共计 1138 个字符，预计需要花费 3 分钟才能阅读完成。
Take a simple advance as example, you might design:
template<class II, class D>
void advance(II& i, D n){
while(n–) ++i;
}
However it have O(n) complexity, which is not acceptable when you have a random_access_iterator. So you may change your design like this:
template<class II, class D>
void advance_II(II& i, D n){
while(n–) ++i;
}
template<class RAI, class D>
void advance_RAI(RAI& i, D n){
i += n;
}
template<class II, class D>
void advance(II& i, D n){
if(is_random_access_iterator(i)) // not yet designed
advance_RAI(i, n);
else
advance_II(i, n);
}
However the version of function to use is decided at run-time, so we try to let compiler decided which method to choose at compile-time. So we give iterators tags. There are five tags:
struct input_iterator_tag {};
struct output_iterator_tag {};
struct forward_iterator_tag : public input_iterator_tag {};
struct bidirection_iterator_tag : public forward_iterator_tag {};
struct random_access_iterator_tag : public bidirection_iterator_tag {};
Now you can do this:
template<class II, class D>
void __advance(II& i, D n, input_iterator_tag){
while(n–) ++i;
}
template<class RAI, class D>
void __advance(RAI& i, D n, random_access_iterator_tag){
i += n;
}
template<class II, class D>
void advance(II& i, D n){
__advance(i, n, iterator_traits<II>::iterator_category());
}
