From cppreference.com
template<class...Ts>structcommon_comparison_category{usingtype=/* 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...Tn-1 is defined as follows:
If any Ti is not a comparison category type (
,
,
), U is void.
Otherwise, if at least one Ti is
, U is
.
Otherwise, if at least one Ti is
, U is
.
Otherwise (if every Ti is
, or if the list is empty), U is
.
Template parameters
...Ts - a possibly empty list of types Helper template
template<class...Ts>usingcommon_comparison_category_t=common_comparison_category<Ts...>::type;(since C++20)Member types
Member type Definition typethe strongest common comparison category (as defined above) Possible implementation
namespacedetail{template<unsignedint>structcommon_cmpcat_base{usingtype=void;};template<>structcommon_cmpcat_base<0u>{usingtype=std::strong_ordering;};template<>structcommon_cmpcat_base<2u>{usingtype=std::partial_ordering;};template<>structcommon_cmpcat_base<4u>{usingtype=std::weak_ordering;};template<>structcommon_cmpcat_base<6u>{usingtype=std::partial_ordering;};}// namespace detailtemplate<class...Ts>structcommon_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
(C++20)
the result type of 3-way comparison that supports all 6 operators and is substitutable
(class)
(C++20)
the result type of 3-way comparison that supports all 6 operators and is not substitutable
(class)
(C++20)
the result type of 3-way comparison that supports all 6 operators, is not substitutable, and allows incomparable values
(class)