http://wiki.dlang.org/DIP25

[Andrei Alexandrescu via Digitalmars-d]

On 12/28/14 12:04 PM, Peter Alexander wrote:
> On Sunday, 28 December 2014 at 18:16:04 UTC, Andrei Alexandrescu wrote:
>> Very little breakage I can think of. Ranges usually don't own their
>> payload.
>
> I'm thinking more about higher order ranges, e.g. take, filter, cycle,
> retro; over a mutable range with ref front. Even if the underlying range
> (e.g. an array) has the inout, the higher order range will need the
> inout as well, so that it is propagated, no?

Not even those for the most part. The archetypal higher order range:

struct Wrapper(R)
{
     private R payload;
     ref ElementType!R front() {
         return payload.front;
     }
     ...
}

If R.front() returns a ref T, that's all Wrapper needs to know - the 
call just works. That would be the case if e.g. R is T[].

If R.front() is inout-qualified, indeed Wrapper needs to propagate that. 
In this case, however, deduction can take care of it; Walter ensures me 
that deduction of inout is cheap and relatively easy. I need to add a 
section on it to the DIP.

For non-templates, inout would need to be added if a range that owns its 
elements is to be wrapped. But generally ranges don't own stuff (i.e. 
they can disappear and stuff is still there). Containers own stuff.


Andrei