std::common_with - cppreference.com (original) (raw)

| | | | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | ------------- | | 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.

[edit] 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.

[edit] Equality preservation

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

[edit] References

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

• 18.4.6 Concept common_with [concept.common]

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

• 18.4.6 Concept common_with [concept.common]

[edit] See also