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.