gcc 5.4中 string_view和optional

#if defined(__GNUC__) && __GNUC__ < 7
# include <experimental/string_view>
# include <experimental/optional>
# define string_view experimental::string_view
# define optionalexperimental::optional
#else
# include <string_view>
# include <optional>
#endif

string_view在gcc 5.4时可以用to_string成员方法转成std::string
optional 在gcc 5.4时，可以用 opt == std::experimental::nullopt 来判断是否有值