iterator_traits

迭代器类型

针对迭代器型别的标记类：

struct input_iterator_tag {};
struct output_iterator_tag {};
struct forward_iterator_tag {};
struct bidirectional_iterator_tag {};
struct random_access_iterator_tag {};

SGI STL __type_traits

针对 __type_traits 的标记类：

struct __true_type{};
struct __false_type{};