copy constructor

Copy constructor is invoked when you have to assign an instance of the class to another instance of the class. For example in your case,  a sample of the code would be :

SomeClass A, B;

A.Assign(); // Assuming you have a method Assign which fills this.
B = A;        // Now the copy constructor is invoked

Purpose of copy constructor is to make a new object based on an existing object, where both objects belong to the same class.

another example is
SomeClass B( A ) ;

how it is different from assignment operator

1. At the time of object creation
2.At datatype casting time
3. At = assignment operator invokation

1) MyObject a(myObjectInst);
2) MyObject a = myObjectInst;
3) a = (MyObject)myObjectInst;       // if a has been initialised it will call the copy ctor then the assignment op...
                                                    // if not then it will call the copy ctor twice,
                                                         // once for the cast and once for the initialisation of a.