isInfinite isInadequate

[Steven Schveighoffer]

On Tue, 12 Mar 2013 12:32:22 -0400, monarch_dodra <monarchdodra at gmail.com>  
wrote:

> On Tuesday, 12 March 2013 at 16:16:07 UTC, Andrei Alexandrescu wrote:
>> On 3/12/13 11:47 AM, Steven Schveighoffer wrote:
>>> On Tue, 12 Mar 2013 11:25:54 -0400, deadalnix <deadalnix at gmail.com>  
>>> wrote:
>>>
>>>> On Tuesday, 12 March 2013 at 10:49:57 UTC, monarch_dodra wrote:
>>>
>>>>> Having "isInfinite" can also have the advantage of protecting users
>>>>> from stupid calls. For example, calling "count" on an infinite range
>>>>> is forbidden => shifting problems from runtime to compile time is a
>>>>> HUGE gain.
>>>>>
>>>>
>>>> Clearly this is a good point. I however think that a static assert
>>>> within count is much better because it allow to give nicer feedback.
>>>> The problem with InfiniteRange is that it does gives you cryptic error
>>>> message like the count function do not exists.
>>>>
>>>
>>> Hm... is there a way to test for inifinitness without requiring to be  
>>> an
>>> enum?
>>>
>>> Wouldn't this also be valid?
>>>
>>> if(!R.init.empty)
>>>
>>> Essentially, you can evaluate R.init.empty at compile time AND it's
>>> false on an uninitialized range. How can a correctly written
>>> non-infinite range pass that?
>>>
>>> That would make forwarding much easier, as the 'dumb' implementation
>>> still would result in an infinite range.
>>
>> Crossed my mind a few times that fresh non-infinite ranges should be  
>> empty.
>
> s/should/could/
>
> In any case, that's a very dangerous logic to follow. Not-initialized  
> means not initialized. At that point, the concept of empty or not empty  
> is irrelevant, it's a wrong call.

No, ranges can be initialized without a constructor.  Structs are.   
Classes aren't.  But a class with empty as an enum would work.

The idea is that the ultimate underlying source of empty is an enum.   
Since it's an enum, it should be calculable at compile-time, and it should  
always be false, regardless of the state of the range (invalid or valid).

The problem is whether NON-infinite ranges are empty or not.  Looks like  
there are cases where they could be non-empty.

> By the same token, I don't think anybody would expect a call to empty on  
> a null class reference to actually succeed.

If it's an enum, I would.  If it doesn't work, the constraint is false,  
which is what we want.

-Steve