Re: [boost] Projection Iterator Adaptor

From: Jeremy Siek (jsiek_at_[hidden])
Date: 2001-03-17 17:22:50

On Sat, 17 Mar 2001, John E. Potter wrote:
jpotte> 
jpotte> Sorry for being out of contact, but I only have time to
jpotte> delete without reading most of the boost stuff for now.
jpotte> 
jpotte> The action of iter[n] is the same as *(iter + n) and in
jpotte> both cases should be a reference or reference to const.  
jpotte> I know that the standard says convertable to T, but
jpotte> forward iterator has it right and there is an issue.

There is a subtle problem with using reference return for
iterator_adaptor::operator[]. The problem shows up for iterators like
counting_iterator that return a reference to a value object that is part
of the iterator object itself, and has the same lifetime as the iterator.
The expressions (iter + n) creates a temporary iterator object, then
*(iter + n) returns a reference to part of this temporary iterator object.
Upon return from operator[], the temporary iterator goes away and we are
left with a dangling reference.

One option is to say iterator_adaptor can't be used with this kind of
iterator. However, if a user were to go ahead and use such an iterator,
they would not find out about the problem until run time! Even so I'm
slighly in favor of this option, but Dave is strongly against it.

Cheers,
Jeremy

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 Jeremy Siek                        www: http://www.lsc.nd.edu/~jsiek/
 Ph.D. Candidate                  email: jsiek_at_[hidden]
 Univ. of Notre Dame         work phone: [phone]
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