Effective C++ Item 47 请使用 traits classes 表现类型信息

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经验：Traits classes 使得"类型相关信息"在编译期可用。它们以 templates 和 "templates 特化"完成实现

template<...>
class deque{
public:
	class iterator{
	public:
		typedef random_access_iterator_tag iterator_category;
	};
};
//template
template<typename IterT>
struct iterator_traits{
	typedef typename IterT::iterator_category iterator_category;
};
//template 偏特化
template<typename IterT>
struct iterator_traits<IterT *> // template 偏特化，限制 IterT 必须是指针
{
	typedef random_access_iterator_tag iterator_category;
};


//实现 advance
template<typename IterT, typename DistT>
void advance(IterT &iter, DistT d){
	if(typeid(typename std::iterator_traits<IterT>::iterator_category) == typeid(std::random_access_iterator_tag))
	...
}

template<typename IterT, typename DistT>
void advance(IterT &iter, DistT d){
	doAdvance(iter, d, typename std::iterator_traits<T>::iterator_category());
}


template<typename IterT, typename DistT>
void advance(IterT &iter, DistT d, std::random_access_iterator_tag){
	iter += d;
}


template<typename IterT, typename DistT>
void advance(IterT &iter, DistT d, std::bidirectional_iterator_tag){
	if(d >= 0){while(d--) ++iter;}
	else{while(d++) --iter;}
}


template<typename IterT, typename DistT>
void advance(IterT &iter, DistT d, std::input_iterator_tag){
	if(d < 0) throw std::out_of_range("Negative distance");
	while(d--) ++iter;
}

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Effective C++ Item 47 请使用 traits classes 表现类型信息

原文：http://blog.csdn.net/zhengsenlie/article/details/37807459