Copies const, volatile or reference qualifiers from one type to another

copy_traits.hpp

#ifndef UT_COPY_TRAITS
#define UT_COPY_TRAITS

/**
 * Usage:
 *  std >= c++20
 *
 *  using A = int;
 *  using B = float;
 *  using C = copy_cvref_t<A, B>; // decltype(C) == float
 *
 *  using A = int const &;
 *  using B = float;
 *  using C = copy_cvref_t<A, B>; // decltype(C) == float const &
 *
 *  using A = int;
 *  using B = float const &;
 *  using C = copy_cvref_t<A, B>; // decltype(C) == float
 */


#include <type_traits>

namespace ut {

namespace detail {
    template<typename T, template<typename> typename what>
    struct ApplyUnderReference {
        using type = what<T>::type;
    };

    template<typename T, template<typename> typename what>
    requires(std::is_lvalue_reference_v<T>)  //
        struct ApplyUnderReference<T, what> {
        using type = std::add_lvalue_reference_t<typename what<std::remove_reference_t<T>>::type>;
    };

    template<typename T, template<typename> typename what>
    requires(std::is_rvalue_reference_v<T>)  //
        struct ApplyUnderReference<T, what> {
        using type = std::add_rvalue_reference_t<typename what<std::remove_reference_t<T>>::type>;
    };

    template<typename T, template<typename> typename what>
    struct QueryUnderReference {
        static constexpr bool value = what<T>::value;
    };

    template<typename T, template<typename> typename what>
    requires(std::is_lvalue_reference_v<T> || std::is_rvalue_reference_v<T>)  //
        struct QueryUnderReference<T, what> {
        static constexpr bool value = what<std::remove_reference_t<T>>::value;
    };

}  // namespace detail

template<typename From, typename To>
using copy_reference_t = std::conditional_t<std::is_lvalue_reference_v<From>,
    std::add_lvalue_reference_t<To>,
    std::conditional_t<std::is_rvalue_reference_v<From>,
        std::add_rvalue_reference_t<To>,
        std::conditional_t<std::is_reference_v<From>,  //
            void,                                      //
            std::remove_reference_t<To>>>>;

template<typename From, typename To>
using copy_const_t = std::conditional_t<detail::QueryUnderReference<From, std::is_const>::value,  //
    typename detail::ApplyUnderReference<To, std::add_const>::type,
    typename detail::ApplyUnderReference<To, std::remove_const>::type>;

template<typename From, typename To>
using copy_volatile_t = std::conditional_t<detail::QueryUnderReference<From, std::is_volatile>::value,  //
    typename detail::ApplyUnderReference<To, std::add_volatile>::type,
    typename detail::ApplyUnderReference<To, std::remove_volatile>::type>;

template<typename From, typename To>
using copy_cvref_t = copy_reference_t<From, copy_volatile_t<From, copy_const_t<From, To>>>;
}  // namespace ut

/*
This is free and unencumbered software released into the public domain.
Anyone is free to copy, modify, publish, use, compile, sell, or
distribute this software, either in source code form or as a compiled
binary, for any purpose, commercial or non-commercial, and by any
means.
In jurisdictions that recognize copyright laws, the author or authors
of this software dedicate any and all copyright interest in the
software to the public domain. We make this dedication for the benefit
of the public at large and to the detriment of our heirs and
successors. We intend this dedication to be an overt act of
relinquishment in perpetuity of all present and future rights to this
software under copyright law.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
OTHER DEALINGS IN THE SOFTWARE.
For more information, please refer to <https://unlicense.org>
*/

#endif

zzz_description

copy cvref qualifiers from one type to another