std::CommonReference

C++

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Concepts library (C++20)

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Technical Specifications

Concepts library

Core language concepts
Same DerivedFrom ConvertibleTo CommonReference Common	Integral SignedIntegral UnsignedIntegral Assignable SwappableSwappableWith	Destructible Constructible DefaultConstructible MoveConstructible CopyConstructible
Comparison concepts
Boolean	EqualityComparableEqualityComparableWith	StrictTotallyOrderedStrictTotallyOrderedWith
Object concepts
Movable Copyable	Semiregular Regular
Callable concepts
InvocableRegularInvocable	Predicate	Relation StrictWeakOrder

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

The concept CommonReference<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.

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

• std::common_reference_t<T, U>(t) is equal to std::common_reference_t<T, U>(t) if and only if t is an equality-preserving expression; and
• std::common_reference_t<T, U>(u) is equal to std::common_reference_t<T, U>(u) if and only if u is an equality-preserving expression.

In other words, the conversion to the common reference 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_referencebasic_common_reference (C++20)	determines the common reference type of a group of types (class template)
Common (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)