std::common_comparison_category

Defined in header `<compare>`
`template< class... Ts > struct common_comparison_category { using type = /* see below */ ; };`		(since C++20)

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 T₀...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

...Ts

a possibly empty list of types

Helper template

template< class... Ts >
using common_comparison_category_t = typename common_comparison_category<Ts...>::type;

(since C++20)

Member types

Member type	Definition
`type`	the strongest common comparison category (as defined above)

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

strong_ordering (C++20)	the result type of 3-way comparison that supports all 6 operators and is substitutable (class)
weak_ordering (C++20)	the result type of 3-way comparison that supports all 6 operators and is not substitutable (class)
partial_ordering (C++20)	the result type of 3-way comparison that supports all 6 operators, is not substitutable, and allows incomparable values (class)

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https://en.cppreference.com/w/cpp/utility/compare/common_comparison_category

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