Bartosz Milewski seems to like D more than C++ now :)

[QAston]

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.