output ranges: by ref or by value?

[Michel Fortin]

On 2010-01-01 17:54:12 -0500, Andrei Alexandrescu 
<SeeWebsiteForEmail at erdani.org> said:

> Michel Fortin wrote:
>> What I'd do instead is somehow make input ranges non-copyable. They 
>> could be either passed by ref or moved, never copied. This way they 
>> would still behave exactly like array slices, only not copyable, and 
>> you get a compile-time error if you try to copy them which is 
>> infinitely better than a subtle change in behavior.
> 
> I tried that, it makes input ranges next to unusable.

I think I can see why. You can't have ref member and local variables 
like in C++, so it's pretty hard to use references.


> save() is an imperfect but workable solution.

save() is an workable but error-prone solution.

Perhaps we could mitigate this by making people more aware of the 
difference instead. Couldn't we rename "input range" for "input stream"?

Currently you have ranges that behave one way and ranges that behave 
the other way, which is confusing. Having a different name for both 
would emphasize there is a difference. With different names, you're 
guarantied to get the "what's the difference?" question from newbies.

And it's simple to explain: "You can often use ranges and streams 
interchangeably, but for that to work you must use save() when you need 
a copy of the current state. Also, not all streams support save(). It's 
good practice to always use save() so that algorithms work for both for 
ranges and streams."


-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/