Creating an assignment (=) operator for class in C++

Possible Duplicate:
Operator overloading

EDIT 2

I was using insert(...) incorrectly, I didn't actually need a '=' operator. Sorry to waste peoples' time. I have voted to close.. 2 votes remain. Please vote.

EDIT

The reason I want an '=' operator is so I can use the insert(...) function on a vector of Derivation objects. At the moment my compiler says:

/usr/include/c++/4.2.1/bits/stl_algobase.h:283: error: no match for 'operator=' in '* __result = * __first'

I have created '==' and '<' operators for my own classes before but I'm struggling to create an '=' operator. My class looks like this (ignore the silly variable names):

class Derivation {
public:
    string                  rc; 
    ImplementationChoice    Y; 
    vector      X;
    vector          D;       
    vector          C;       
    vector          P, O;   
    vector          B;   


    // various functions
    // ...
};

and I want to know what I need to put in

// What do '=' return?  An object of the class right?
Derivation& operator=(const Derivation &d) const {
    // something....

}

Many thanks.