Howard Hinnant skrev:
On Sep 24, 2009, at 4:26 PM, Ion Gaztañaga wrote:
Howard Hinnant escribió:
This is effectively the same thing as: typedef ... map; map m; map::node_ptr p = m.remove(m.emplace_hint(m.begin(), my_special_cheap_key_value, mv1, mv2)); p->first = modify( p->second ); m.insert( boost::move(p) );
Removing something just after inserting it seems a bit useless. Even if we insert in the first position we need to do a comparison to make sure the hint is correct.Why not:
map::node_ptr p = m.create_node(my_special_cheap_key_value, mv1, mv2); //We would need to unconst-cast... const_cast<Key&>(p->first) = modify( p->second ); m.insert( boost::move(p) );
FWIW, I like this. My only worry is about the const_cast: is it not undefined behavior to do that here (since the .first member is constructed as a const object)?
One could do that but the node allocation and construction is typically the expensive part of the insertion. Once you've got that done, and if especially if your hint is correct, the insertion is very fast. So it seems to me like m.create_node would be interface aimed at a very rare use case and for an optimization that is going to be quite minor (percent-wise), perhaps not even measurable.
In my case it is not possible to provide a hint fot the new node. But you are right, that emplace/remove and create_node might not be that different in performace. I find create_node slightly more to the point. -Thorsten