VLERange: a range in between BidirectionalRange and RandomAccessRange

[Andrei Alexandrescu]

On 1/11/11 4:41 AM, Michel Fortin wrote:
> On 2011-01-10 22:57:36 -0500, Andrei Alexandrescu
> <SeeWebsiteForEmail at erdani.org> said:
>> In addition to these (and connecting the two), a VLERange would offer
>> two additional primitives:
>>
>> 1. size_t stepSize(size_t offset) gives the length of the step needed
>> to skip to the next element.
>>
>> 2. size_t backstepSize(size_t offset) gives the size of the _backward_
>> step that goes to the previous element.
>
> I like the idea, but I'm not sure about this interface. What's the
> result of stepSize if your range must create two elements from one
> underlying unit? Perhaps in those cases the element type could be an
> array (to return more than one element from one iteration).
>
> For instance, say we have a conversion range taking a Unicode string and
> converting it to ISO Latin 1. The best (lossy) conversion for "œ" is
> "oe" (one chararacter to two characters), in this case 'front' could
> simply return "oe" (two characters) in one iteration, with stepSize
> being the size of the "œ" code point. In the same conversion process,
> encountering "e" followed by a combining "´" would return pre-combined
> character "é" (two characters to one character).

In the design as I thought of it, the effective length of one logical 
element is one or more representation units. My understanding is that 
you are referring to a fractional number of representation units for one 
logical element.

>> Writing to such ranges is not addressed by this design. Ideas are
>> welcome.
>
> Writing, as in assigning to 'front'? That's not really possible with
> variable-length units as it'd need to shift everything in case of a
> length difference. Or maybe you meant writing as in having an output
> range for variable-length elements... I'm not sure

Well all of the above :o). Clearly assigning to e.g. front or back 
should not work. The question is what kind of API can we provide beyond 
simple append with put().


Andrei