如何定义is_iterator类型trait？ [英] How to define is_iterator type trait?

问题描述

• 我尝试编写 is_iterator< T> 类型特征。其中 T 是迭代器类型 is_iterator< T> :: value == true ，否则为 is_iterator< T> :: value == false 。




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  template< class，class Enable = void> 
 struct is_iterator：std :: false_type {}; 
 
 template< typename T> 
 struct is_iterator< T，typename std :: enable_if< std :: is_pointer< typename 
 std :: iterator_traits< T> :: pointer> :: value> :: type> ：std :: true_type {}; 
  

Q：是否有更适当的方法来定义 is_iterator 类型特征？

解决方案

正如我在评论中所说，这里提出的解决方案依赖于在一些实现中 iterators_traits 的非便携性属性。根据C ++ 03和C ++ 11标准， iterator_traits 只定义为迭代器（和指针的特殊情况），所以任何其他用途是未定义的行为。特别地，在SFINAE上下文中使用 iterator_traits< T> :: pointer 不能保证工作，因为实例化 iterator_traits< T> 将引用 T :: value_type ， T :: pointer ， T： ：iterator_category 等，并且发生在不适用SFINAE的立即上下文之外。

C ++ 14 <修复了应该修复它（它发生在C ++ 14之后的 DR 2408 ）但对于C ++ 11，定义 is_iterator 的安全方法是写一个trait，该trait检查迭代器必须定义的所有必需的操作。所有迭代器需要支持的唯一操作是 operator * 以及前和后递增。不幸的是，可能有类型定义那些不是有效迭代器的操作，所以编写正确的trait是很难的。

• I'm trying to code a is_iterator<T> type trait. Where when T is an iterator type is_iterator<T>::value == true otherwise is is_iterator<T>::value == false.

• What I tried so far:

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template <class, class Enable = void> 
struct is_iterator : std::false_type {};

template <typename T> 
struct is_iterator<T, typename std::enable_if<std::is_pointer<typename
     std::iterator_traits<T>::pointer>::value>::type> : std::true_type {};

LIVE DEMO

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Q: Is there a more proper way to define a is_iterator type trait than the one displayed above?

解决方案

As I said in comments, the solutions presented here rely on non-portable properties of iterators_traits in some implementations. According to the C++03 and C++11 standards, iterator_traits is only defined for iterators (and the special case of pointers) so any other use is undefined behaviour. Specifically, using iterator_traits<T>::pointer in a SFINAE context isn't guaranteed to work, because instantiating iterator_traits<T> will refer to T::value_type, T::pointer, T::iterator_category etc. and that happens outside the "immediate context" where SFINAE doesn't apply.

C++14 will fix that was supposed to fix that (it happened post-C++14 with DR 2408), but for C++11 the safe way to define is_iterator is to write a trait that checks for all the required operations an iterator must define. The only operations that all iterators are required to support are operator* and pre- and post-increment. Unfortunately, there can be types that define those operations which are not valid iterators, so writing a correct trait is quite hard.

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