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
示例:
template<...>
class deque{
public:
class iterator{
public:
typedef random_access_iterator_tag iterator_category;
};
};
//template
template
struct iterator_traits{
typedef typename IterT::iterator_category iterator_category;
};
//template 偏特化
template
struct iterator_traits
// template 偏特化,限制 IterT 必须是指针 { typedef random_access_iterator_tag iterator_category; }; //实现 advance template
void advance(IterT &iter, DistT d){ if(typeid(typename std::iterator_traits
::iterator_category) == typeid(std::random_access_iterator_tag)) ... }
解析:上面 iterator_traits
::iterator_category 在编译期就可确定,但 if 语句却是在运行期才会核定,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执行 if...else 测试
template
void advance(IterT &iter, DistT d){
doAdvance(iter, d, typename std::iterator_traits
::iterator_category()); } template
void advance(IterT &iter, DistT d, std::random_access_iterator_tag){ iter += d; } template
void advance(IterT &iter, DistT d, std::bidirectional_iterator_tag){ if(d >= 0){while(d--) ++iter;} else{while(d++) --iter;} } template
void advance(IterT &iter, DistT d, std::input_iterator_tag){ if(d < 0) throw std::out_of_range(Negative distance); while(d--) ++iter; }