不重复用作模板参数的类型

Question

鉴于以下代码是否有更好的方法来纠正它，不会重复typename std::iterator_traits<T>::iterator_category两次？

template<class T, class T2>
struct foo :
    bar<
        foo<T, T2>, typename 
        std::conditional<
            std::is_same<typename 
                std::iterator_traits<T>::iterator_category,  //Repeated
                std::random_access_iterator_tag
            >::value,
            std::bidirectional_iterator_tag, typename 
            std::iterator_traits<T>::iterator_category       //Repeated
        >::type
    >
{}

Answer 1

将其拆分（无论如何应该是这样）：

// put in detail namespace/file or something in real code
template<class T, class T2>
struct foo_base
{
    typedef foo<T, T2> foo_type;
    typedef typename std::iterator_traits<T>::iterator_category category_type;

    static const bool random_access = std::is_same<category_type,
                                        std::random_access_iterator_tag>::value;
    typedef typename std::conditional<random_access,
                                        std::bidirectional_iterator_tag,
                                        category_type>::type tag_type;

    typedef bar<foo_type, tag_type>::type base_type;
}

template<class T, class T2>
struct foo :
   foo_base<T, T2>::base_type
{};

即使没有重复位，您仍应将其拆分以使基类型逻辑与实际继承基类型分开。

Answer 2

你可以typedef：

typedef std::iterator_traits<T>::iterator_category it_cat;