std::common_reference_with

 

C++

Compiler support

Freestanding and hosted

Language

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Concepts library

Core language concepts same_as derived_from convertible_to common_reference_with common_with integral signed_integral unsigned_integral floating_point swappableswappable_with destructible constructible_from default_initializable move_constructible copy_constructible assignable_from

Comparison concepts equality_comparableequality_comparable_with totally_orderedtotally_ordered_with Object concepts movable copyable semiregular regular Callable concepts invocableregular_invocable predicate relation equivalence_relation strict_weak_order

 

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 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.

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).

In specification of standard concepts, operands are defined as the largest subexpressions that include only:

• an id-expression, and
• invocations of std::move, std::forward, and std::declval.

The cv-qualification and value category of each operand is determined by assuming that each template type parameter denotes a cv-unqualified complete non-array object type.

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_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)