Question

This is my code:

map<string, int> errs;
struct Compare {
    bool operator() (map<string, int>::const_iterator l, 
        map<string, int>::const_iterator r) { 
        return ((*l).second < (*r).second); 
    }
} comp;
sort(errs.begin(), errs.end(), comp);

Can’t compile. This is what I’m getting:

no matching function for call to ‘sort(..’

Why so? Can anyone help? Thanks!

Answer 1

Maps are, by definition, sorted by their keys, so you can’t resort a map by its values.

You can provide an alternate comparison function as the third template parameter to a map, if you want to sort the keys by a non-default order.

If you’re trying to sort a map by its values, then perhaps you could try using Boost.MultiIndex to make a bidirectional map instead?