Associating and iterating in C++

Question

I've got a situation where I want to use an associative container, and I chose to use a std::unordered_map, because it's perfectly feasible that this container could be used to hold millions or more of elements. But now I also need to iterate in order. I considered having the value types link to each other in a list, but now I'm going to have issues with memory management.

Should I change container, say to a std::map? Or just iterate once through my unordered_map, insert into a vector, and sort, then iterate? It's pretty unlikely that I will need to iterate in an ordered fashion repeatedly.

Answer 1

You could use Boost.MultiIndex , specifying the unordered (hashed) index as well as an ordered one, on the same underlying object collection.

Possible issues with this - there is no natural mapping from an existing associative container model, and it might be overkill if you don't need the second index all the time.

Answer 2

Well, you know the O() of the various operations of the two alternatives you've picked. You should pick based on that and do a cost/benefit analysis based on where you need the performance to happen and which container does best for THAT.

Of course, I couldn't possibly know enough to do that analysis for you.