NAME

std::common_comparison_category - std::common_comparison_category

Synopsis

Defined in header <compare>
template<class... Ts>

struct common_comparison_category { (since C++20)
using type = /*see below*/ ;

};

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...Tn-1 is defined as
follows:

* If any Ti is not a comparison category type (std::partial_ordering,
std::weak_ordering, std::strong_ordering), U is void.
* Otherwise, if at least one Ti is std::partial_ordering, U is
std::partial_ordering
* Otherwise, if at least one Ti is std::weak_ordering, U is std::weak_ordering
* Otherwise (if every Ti 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 (since C++20)
common_comparison_category<Ts...>::type;

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