output ranges: by ref or by value?

[Andrei Alexandrescu]

Rainer Deyke wrote:
> Andrei Alexandrescu wrote:
>> If the implementor of consume() forgets to call save(), the situation is
>> unpleasant albeit not catastrophic: for most struct ranges things will
>> continue to work, but for class ranges the function will fail to perform
>> to spec. I don't know how to improve on that.
> 
> Require that all ranges are structs.  If you want to implement a range
> as a class, use a wrapper struct that creates a new object in its
> postblit function.  The wrapper struct can be made generic and placed in
> the standard library.

That's a good idea, but it doesn't work with covariant return types. 
Those are needed for the container hierarchy that I'm working on.


Andrei