C++ 中结构体的继承关系

今天才发现，C++中结构体不仅能够定义方法，还能有继承关系。如下面代码所示：

    namespace std{
        struct output_iterator_tag{};
        struct input_iterator_tag{};
        struct forward_iterator_tag
                :public input_iterator_tag{
        };
        struct bidirectional_iterator_tag
                :public forward_iterator_tag{
        };
        struct random_access_iterator_tag
                :public bidirectional_iterator_tag{
        };
    }

不可思议！