std:: common_comparison_category

The class template std::common_comparison_category provides an alias (as the member typedef type ) for the strongest comparison category to which all of the template arguments Ts... can be converted.

In detail, the common comparison type of a list of n types T ₀ ... T _n-1 is defined as follows:

• If any T _i is not a comparison category type ( std::partial_ordering , std::weak_ordering , std::strong_ordering ), U is void .
• Otherwise, if at least one T _i is std::partial_ordering , U is std::partial_ordering .
• Otherwise, if at least one T _i is std::weak_ordering , U is std::weak_ordering .
• Otherwise (if every T _i is std::strong_ordering , or if the list is empty), U is std::strong_ordering .

Template parameters

Helper template

Member types

Possible implementation

namespace detail
{
    template<unsigned int>
    struct common_cmpcat_base     { using type = void; };
    template<>
    struct common_cmpcat_base<0u> { using type = std::strong_ordering; };
    template<>
    struct common_cmpcat_base<2u> { using type = std::partial_ordering; };
    template<>
    struct common_cmpcat_base<4u> { using type = std::weak_ordering; };
    template<>
    struct common_cmpcat_base<6u> { using type = std::partial_ordering; };
} // namespace detail
 
template<class...Ts>
struct common_comparison_category :
    detail::common_cmpcat_base<(0u | ... |
        (std::is_same_v<Ts, std::strong_ordering>  ? 0u :
         std::is_same_v<Ts, std::weak_ordering>    ? 4u :
         std::is_same_v<Ts, std::partial_ordering> ? 2u : 1u)
    )> {};

Example

See also