std::common_reference_with

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>>;

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