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Message: [QUOTE="Erik WikstrÃ¶m, post: 3405118"] Yes, if you use two maps you have to store the key twice, and when retrieving items you have to do the lookup twice too. If you always handle both A and B together there is not reason to keep them in separate maps. Consider also the implications on exception-safety, if you have two maps and want to insert a new pair of A and B and you successfully insert the A into its map but an exception is thrown when inserting B you must remove the A from its map. Yes, at the very least you need the extra memory that is used to store the pointers in addition to the space for the pair. Of course you might be able to save memory using the pointer if multiple entries in the map refer to the same pair of A and B. Depends on how you plan to use C, perhaps you should offer both? [/QUOTE]