Question

I have a bunch of collection-like COM interfaces that I’m trying to write an STL-like iterator for. I’ve got the iterator working and specialized begin() and end() to return my iterator. Everything works perfectly! Except, when I try to use begin(std::vector), it’s using my very general specialization of begin(). Since these COM objects don’t extend from a base collection object, I first tried:

template< class CollType >
CollectionIterator<CollType> begin( CollType coll )

I see why the overload resolution isn’t picking the right begin() for std::vector, but I’m not sure how to fix this.

Unfortunately, I don’t have a base collection class to specialize the coll parameter against. I’m assuming I need something like SFINAE to only resolve these specializations if the right members exist. I tried:

template<
    class CollType,
    typename std::enable_if< std::is_member_pointer< decltype(&CollType::GetItem) >::value >::type
>
CollectionIterator<CollType> begin( CollType coll )

(where GetItem is a method of CollType)

As well as a handful of variations, to no avail. To make matters worse, these collections are COM smart pointers, so I’m not sure if GetItem will actually register as a member of the smart pointer.

Any insights into the right direction would be great, I’ve been mostly running in circles with this.

Answer 1

If you always use some specific COM smart pointer for your tricky collections (e.g. _com_ptr_t<T>), you could define begin()specialization this way:

template<class T> your_iterator_type<T> begin(_com_ptr_t<T>& collection)

If this is not acceptable try another possible way (only if you use Visual C++ and don’t care to be non-standard) – using Microsoft specific extensions – __if_exists/__if_not_exists statements:

template<class T> typename T::iterator begin_ex(T& collection)
{
  __if_exists(T::GetItem) {
     return my_collection_begin(collection); // Use custom impl
  }

  __if_not_exists(T::GetItem) {
     return std::begin(collection); // Use standard impl
  }
}