在std :: map中使用2个键

Question

I was trying to create a std::map with 2 keys. I can do it with std::pair or create a struct and use it as the key.

In my software, there's a search function that is being called a lot. This function searches by the 1st or the 2nd key.

If I had about 1000 items in the map, I'm guessing it will take some time if I wanted to search it. So I thought that if I make another std::map that holds the 2nd key and the value is 1st key, then I can take the value and search in the other map to get the real value.

But my guess is that this will take more memory. What is the best option in this scenario?

Answer 1

Brute force approach

You can store your items in std::vector and have two maps: the first with your first key and pointers (or indices) to vector items and the second with your second key and pointers (or indices) to vector items. The problem is to maintain all three containers when your set is modified.

Pointers vs. indices: Pointers are dangerous as pointed correctly in the comment, but simpler if you're going to delete items from the vector. Otherwise indices are safer.

Smart approach

You can use Boost.MultiIndex container that was designed for cases exactly like yours.

Answer 2

This is an engineering decision that you'll have to resolve.

Multiple maps only makes sense (IMHO) if you know that the sets of key1 and key2 keys conflict. Otherwise, why not just insert both keys into the same map, each corresponding value referring to your object?

You don't want to duplicate your object, so you might put them in a vector, and put the vector index as the mapped value. Or use a map of key-to-pointer, etc.

1k items is not really that many, so I'm not concerned about the memory use here, but using a map instead of an unordered_map might be a concern (rb-tree vs. hash table).

Also, if you remove items from the map, you'll need to be able to remove both keys together, so be sure to account for that.