one_of.hh

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#ifndef ONE_OF_HH
#define ONE_OF_HH
 
#include <tuple>
 
template<typename... Ts>
class one_of {
public:
        constexpr one_of(Ts... ts) : tup(ts...) {}
 
        template<typename T>
        friend constexpr bool operator==(const T& t, const one_of& o) {
                return std::apply([&](const Ts& ... ts) { return (... || (t == ts)); }, o.tup);
        }
 
        template<typename T>
        friend constexpr bool operator!=(const T& t, const one_of& o) {
                return !(t == o);
        }
 
private:
        std::tuple<Ts...> tup;
};
 
#endif