range and algorithm-related stuff

[Andrei Alexandrescu]

Steven Schveighoffer wrote:
> "Andrei Alexandrescu" wrote
>> If Retro.empty is const and Range.empty is not, that won't compile. If 
>> Retro.empty is non-const and Range.empty is const, it will compile, but 
>> passing a const Retro won't work as well as passing a const Range.
> 
> Hm... you need the template code to take the place of the designer who looks 
> at code and decides that something should or should not be const.
> 
> For the case of Retro, you'd need some sort of const detection on Range's 
> empty function.

Exactly. A sort of "auto-qualify". But I guess at that point some people 
will throw their hands in the air, and those who'd thrown their hands 
already will now throw their feet too. :o)

> But in general, when is it necessary for Range.empty to be const (and 
> therefore Retro.empty to be const)?  If Range is working with a const 
> container, wouldn't you expect that Range itself would not be const, just 
> the reference to the container is const?  A range that doesn't mutate 
> doesn't seem like a very useful idiom.

I agree, but then I don't want to make many assumptions about future 
designs. In general I'm not sure we can always assume the type of the 
range and the environment make it easy to obtain mutable ranges for 
immutable elements. Probably many people would expect to just write this:

void foo(in SomeRange range) { ... }

and have foo look at the range and its elements no problem, without 
actually iterating through the range.

>>> For example, an array range might have empty be const, but a stream range 
>>> might not.  What matters is what functions you can use those ranges in, 
>>> but those are generally templated functions, so the compiler will tell 
>>> you whether it can be used or not when it tries to compile it.
>>>
>>> Personally, I see no benefit to having empty() be const.  What benefits 
>>> do you gain by specifically making empty const and the other functions 
>>> not const?  Presumably, the underlying container must be not const in 
>>> order for head, next, etc. to work properly, so there is no requirement 
>>> there.
>> If you have a constant range with random access, empty, length, and 
>> opIndex should be enough for you to look at anything you want without 
>> altering the range itself.
> 
> Why not just pass the container itself if the range isn't going to mutate? 
> The container probably already has opIndex, and length (empty AFAIK plays no 
> role here).  If it doesn't it seems like a bad design to me.
> 
> If you have a good example where this is useful, I'd like to hear it.

I need to think that through.


Andrei