Primary Ranges of Containers

[Timon Gehr]

On 06/20/2012 12:29 PM, Matthias Walter wrote:
> On 06/19/2012 04:04 PM, Timon Gehr wrote:
>> On 06/19/2012 02:54 PM, Christophe Travert wrote:
>>> Jonathan M Davis , dans le message (digitalmars.D:170054), a écrit :
>>>>> I'd propose to always add a bool template parameter (maybe isConst?) to
>>>>> the range since most of the write-functionality can be "removed" by a
>>>>> static if statement in order to make the range read-only.
>>>>>
>>>>> Any suggestions?
>>>
>>> Boolean parameters are very obscure.
>>> How do you guess what is the meaning of false in:
>>> Range!false;
>>>
>>> Range!IsConst.no would be better.
>>>
>>>> struct ArrayRange(bool isConst) {...}
>>>> alias ArrayRange!false Range;
>>>> alias ArrayRange!true ConstRange;
>>>
>>> Range and ConstRange seems a good thing to have, just like c++
>>> containers have iterator and const_iterator.
>>>
>>
>> Something along these lines seems to be a superior design:
>>
>> struct ArrayRange(T){ ... }
>>
>> class Array(T){
>>      ...
>>      ArrayRange!T opSlice(){ ... }
>>      ArrayRange!(const(T)) opSlice()const{ ... }
>>      ArrayRange!(immutable(T)) opSlice()immutable{ ... }
>>      ...
>> }
>>
>> (Where the opSlice functions can be generated automatically.)
>
> I like the design. I looked at the actual implementation of
> std.container.Array and realized the following problem:
>
> The payload is stored in a RefCounted object. The range returned by
> opSlice() const must obviously somehow access the payload. By
> documentation of RefCounted, "no references to the payload should be
> escaped outside the RefCounted object." which implies that we need to
> create a copy of the RefCounted variable to have proper access.
>
> On the other hand, given the constness of opSlice() we only have access
> to a const version of the RefCounted variable which cannot be used to
> create another reference since the reference counter is const as well.
>
> So how can a ConstRange be implemented *somehow*?
>
> Matthias

Escape the payload anyway and document that the range is invalid after 
the container has been destroyed.

But I question the usefulness of having a const or immutable RefCounted 
object at all.

Maybe it is better to have something like this:

struct ArrayRange(T){
     ArrayRange!(const(StripImmutable!T)) constView() const { ... };
     alias constView this;
}

class Array(T){
     ArrayRange!T opSlice(){ ... }
}