.
- The outputs of an expression consist of its result and all operands modified by the expression (if any).

Every expression required to be equality preserving is further required to be stable: two evaluations of such an expression with the same input objects must have equal outputs absent any explicit intervening modification of those input objects.

### See also

common_type | determine the common type of a set of types (class template)
common_reference | determine the common reference type of a set of types (class template)
CommonReference | specifies that two types share a common reference type (concept)

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