Question

I can define a compare class for a map like this:

struct classcomp {
    bool operator() (const string& lhs, const string& rhs) const
    {
        if(lhs < rhs)
            return true;
        else 
            return false;
    }
};

but here lhs and rhs represent keys. What if I want to compare by values instead of key?
How will I do that?

Answer 1

It’s not about what you want; it’s about what std::map wants. The ordering is based on the layout of elements in memory (usually in a tree structure) and it is this process that uses the comparator. Wishing this were not the case does not make it so!

It sounds to me like std::map as a container choice does not fit your requirements. Consult a container choice flow chart to decide what to do next.