Re: [boost] is_range metafunction

Subject: Re: [boost] is_range metafunction
From: David Abrahams (dave_at_[hidden])
Date: 2008-10-05 18:47:35

The answer is yes.

Sent from my iPhone

On Oct 5, 2008, at 6:06 PM, "Daniel Walker"  
<daniel.j.walker_at_[hidden]> wrote:

> On Fri, Oct 3, 2008 at 5:59 PM, Steven Watanabe  
> <watanabesj_at_[hidden]> wrote:
>> AMDG
>>
>> Mathias Gaunard wrote:
>>>
>>> This concept checking isn't really done at compile-time, so you  
>>> cannot use
>>> its result with SFINAE.
>>> That's one of the major drawbacks of the concept checking library.
>>
>> It is done at compile time.  It just gives a compiles/doesn't  
>> compile answer
>> rather than a boolean result that can be tested.
>
> Is there any other option? I mean if t<x> is ill-formed, because the
> type x doesn't support some expression used in the template t, then is
> there some context where t could be instantiated with x without making
> the whole program ill-formed? I can't think of one.
>
> If there were such a context, then you might be able associate t<x>'s
> ill-formation with some boolean constant, and you would have a way to
> implement tests based on concept checking templates - a sort of
> concept-based type introspection for C++03, which would be way cool.
> However, without such a context, the best you can do is fail
> gracefully with informative compilation errors, which is the purpose
> of the concept checking library.
>
> This brings up an interesting question regarding the proposed concept
> language extensions for C++0x: If a function in an overload set
> requires a concept that its argument doesn't model but some other
> function in the set accepts the argument, does overload resolution
> succeed? In other words, does the proposal support Concept Requirement
> Failure Is Not An Error? CRFINAE. ;-)
>
> Daniel Walker
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