# std::common_comparison_category

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< cpp‎ | utility

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 `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

...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

| This section is incomplete Reason: no example

### See also

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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