How to pass a function as argument for a class constructor in c++

I'm trying to implement an ordered multi map. But I need to give a function to compare whatever the multimap contain.

Ex:

bool compare(string a, string b) { return a<b; }
bool compare2(string a, string b) { return a.size()<b.size(); }


class MultiMap
{
 private:
    toCompare( string a, string b); 

 public:
    MultiMap(function) { toCompare = function };


}

So I would like to make set the order in the multimap by a given function.

Is there a way, maybe some a pointer to a function. Hope you understand what I want to do, if not feel free to ask.

Also, notice this is not the real code, is just something I've writen to give an idea.