如何检查传递的 Iterator 是否为随机访问迭代器?

问题描述

我有以下代码，它执行一些迭代器算法:

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，则

If Iterator is a random access iterator, then

std::iterator_traits<Iterator>::iterator_category

将是 std::random_access_iterator_tag.最简洁的实现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 在其中一本Effective C++ 书中讨论了这种技术(我不记得是哪一本).

Scott Meyers discusses this technique in one of the Effective C++ books (I don't remember which one).

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