Re: [boost] [iterator] iterator_facade reference type problem

Subject: Re: [boost] [iterator] iterator_facade reference type problem
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2010-01-14 12:47:46

Phil Endecott wrote:
> But what do I do about the reference type?  I can't use std::pair<const 
> KEY,VALUE>& because I'm not storing the keys and values in that way.  
> It's almost possible to use std::pair<KEY,VALUE> for a const_iterator, 
> but not if you want to allow the user to change the value through the 
> reference.  What I think I want to use is std::pair<KEY,VALUE&> i.e. 
> the key is returned by value and only the value is returned by 
> reference.  But when I try this I get various cryptic errors about 
> references to references and mpl things.

I've done some more experiments, and it seems to work with 
std::pair<const KEY, value&> if i write (*iter).second rather than iter->second.

Looking at the iterator_facade source, operator-> calls operator* and 
passes the result to operator_arrow_result::make().  This takes its 
address and, because std::pair<const KEY,VALUE&> is not a reference, it 
tries to convert it to operator_arrow_proxy<...>.  This fails because 
std::pair<const KEY,VALUE&> is not convertible to std::pair<KEY,VALUE>.

I think I just want operator-> to return a pointer to my reference 
type, but the operator_arrow_result stuff is trying to do something 
more complicated.  In what case is the operator_arrow_proxy useful?  Is 
there some way to get what I want using iterator_facade?

(Thanks to er for the comments about zip_iterator.  It's interesting 
that zip_iterator uses iterator_facade internally and doesn't appear to 
do anything special to avoid this problem.  Thanks also to Jeffrey for 
the comments about std::pair's deficiencies wrt references; I think 
that by fixing that I could make pair<A,B&> convertible to pair<A,B> 
and get the code to compile, but it would still be broken due to a 
reference to a temporary.)

BTW I have been testing with an oldish version of Boost, but I've 
looked at the source in svn and it doesn't seem to have changed much.

Cheers,  Phil.