Difference between input range and forward range

Steven Schveighoffer via Digitalmars-d digitalmars-d at puremagic.com
Thu Nov 12 07:29:19 PST 2015

On 11/11/15 4:20 AM, Dominikus Dittes Scherkl wrote:
> On Tuesday, 10 November 2015 at 18:57:31 UTC, Steven Schveighoffer wrote:
>> IMO, that shouldn't be a forward range. But in any case, the correct
>> mechanism is:
>> forward range -> a = b works and makes a copy of the iteration.
>> non-forward range -> a = b fails, you'd have to use a = b.getRef or
>> something like that -or- a = b is a moving operation (a is no longer
>> usable)
> Hmm. You mean "b is no longer usable", right?

Heh, right :)

> So, any algorithm requiring a copy should better do something like
> static if(isForwardRange!range)
>     copy = range;
> else
>     copy = range.save();

No, it should do this:

static if(!isForwardRange!range)
    assert(0, "I need a forward range")

save doesn't enter the picture, it would be eliminated. But again, this 
isn't going to happen. Note my specification above is for a fictional 
proposal of what I would have done.

The problem with the current save regime is that simple assignment for 
forward ranges works too, but it doesn't always do what you want (and 
diabolically, 99% of the time it DOES do what you want).


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