std::common_reference_with (since C++20)

 

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Concepts library

Core language concepts same_as (C++20) derived_from (C++20) convertible_to (C++20) common_reference_with (C++20) common_with (C++20) integral (C++20) signed_integral (C++20) unsigned_integral (C++20) floating_point (C++20) swappableswappable_with (C++20)(C++20) destructible (C++20) constructible_from (C++20) default_initializable (C++20)   move_constructible (C++20) copy_constructible (C++20) assignable_from (C++20)

Comparison concepts equality_comparableequality_comparable_with (C++20)(C++20) totally_orderedtotally_ordered_with (C++20)(C++20) Object concepts movable (C++20) copyable (C++20) semiregular (C++20) regular (C++20) Callable concepts invocableregular_invocable (C++20)(C++20) predicate (C++20) relation (C++20) equivalence_relation (C++20) strict_weak_order (C++20)

Exposition-only concepts boolean-testable  (C++20)

 

Defined in header <concepts>
template< class T, class U > concept common_reference_with =     std::same_as<std::common_reference_t<T, U>, std::common_reference_t<U, T>> &&     std::convertible_to<T, std::common_reference_t<T, U>> &&     std::convertible_to<U, std::common_reference_t<T, U>>;		(since C++20)

The concept common_reference_with<T, U> specifies that two types T and U share a common reference type (as computed by std::common_reference_t) to which both can be converted.

Semantic requirements

T and U model std::common_reference_with<T, U> only if, given equality-preserving expressions t1, t2, u1 and u2 such that decltype((t1)) and decltype((t2)) are both T and decltype((u1)) and decltype((u2)) are both U,

• std::common_reference_t<T, U>(t1) equals std::common_reference_t<T, U>(t2) if and only if t1 equals t2; and
• std::common_reference_t<T, U>(u1) equals std::common_reference_t<T, U>(u2) if and only if u1 equals u2.

In other words, the conversion to the common reference type must preserve equality.

References

• C++23 standard (ISO/IEC 14882:2024):

• 18.4.5 Concept common_reference_with [concept.commonref]

• C++20 standard (ISO/IEC 14882:2020):

• 18.4.5 Concept common_reference_with [concept.commonref]

See also

common_referencebasic_common_reference (C++20)	determines the common reference type of a group of types (class template)
common_with (C++20)	specifies that two types share a common type (concept)
common_type (C++11)	determines the common type of a group of types (class template)