std::ring_span

This code came out of the SG14 working meeting at CppCon 2015. Guy Davidson proposed std::fixed_ring and std::dynamic_ring:

• P0059R0 "A proposal to add a ring adaptor to the standard library"

This code proposes just a single, lower-level primitive: std::ring_span, as described in the current revision of P0059:

• P0059R1 "A proposal to add a ring span to the standard library"

A ring_span doesn't manage its own memory nor its own objects; it is merely a non-owning (yet mutable) view onto an array of existing objects, providing the following facilities:

• push_back() assigns a new value to *end() and increments end(). If the ring-buffer is at capacity, this will cause begin() also to be incremented; that is, the oldest item in the ring-buffer will be forgotten. Regardless, push_back() invalidates iterators.

• pop_front() performs a customizable action and increments begin(). It also invalidates iterators.

• Iteration of the whole buffer (from front to back) is possible via the usual begin() and end() iterators, or via range-based for loop.

• The ring_span itself is a lightweight value type; you can copy it to get a second (equivalent) view of the same objects. As with array_view and string_view, any operation that invalidates a pointer in the range on which a view was created invalidates pointers and references returned from the view's functions.

For the array_view proposal, see N4177 "Multidimensional bounds, index and array_view"