问题描述
我有以下代码，它执行一些迭代算法:
I have the following code, which does some iterator arithmetic:
template<class Iterator>
void Foo(Iterator first, Iterator last) {
typedef typename Iterator::value_type Value;
std::vector<Value> vec;
vec.resize(last - first);
// ...
}
(last - first) 表达式 (AFAIK) 仅适用于随机访问迭代器(例如来自 vector 和 deque 的迭代器).如何检查传递的迭代器满足此要求的代码?
The (last - first) expression works (AFAIK) only for random access iterators (like the ones from vector and deque). How can I check in the code that the passed iterator meets this requirement?
推荐答案
如果Iterator是随机访问迭代器，那么
If Iterator is a random access iterator, then
std::iterator_traits<Iterator>::iterator_category
将是 std::random_access_iterator_tag.实现这一点的最简洁方法可能是创建第二个函数模板并让 Foo 调用它:
will be std::random_access_iterator_tag. The cleanest way to implement this is probably to create a second function template and have Foo call it:
template <typename Iterator>
void FooImpl(Iterator first, Iterator last, std::random_access_iterator_tag) {
// ...
}
template <typename Iterator>
void Foo(Iterator first, Iterator last) {
typedef typename std::iterator_traits<Iterator>::iterator_category category;
return FooImpl(first, last, category());
}
这样做的好处是您可以根据需要为不同类别的迭代器重载 FooImpl.
This has the advantage that you can overload FooImpl for different categories of iterators if you'd like.
Scott Meyers 在一本Effective C++ 书中讨论了这种技术(我不记得是哪一本了).
Scott Meyers discusses this technique in one of the Effective C++ books (I don't remember which one).
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