for(auto i : c) -- Is there a short way to do it in reverse direction?

Question

I have a custom container class and defined iterators, so I can do:

for (auto i : c)

but is there something to iterate in reverse direction?

Something like:

for_reverse (auto i : c)

Answer 1

1. You can use boost:

   #include <boost/range/adaptor/reversed.hpp>
   
   using namespace boost::adaptors;
   for (auto i : c | reversed)
       ...
   

   Or if you don't like operator overloading:

   #include <boost/range/adaptor/reversed.hpp>
   
   using namespace boost::adaptors;
   for (auto i : reverse(c))
       ...
   

2. You can define a similar helper function using std::reverse_iterator so you don't depend on boost:

   template<typename It>
   class Range
   {
       It b, e;
   public:
       Range(It b, It e) : b(b), e(e) {}
       It begin() const { return b; }
       It end() const { return e; }
   };
   
   template<typename ORange, typename OIt = decltype(std::begin(std::declval<ORange>())), typename It = std::reverse_iterator<OIt>>
   Range<It> reverse(ORange && originalRange) {
       return Range<It>(It(std::end(originalRange)), It(std::begin(originalRange)));
   }
   

   Then you can write:

   for (auto i : reverse(c))
       ...
   

   Example

Answer 2

A one-off pain

namespace details {
    template <class T> struct _reversed { T& t; _reversed(T& _t): t(_t) {} };
}

template <class T> details::_reversed<T> reversed(T& t) { return details::_reversed<T>(t); }
template <class T> details::_reversed<T const> reversed(T const& t) { return details::_reversed<T const>(t); }

namespace std {
    template <class T> auto begin(details::_reversed<T>& r) -> decltype(r.t.rbegin()) { return r.t.rbegin(); }
    template <class T> auto end(details::_reversed<T>& r) -> decltype(r.t.rend()) { return r.t.rend(); }

    template <class T> auto begin(details::_creversed<T> const& cr) -> decltype(cr.t.rbegin()) { return cr.t.rbegin(); }
    template <class T> auto end(details::_creversed<T> const& cr) -> decltype(cr.t.rend()) { return cr.t.rend(); }
}

and then you are able to do the below

for (auto x: reversed(c)) ...