Assigning objects

The assignment operator is called when an already created object is assigned to the value of another one. By default, you will get a copy assignment operator that will copy all the data members. This is not necessarily what you want, particularly if the object has a data member that is a pointer, in which case your intention is more likely to do a deep copy and copy the data pointed to rather than the value of the pointer (in the latter case, two objects will point to the same data).

If you define a copy constructor, you will still get the default copy assignment operator; however, it makes sense that if you regard it important to write your own copy constructor, you should also provide a custom copy assignment operator. (Similarly, if you define a copy assignment operator, you will get the default copy constructor unless you define it.)

The copy assignment operator is typically a public member of the class and it takes a const reference to the object that will be used to provide the values for the assignment. The semantics of the assignment operator are that you can chain them, so, for example, this code calls the assignment operator on two of the objects:

    buffer a, b, c;              // default constructors called 
    // do something with them 
    a = b = c;                   // make them all the same value 
    a.operator=(b.operator=(c)); // make them all the same value

The last two lines do the same thing, but clearly the first is more readable. To enable these semantics, the assignment operator must return a reference to the object that has been assigned. So, the class buffer will have the following method:

    class buffer 
    { 
        // data members 
    public: 
        buffer(const buffer&);            // copy constructor 
        buffer& operator=(const buffer&); // copy assignment 
    };

Although the copy constructor and copy assignment methods appear to do similar things, there is a key difference. A copy constructor creates a new object that did not exist before the call. The calling code is aware that if the construction fails, then an exception will be raised. With assignment, both objects already exist, so you are copying the value from one object to another. This should be treated as an atomic action and all the copy should be performed; it is not acceptable for the assignment to fail halfway through, resulting in an object that is a bit of both objects.

Furthermore, in construction, an object only exists after the construction is successful, so a copy construction cannot happen on an object itself, but it is perfectly legal (if pointless) for code to assign an object to itself. The copy assignment needs to check for this situation and take appropriate action.

There are various strategies to do this, and a common one is called the copy-and-swap idiom because it uses the Standard Library swap function that is marked as noexcept, and will not throw an exception. The idiom involves creating a temporary copy of the object on the right-hand side of the assignment and then swapping its data members with the data members of the object on the left-hand side.