确定（c ++）迭代器是否相反

问题描述

我想要一个机制来确定在编译时是否迭代器是否反转。

I want a mechanism to determine in compile time whether an iterator is reverse or not.

只能帮助迭代器类型的类别，而我需要的是以下行：

Iterator traits can only help with the category of an iterator type and what I need is something in the lines of:

template<typename IterType>
struct IsReverseIterator
{
    enum { Yes = /* Implementation of the mechanism */ };
}

我有一个解决方案， >虽然，容器类型也必须提供：

I have a solution, that has a small drawback though, the container type has to be provided as well :

typedef char     TrueT;
typedef struct { TrueT _[2]; } FalseT;

template<typename Cont> TrueT  IsReverseIterator(typename Cont::const_reverse_iterator);    
template<typename Cont> FalseT IsReverseIterator(...);

它显然使用SFINAE，可以像这样使用：

It uses SFINAE obviously and can be utilized like so :

std::vector<int> v;

std::cout << (sizeof(IsReverseIterator<std::vector<int>>(v.begin())) ==  sizeof(TrueT)) << std::endl;
std::cout << (sizeof(IsReverseIterator<std::vector<int>>(v.rbegin())) ==  sizeof(TrueT)) << std::endl;    

任何想法？

EDIT

为了解释我要搜索的内容，以下面的代码为例

To explain what I'm searching for, take for example the following code

template<typename Cont, typename It>
bool points_to_last_element(Cont const &container, It iter)
{
    return iter == container.rend(); 
    // here I would like to dispatch to 2 implementations
    // one for reverse iterators and one for forward where this would happen
    // return iter == container.end();
}

这是一个假的例子，请不要抓住这个事实我已经处理它或者我可以有两个重载，一个采取 Cont :: reverse_iterator 和一个采取 Cont :: iterator 。我不/不能改变这种设计，我只是试图一个更优雅的方式（如果有任何）在内部处理它。我再次重申，这是一个假的例子。

It is a dummy example, please don't get caught on the fact that the code I have already handles it or that I could have two overloads, one taking Cont::reverse_iterator and one taking Cont::iterator. I don't/can't change that design, I'm just trying a more elegant way (if there's any) to handle it internally. Again I repeat that was a dummy example.

推荐答案

#include <iterator>
#include <type_traits>

template<typename Iter>
struct is_reverse_iterator : std::false_type { };

template<typename Iter>
struct is_reverse_iterator<std::reverse_iterator<Iter>>
: std::integral_constant<bool, !is_reverse_iterator<Iter>::value>
{ };

您还可以为任何用户定义的反向迭代器迭代器专门化trait。

You could also specialize the trait for any user-defined iterators that are reverse iterators.

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