utility.h

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#ifndef GUL14_UTILITY_H_
#define GUL14_UTILITY_H_
 
#include <functional>
 
namespace gul14 {
 
struct in_place_t
{
    explicit in_place_t() = default;
};
 
static constexpr in_place_t in_place{};
 
template <typename T>
struct in_place_type_t {
    explicit in_place_type_t() = default;
};
 
template <typename T>
static constexpr in_place_type_t<T> in_place_type{};
 
template <std::size_t I>
struct in_place_index_t { explicit in_place_index_t() = default; };
 
template <std::size_t I>
static constexpr in_place_index_t<I> in_place_index{};
 
struct monostate {};
inline constexpr bool operator<(monostate, monostate) noexcept { return false; }
inline constexpr bool operator>(monostate, monostate) noexcept { return false; }
inline constexpr bool operator<=(monostate, monostate) noexcept { return true; }
inline constexpr bool operator>=(monostate, monostate) noexcept { return true; }
inline constexpr bool operator==(monostate, monostate) noexcept { return true; }
inline constexpr bool operator!=(monostate, monostate) noexcept { return false; }
 
 
} // namespace gul14
 
namespace std {
 
template <>
struct hash<gul14::monostate>
{
    using argument_type = gul14::monostate;
    using result_type = std::size_t;
 
    inline result_type operator()(const argument_type&) const noexcept
    {
        return 66740831; // random value (as proposed in https://github.com/mpark/variant)
    }
};
 
} // namespace std
 
#endif