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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"完毕实现

演示样例：

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))

	...

}

解析：上面 iterator_traits<IterT>::iterator_category 在编译期就可确定。但 if 语句却是在执行期才会核定，浪费时间，也造成可执行文件的膨胀。

纠正：融合重载技术后。traits classes 有可能在编译期对类型运行 if...else 測试

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;

}