Method is_string

example/json.hpp:292–292

        }
        bool is_number() const noexcept { return kind() == value_kind::number; }
        bool is_string() const noexcept { return kind() == value_kind::string; }
        bool is_object() const noexcept { return kind() == value_kind::object; }
        bool is_array() const noexcept { return kind() == value_kind::array; }