Question

I’ve got a multimap<pair<string,string>,vector> > mmap;
and I would like to iterate over it with two loops:

1. on the first element of the key: key.first
2. then key.second

Does the equal_range method work in this case?
How to write it?

 pair<multimap<pair<string,string>,vector>::iterator,
 multimap<pair<string,string>,vector>::iterator> key_range = mmap.equal_range( ?? );

Answer 1

multimap only supports a single ordering, which for pair would be by default the lexicographical ordering over two elements:

(A, A) (A, B) ... (A, Z) (B, A) (B, B) ... (B, Z) ... (Z, A) ... (Z, Z)

If you need to index by two different keys you’re best off using another container e.g. Boost.MultiIndex. You could achieve the same effect yourself by e.g. storing items in a list and holding maps from a key to list iterators:

list<vector> items;
map<string, list<vector>::iterator> index1;
map<string, list<vector>::iterator> index2;