如何检查传递的迭代器是一个随机访问迭代器？ [英] How to check that the passed Iterator is a random access iterator?

问题描述

我有以下代码，它执行一些迭代器算术：

  template< class Iterator> 
 void Foo（Iterator first，Iterator last）{
 typedef typename Iterator :: value_type Value; 
 std :: vector< Value> vec; 
 vec.resize（last  -  first）; 
 // ... 
} 
  

（例如向量和 deque c）的表达式（最后一个） / code>）。如果迭代器

解决方案

code>是随机存取迭代器，然后

  std :: iterator_traits< Iterator> :: iterator_category 
  

将会是 std :: random_access_iterator_tag 。实现这个的最简单的方法可能是创建第二个函数模板，并且 Foo 调用它：

  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（））; 
} 
  

这样做的好处是可以重载 FooImpl



Scott Meyers在有效的C ++ 之一中讨论了这种技术。书（我不记得哪一个）。

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);
  // ...
}

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?

解决方案

If Iterator is a random access iterator, then

std::iterator_traits<Iterator>::iterator_category

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());
}

This has the advantage that you can overload FooImpl for different categories of iterators if you'd like.

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

这篇关于如何检查传递的迭代器是一个随机访问迭代器？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！