条款47：请使用traits classes表现类型信息

STL迭代器可分为五类：

 

struct input_iterator_tag{};

struct output_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{};

iterator_traits的两个部分实现：

 

template<typename IterT>

struct iterator_traits;

示例代码：

template<...>class deque{public:class iterator{public:typedef random_access_iterator_tag iterator_category;...};...};//list迭代器可双向行进,所以应该这样:template<...>class list{public:class iterator{public:typedef bidirectional_iterator_tag iterator_category;...};...};//对于iterator_traits只要响应iterator class的嵌套式typedef即可:template <typename IterT>struct iterator_traits{typedef typename IterT::iterator_category iterator_category;...};//以iterator_traits为指针指定的迭代器类型如下:template<typename IterT>struct iterator_traits<IterT*>{typedef random_access_iterator_tag iterator_category;...}; 

STL算法里面的函数advance的实现,其作用就是将某个迭代器移动某个距离，示例代码：

//这份实现用于random access迭代器template<typename IterT,typename DistT>void doAdvance( IterT& iter, DistT d, std::random_access_iterator_tag){iter += d;}//这份实现用于bidirectional迭代器template<typename IterT,typename DistT>void doAdvance( IterT& iter, DistT d, std::bidirectional_iterator_tag){if( d >= 0 ){while( d-- ){++iter;}} else {while( d++ ){--iter;} }}//这份实现用于input迭代器template<typename IterT,typename DistT>void doAdvance( IterT& iter, DistT d, std::input_iterator_tag){if( d < 0 ){throw std::out_of_range( "Negative distance" );}while( d-- ){++iter;}}//重载机制调用适当的实现代码template<typename IterT,typename DistT>void advance( IterT& iter, DistT& d ){doAdvance( iter, d, typename std::iterator_traits<IterT>::iterator_category() );}