What's the advantage of multimap over map of vectors?

Question

I don't understand why multimap exists if we can create map of vectors or map of sets. For me only differences are:

• using equal_range in multimap for getting elements of a key and in map of vectors we simply use [] operator and have vector of elements.
• using multimap.insert(make_pair(key,value)) in multimap for adding elements and map_of_vectors[key].push_back(value) in map of vectors.

So why use multimap? For me it's better to have a vector than two iterators to get all values of a key.

This question applies also to unordered_map of vectors and unordered_multimap.

Answer 1

I would say it depends on whether all the values with same key have a relationship that you want to address.

So for example, do you often go through all elements with key X, or pass them to a function, and so on? Then it is more convenient to already have them in their separate container, that you can address directly.

However, if you just have a collection of items, that may share same key value, or not, why use vectors in between? It is more convenient to run through the multimap with iterators than have a nested for-loop for the map, vector case.

Another way of looking at this: If multiple entries per key is very common, your structure is more efficient in the map, vector case. If they seldomly happen, it is the opposite.

Answer 2

There are many important differences between multimap<x, y> and map<x, vector<y>>

Once you had inserted a value into multimap, you know that the iterator would remain valid until you remove it and this is very strong property, you can't have it with map of vectors.

multimap<x,y>::iterator p=mymap.insert(make_pair(a,b));

The iterator remains valid until it is erased from map, while in second case, it would be invalidated each time you add new entry to the vector.

Also note that map<x, vector<y>> may have an empty value set with existing key, while multimap does not.

These are different things that behave differently.

And to be honest I miss multimap in some languages that do not provide it in their library.