range and algorithm-related stuff

[Christopher Wright]

Andrei Alexandrescu wrote:
> Sergey Gromov wrote:
>> Sat, 24 Jan 2009 17:09:07 -0800, 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.
>>
>> I have a hard time imagining a use for a const range.
> 
> Read-only arrays for example.

A range is essentially an iterator. It has to change its internal state 
to move to the next element. So a const range will not allow you to 
iterate over the members of a const array. It will allow you to iterate 
over a single element, either once or an infinite number of times.

You could change ranges to have "Range next()" rather than "void 
next()", and that would allow you to use a const Range for reasons other 
than checking whether it's empty. Iterating over a range would then look 
like:
for (auto range = getRange(); !range.empty; range = range.next) {}

I don't see much purpose to this. If you want polymorphic ranges, you're 
going to use a class for ranges. This will incur a lot of allocations, 
which will be dog slow. The current design would only allocate once if 
your range is a class.