How can I use std::maps with user-defined types as key?

Question

I\'m wondering why I can\'t use STL maps with user-defined classes. When I compile the code below, I get the following cryptic error message. What does it mean? Also, why is

Answer 1

You need to define operator < for the Class1.

Map needs to compare the values using operator < and hence you need to provide the same when user defined class are used as key.

class Class1
{
public:
    Class1(int id);

    bool operator <(const Class1& rhs) const
    {
        return id < rhs.id;
    }
private:
    int id;
};