std::common_with

 

C++

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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

Exposition-only concepts boolean-testable

 

Defined in header <concepts>
template <class T, class U> concept common_with =   std::same_as<std::common_type_t<T, U>, std::common_type_t<U, T>> &&   requires {     static_cast<std::common_type_t<T, U>>(std::declval<T>());     static_cast<std::common_type_t<T, U>>(std::declval<U>());   } &&   std::common_reference_with<     std::add_lvalue_reference_t<const T>,     std::add_lvalue_reference_t<const U>> &&   std::common_reference_with<     std::add_lvalue_reference_t<std::common_type_t<T, U>>,     std::common_reference_t<       std::add_lvalue_reference_t<const T>,       std::add_lvalue_reference_t<const U>>>;		(since C++20)

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

Semantic requirements

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

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

Equality preservation

Expressions declared in requires-expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).

See also

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