"the last change" for ranges

[Andrei Alexandrescu]

Bill Baxter wrote:
> On Wed, May 20, 2009 at 9:19 AM, Andrei Alexandrescu
> <SeeWebsiteForEmail at erdani.org> wrote:
> 
>> I'm thinking a better design is to require any range that's forward or
>> better to define a function save(). Ranges that don't implement it are input
>> ranges; those that do, will guarantee a brand new range is returned from
>> save(). So then adjacentFind would look like this:
>>
>> R adjacentFind(R)(R r)
>> {
>>    if (r,empty) return r;
>>    R last = r.save;
>>    r.popFront;
>>    for (; !r.empty && last.front != r.front; last.popFront, r.popFront)
>>    {
>>    }
>>    return r;
>> }
>>
>> Obviously, when you pass a range that doesn't support save, adjacentFind
>> will not compile, which is what we want.
> 
> The only other alternative that comes to mind would be forcing input
> ranges to hide their copy constructor, or whatever the D equivalent
> is, making R last = r; fail.  But that would make input ranges very
> difficult to use.

Exactly. I thought of that design, and it was difficult to even pass a 
range to a function.

> So, of those two options at least, requiring a .save sounds like the
> better choice.
> 
> The down side is you will get no error if you write the code the first
> way, without a .save.   I see this as turning into tip #5 in
> "Effective D" -- "Know when to use .save"   It would be nice if that
> potential mistake could be eliminated somehow.  You could perhaps
> require input ranges to implement transfer semantics, and have them
> implement a .clone for cases when you really do want to make an
> aliasing copy.

Good point. I don't have a solution for that. Giving ranges move 
semantics would probably make for another Effective D tip (or perhaps 
more... move semantics are pretty brutal).

Another partial solution is to define a different interface for input 
ranges, one that combines front() and popFront(). Something like 
popNext. That way, people who use only the primitives empty() and 
popNext() know they are using a forward range and with hope they'll 
remember they can't really save copies of it and expect them to 
"remember" where they are in the input.


Andrei