Determine if a (c++) iterator is reverse

Question

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

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 */ };
}

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(...);

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;    

Any ideas?

EDIT

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();
}

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.

Answer 1

#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>
{ };

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

Answer 2

A simple method. It is easy to use and can determine if something is a reverse iterator or not, and can differentiate between const reverse iterators if needed.

#include <iostream>

template<typename Container, typename T>
using IsRegRevIter = std::is_same<T,typename Container::reverse_iterator>;

template<typename Container, typename T>
using IsConstRevIter = std::is_same<T,typename Container::const_reverse_iterator>;

template<typename Container, typename T>
struct IsRevIter
{
     const static bool value = IsRegRevIter<Container,T>::value
          || IsConstRevIter<Container,T>::value;
};

int main()
{
     using Container = std::list<int>;
     Container myList = {1,2,3,4,5,6};

     auto RI = myList.rbegin();
     auto I = myList.begin();

     std::cout << std::endl << std::boolalpha << IsRevIter<Container,decltype(RI)>::value; //true
     std::cout << std::endl << std::boolalpha << IsRegRevIter<Container,decltype(RI)>::value; //true
     std::cout << std::endl << std::boolalpha << IsConstRevIter<Container,decltype(RI)>::value; //false (it is not const).
     std::cout << std::endl << std::boolalpha << IsRevIter<Container,decltype(I)>::value; //false

return 0;

}

output:
false
true
false
false