On Friday, 20 September 2013 at 08:20:48 UTC, Szymon Gatner wrote:
The struct was only an example, obviously you can use proper
class implementations with data hiding etc. You will also get
at least 1 virtual call because a "concept" is always an
abstract class.
Direct use of the value type has no indirections.
That being said, I've been using this technique in other places
in my code this far too, for example: my C++ ranges (yeah idea
stolen from D) are templates so can be composed for free but
there are abstractions like InputRange<> which "erase" (this
word is stupid btw, no type is erased really) underlying type
and provide convenient abstraction in places. I also use this
technique for my std containers. Vector is a std::vector with
polymorphic allocator, again it holds Allocator<> by value but
underlying implementation is (often composition) of allocator
templates. In allocator's case I do explicitly want container
alloc to be polymorphic type so that my interfaces never depend
on allocator type.
Difference now for me is that I did sometimes turn value types
into polymorphic types using this technique (like with
allocators) but it never occurred to me that other wan can be
beneficial too.
Well Adapter is suppose to *change* interface to adapt to
client needs so this is a bit of a strech to call it that here,
but yeah, this technique called ("external polymorphism" idiom)
can be used to adapt . This really just is a (smart)pointer
with a full interface of underlying object (which also adds
benefit or using "." instead of "->" on target).
Well, I won't argue about naming, for me when a type is a wrapper
which provides desired interface to that type is an adapter :P.
I agree that this idiom is useful and I use it as needed too.
