Re: [boost] AlRangeExandrescu?

Subject: Re: [boost] AlRangeExandrescu?
From: Andrei Alexandrescu (andrei_at_[hidden])
Date: 2009-07-28 12:54:20

Thorsten Ottosen wrote:
> Andrei Alexandrescu skrev:
>> Thorsten Ottosen wrote:
>>> Stewart, Robert skrev:
>>>> Steven Watanabe wrote:
>>>
>>>>>> How about another try: traversing a range with changing criteria?
>>>>>> A particular range class can take a predicate or some other
>>>>>> traversal influencing argument that can change with each
>>>>>> advance.  How would you express that with iterator pairs?
>>>>> Easily.  First of all, consider what's possible if the two iterators
>>>>> are allowed to be different types.  Then all the actual state can
>>>>> be put in the first iterator and comparison to the second iterator
>>>>> can just check whether the range is empty.  Once you have that,
>>>>> you can use boost::variant to store both iterators as the same type.
>>>>> This will not be efficient, but it will work.
>>>>
>>>> That may well work, but the range version would certainly be more 
>>>> straightforward and, 
>>>  >as you noted, efficient.  Of course, once you permit the iterators 
>>> to have different types,
>>>  >you also increase the opportunity for mixing them incorrectly, 
>>> which won't happen with an Alexandrescu range.
>>>
>>> I think we all agree that Alexandrescu ranges are conceptually 
>>> simpler to implement and specify; however, from a user perspective 
>>> the two approaches should be be similar.
>>
>> Well ranges seem to make functional composition easier because one 
>> object represents what you need, not two.
> 
> By the other approach I meant what Boost.Range and RangeEx is offering. 
> Not the iterator approach.

Oh, I see. Sorry. I think with Boost::Range you can indeed do what you 
can do with a primitive range. The downsides that I see are (a) you need 
to mind the occasional wart, e.g. defining dummy iterators; (b) given 
that users can always tease out the iterators, you need to define 
iterators anyway, with the occasional loss in safety or efficiency; (c) 
you may occasionally waste some space by holding an extra iterator when 
not needed. The upside is that you have access to a few idioms that are 
gauche with primitive ranges.

One possible way to combine the advantages of the two is to define a 
flexible interface for Boost::Range(Ex). Say for example that 
Boost::RangeEx defines empty, front, pop_front, and optionally the 
obvious extensions for bidir and random-access (back, pop_back, and 
operator[]). So far so good. People who only need these operations will 
find them convenient.

Now here comes an interesting possibility. Boost::RangeEx also defines a 
typename Boost::RangeEx::iterator, and begin() and end() that return 
iterators. But it makes them optional! Meaning, as much as there are 
forward vs. bidirectional ranges, there are ranges that cloak iterators 
versus ranges that don't. Then algorithms that do use begin() and end() 
can only work with "iterator ranges" and those that don't can work with 
iterator ranges but also with "primitive ranges". Then, whoever wants to 
define a range either goes through the iterator route or just defines 
the range primitives.

Andrei