Match iterable types (arrays and classes with begin()/end())

Question

I wrote type traits like classes that can be used test if a given type is \"iterable\". This is true for arrays (for T[N], not for T[]) and for cla

Answer 1

First, some boilerplate to do easy argument dependent lookup of begin in a context where std::begin is visible:

#include <utility>
#include <iterator>
namespace adl_details {
  using std::begin; using std::end;
  template<class R>
  decltype(begin(std::declval<R>())) adl_begin(R&&r){
    return begin(std::forward<R>(r));
  }
  template<class R>
  decltype(end(std::declval<R>())) adl_end(R&&r){
    return end(std::forward<R>(r));
  }
}
using adl_details::adl_begin;
using adl_details::adl_end;

This is required to reasonably emulate how range-based for(:) loops find their begin/end iterators. By packaging it up like this, we reduce boilerplate below.

Next, some C++1y style utility aliases:

template<class>struct sink {using type=void;};
template<class X>using sink_t=typename sink<X>::type;
template<bool b, class T=void>using enable_if_t=typename std::enable_if<b,T>::type;

sink_t takes any type, and throws it away replacing it with void.

enable_if_t removes annoying typename spam below.

In an industrial strength library, we'd put this in details, and have a 1-type-argument version that dispatches to it. But I don't care:

template<class I,class=void> struct is_iterator:std::false_type{};
template<> struct is_iterator<void*,void>:std::false_type{};
template<> struct is_iterator<void const*,void>:std::false_type{};
template<> struct is_iterator<void volatile*,void>:std::false_type{};
template<> struct is_iterator<void const volatile*,void>:std::false_type{};
template<class I>struct is_iterator<I,
  sink_t< typename std::iterator_traits<I>::value_type >
>:std::true_type{};

is_iterator doesn't do heavy auditing of the iterator_traits of I. But it is enough.

template<class R>
using begin_t=decltype(adl_begin(std::declval<R&>()));
template<class R>
using end_t=decltype(adl_end(std::declval<R&>()));

These two type aliases make the stuff below less annoying.

Again, in industrial strength libraries, put 2-arg-with-void into details:

template<class R,class=void> struct has_iterator:std::false_type{};
template<class R>
struct has_iterator<
  R,
  enable_if_t<
    is_iterator<begin_t<R>>::value
    && is_iterator<end_t<R>>::value
    // && std::is_same<begin_t<R>,end_t<R>>::value
  >
>:std::true_type{};

Note the commented out line in the enable_if_t above. I left that out to allow asymmetric iteration to work, where the end is a type that has a different operator== overload. Such is being considered for C++17: it allows really, really efficient algorithms on null-terminated strings (for example).

Finally, the final output:

template<class R>using iterator_t=enable_if_t<has_iterator<R>::type, begin_t<R>>;

which evaluates to the iterator of the iterable range R iff it has one.

There are cases where this won't work, but they are pathological.

live example