type_identity.hpp

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#ifndef EAGINE_TYPE_IDENTITY_HPP
#define EAGINE_TYPE_IDENTITY_HPP
 
#include <type_traits>
 
namespace eagine {
 
template <typename T>
struct type_identity {
    using type = T;
 
    template <
      typename X,
      typename = std::enable_if_t<std::is_same_v<T, X> && !std::is_array_v<X>>>
    constexpr auto operator()(X v) const -> X {
        return v;
    }
};
 
template <typename T, typename F>
struct get_type {
    template <typename X>
    static auto _get(X*) -> typename X::type;
 
    template <typename X>
    static auto _get(...) -> F;
 
    using type = decltype(_get<T>(static_cast<T*>(nullptr)));
};
 
template <typename T, typename F = void>
using type_t = typename get_type<T, F>::type;
 
template <typename T>
static constexpr auto make_identity(const T&) noexcept -> type_identity<T> {
    return {};
}
 
} // namespace eagine
 
#endif // EAGINE_TYPE_IDENTITY_HPP