Assignment operator Vs. Copy constructor

The copy constructor is for creating a new object. It copies a existing object to a newly constructed object.

The assignment operator is to deal with an already existing object.

class MyObject
{
    public:

        MyObject(); // Default constructor
        MyObject(MyObject const & a); // Copy constructor

        // Assignment operator
        MyObject & operator = (MyObject const & a)
};

The copy constructors play an important role, as they are called when class objects are passed by value, returned by value, or thrown as an exception.

// A function declaration with an argument of type MyObject,
// passed by value, and returning a MyObject

MyObject f(MyObject x)
{
    MyObject r;
    ...
    return(r); // Copy constructor is called here
}

// Calling the function :

MyObject a;

f(a); // Copy constructor called for "a"

It should be noted that the C++ syntax is ambiguous for the assignment operator.

MyObject x; x=y;

and

MyObject x=y;

have different meaning.

MyObject a; // default constructor call

MyObject b(a); // copy constructor call

MyObject bb = a; // identical to bb(a) : copy constructor call

MyObject c; // default constructor call

c = a; // assignment operator call