Re: [boost] Problem with range (make_iterator_range)?

From: Eric Niebler (eric_at_[hidden])
Date: 2006-05-30 14:49:01

Thorsten Ottosen wrote:
> Eric Niebler wrote:
>> Thorsten Ottosen wrote:
>>
>>> Eric Niebler wrote:
> 
>>>> If make_iterator_range() returned a const-qualified iterator_range<>, 
>>>> this would work. Thorsten?
>>> Isn't the problem thet out is a reference?
> 
>> out is a reference, which means that non-const rvalues will not bind to 
>> it. But const rvalues will because out_t will be deduced to be 
>> "iterator_range<> const". Try it and see.
> 
> I see. I certainly didn't know that. I could get it to work on
> gcc, but not vc8.
> 
> On what compilers does it actually work?

All of them, as far as I know. I just tried on VC8 and it worked:

template<typename T> void foo(T &) {}
struct empty {};
empty const bar() { empty e; return e; }

int main()
{
   foo(bar());
   return 0;
}

Without thinking about this deeply, it seems that lightweight proxy 
objects such as iterator_range<> should always be returned as a 
const-qualified rvalues for precisely this reason.

(Note that cv-qualifications on intrinsic return types are ignored, so 
"int foo()" and "int const foo()" are the same. Perhaps that's the 
problem you were seeing?)

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com