Question

I have a class where I overload all the comparison operators. I load a bunch of these objects into a map as keys and I expect the map to sort them least to greatest.

However, the map is actually sorted by greatest to least. Why is this? Does it not use the comparison operators on the key when it sorts the map? How can I make it do this?

Answer 1

If you look at the definition for std::map, the 3rd parameter is the comparator – std::less< Key > by default. The default implementation of which is just to invoke operator<.

You could try defining a new version in the header where YourType is defined, like so:

template<> std::less< YourType >
{
    bool operator()(
        const YourType& lhs,
        const YourType& rhs)
    {
        if(!(lhs.member1 < rhs.member1))
        {
           return false;
        }

        if(!(lhs.member2 < rhs.member2))
        {
           return false;
        }

        return true;
    }
}

Remember to friend std::less< YourType > in your class.

The actual test must comply with the following or you’ll get all kinds of problems:

• lhs < rhs = true
• rha < lhs = false
• lhs < lhs = false

One important thing to note (that I discovered quite recently) is that if your compiler starts complaining that you’ve redefined the meaning of std::less or other funky things, it could be that you’ve only forward declared your YourType when you declared your std::map< YourType >.

Hope this helps!