On 11/28/2010 11:44 PM, Alexander Fokin wrote:
Three things: - value_type should just be pair< Key, T> (no reference or const qualifications on Key or T) - reference should be pair< Key const&, T& >, with T possibly const-qualified (as you've done) - you can (if you want) use iterator_adaptor instead of iterator_facade to automatically get a correct equal, increment, and decrement. I have modified the code as you have suggested and now it compiles OK. The modified code is available at http://codepad.org/AJFIVbdY.
However, there is one thing I'm concerned about. Consider the example at http://codepad.org/plyTCBpx. The following lines do not call the copy constructor of class A: BOOST_FOREACH(auto pair, m) { const A&key = pair.first; }
Good.
However, when I use range adaptors, the copy constructor is called. That was exactly what I was trying to avoid by using references in value_type.
It's certainly a reasonable thing to avoid. I'm not sure why A's copy constructor would be called. Perhaps you should run it through a debugger and set a break point in A's copy constructor. I don't have Qt installed so it would take me a little more effort to help you more than that ;) It could be the case that the iterator for the range produced by (m | map_keys) returns by value upon dereferencing, rather than by reference. Might be worth checking...
I guess I'll try to fix this by myself and then file a patch.
Good luck! - Jeff