range and algorithm-related stuff

[Andrei Alexandrescu]

Steven Schveighoffer wrote:
> "Andrei Alexandrescu" wrote
>> I'm working on the new range stuff and the range-based algorithm. In all 
>> likelihood, you all might be pleased with the results.
>>
>> I wanted to gauge opinions on a couple of issues. One is, should the 
>> empty() member function for ranges be const? On the face of it it should, 
>> but I don't want that to be a hindrance. I presume non-const empty might 
>> be necessary sometimes, e.g. figuring out if a stream is empty effectively 
>> means fetching an element off it.
>>
> 
> Ranges are structs.  It should not matter if you want to make some const and 
> some non-const.  Basically, it depends on the range implementation.  If you 
> can make it const, make it const, if not, don't make it const.  It shouldn't 
> break any APIs.

The problem is "higher-order" ranges - ranges that take other ranges as 
argument. For example, consider Retro, a range that iterates another 
range backwards.

struct Retro(Range)
{
    Range _input;
    ...
    bool empty() { return _input.empty; }
}

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.

> 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.


Andrei