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

Subject: Re: [boost] [iterator] iterator_facade reference type problem
From: Jeffrey Hellrung (jhellrung_at_[hidden])
Date: 2010-01-14 14:22:33

Phil Endecott wrote:
> 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?
<snip>
> 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.
> 

(Also looking at the code) operator_array_proxy should be probably be 
instantiated with the Reference, not ValueType...???  Would that be 
correct?  I.e., operator_array_proxy should wrap a reference (which is 
not a real C++ reference), not a value...

You *could* just overload operator-> in the derived class to do 
something else (in this case, I would say try using a proxy that wraps a 
reference).

Also, just to be clear, the operator_array_proxy stuff in 
iterator_facade fails not because std::pair< const KEY, VALUE& > isn't 
convertible to std::pair< KEY, VALUE >, but because their pointers 
aren't convertible.  At least that's what I'm seeing...

Seems like I stumbled on this in the past and rather than figure out 
what was wrong, I just redefined operator-> in the derived class to work...

- Jeff