Re: [Boost-users] [Range] Strange!

Subject: Re: [Boost-users] [Range] Strange!
From: Krzysztof Czainski (1czajnik_at_[hidden])
Date: 2014-08-28 17:28:49

2014-08-28 23:22 GMT+02:00 Neil Groves <neil_at_[hidden]>:

> On 28 August 2014 20:50, Robert Jones <robertgbjones_at_[hidden]> wrote:
>
>> Can anyone explain why my last output line is garbage?
>>
>> int main( )
>> {
>>     using boost::adaptors::reversed;
>>     using boost::adaptors::transformed;
>>     using boost::counting_range;
>>     using boost::phoenix::arg_names::_1;
>>
>>     print_it( counting_range( 1, 5 ) );
>>     print_it( counting_range( 1, 5 ) | reversed );
>>
>>     print_it( counting_range( 1, 5 ) |            transformed( doubled )
>> );
>>     print_it( counting_range( 1, 5 ) | reversed | transformed( doubled )
>> );
>>
>>     print_it( counting_range( 1, 5 ) |            transformed( bind(
>> doubled, _1 ) ) );
>>     print_it( counting_range( 1, 5 ) | reversed | transformed( bind(
>> doubled, _1 ) ) );
>> }
>>
>>
> Same result on VS12 (2013).

> The counting_range instance is a unnamed temporary, since the adaptors are
> applied to this temporary the adapted iterators may live beyond the
> lifetime of the source. If you where to add the line:
>
> const auto& rng = counting_range(1, 5)
>
> and then you did:
> print_it(rng | reversed | transformed(bind(doubled, _1)));
>
> it should then work since there are no issues with chaining the adaptors
> since the iterators are always copied from the base range instances during
> the chaining process. One has to manage the lifetime of the source to
> ensure it lives beyond the lifetime of the iterators.
>
>
Actually, I get garbage with the above too. I don't think the problem is
counting_range being a temporary, because it is guaranteed to outlive the
call to print_it() here.

My explanation: dereferencing a reverse_iteraotr(counting_iterator(5))
works like this:
1. make a local copy of counting_iterator(5)
2. decrement the local copy
3. dereference the local copy, which returns a reference
4. return the reference
5. destroy the local copy, laving the returned reference dangling

Regards,
Kris