What is the virtual function and friend function?

Answer 1

I have not used friend concept in C#. Actually it may not exist in C#.

Based on OO principles, the friend feature in C++ violates the privacy or encaptualization of private data memeber.

Would you allow your friend (a friend class) to access your wallet or bank account (private member)?

Answer 2

Virtual Function

A virtual function is a class method that is intended to be overridden by derived classes. This is achieved be preceding the function declaration with the keyword 'virtual'. If you fail to do this, derived classes can still override the method, but they will effectively hide the base class method. Moreover, when dealing with references or pointers to the base class of a derived class, calling the method will execute the base class method, not the override. By declaring the method virtual, you ensure expected behaviour. The derived class can still call the base class method explicitly, if required, but the base class will always call the override without the need to include runtime code to determine the actual type of the derived class. The virtual-table (or vtable) takes care of all that for you and costs very little in terms of memory and speed compared to using runtime information. In fact, the use of runtime information to access derived class behaviour is a clear indication of poor class design.

As a side-note, a pure-virtual function is one that is declared to be virtual but that MUST be overridden by derived classes. You achieve this by appending '=0' to the function declaration, before the semi-colon. When you do this, you are signifying that your class is intended to act as a common interface to all its derived classes, which will themselves provide the specific implementations of those methods. You also signify that your class is abstract; you cannot instantiate an abstract class other than by derivation. Derived classes that do not fully implement all the pure-virtual methods inherited from their base classes become abstract themselves. However, derived classes can inherit implementations defined in higher base classes, other than the base class that originally declared the method pure-virtual. Typically, abstract classes have none or few member variables, and little if any implementation; they are conceptual classes rather than concrete classes. By way of an example, circles and squares are types of shape. You cannot draw a shape without knowing its type, but since all shapes can be drawn, all shapes have a common interface: Draw(). Thus the Shape class can be declared abstract with a pure-virtual Draw() method. All classes derived from Shape (such as Circle and Square) must therefore implement the Draw() method. You can then create collections of shapes without regard to their actual type; calling the Draw() method will invoke the appropriate override. While this behaviour is really no different to the way in which a standard virtual function works, the difference is that you cannot instantiate a Shape class (other than by derivation) and the Shape class need not provide any implementation. Any implementation that is provided by the abstract class must be called explicitly, just as it would if it were declared virtual rather than pure-virtual.

Friend Function

A friend function is any function that is external to a class (including methods of other classes) that is declared to be a friend of the class. Entire classes can also be declared friends of other classes, if required. This is achieved by including the function prototype (exactly as it appears in the function's declaration) within the class declaration, preceded by the keyword 'friend'. Friend classes can be declared in their entirety by preceding the class name with the keywords 'friend class' in the class declaration.

Friends of a class are treated as extensions to the class interface and have the same access rights as the class itself. That is; unrestricted access to the private members of the class. As such they are highly privileged and should be used with a measure of caution. If misused, they can potentially undermine the encapsulation of your class. As a general rule, if the class interface is such that the public interface is sufficient for the external function to achieve its purpose, there is absolutely no need to declare the function a friend of the class. But if the function requires access to private and/or protected members of the class and exposing those members to public access would itself undermine the encapsulation of the class, then a friend function is the simplest solution. Unfortunately, there is no way to expose individual members of a class to a friend function -- it's all or nothing. However, friends are not subject to inheritance. That is, if Tom is derived from Dick and Harry is Dick's friend, Tom is not a friend of Harry unless Harry is specifically declared a friend of Tom.

Friend classes are often declared when two classes work closely together as a single unit. In such cases, one class may be more dominant than the other, but private access may still be required in both directions. Therefore the dominant class must be declared a friend of the other class (allowing the dominant class complete access to the other class) while specific methods of the other class can be declared friends of the dominant class, allowing only those specific methods access to the dominant class. This is the only way to achieve bi-directional access to private members. You cannot declare specific friend functions in both as the classes will not compile. At least one class must be late-bound to the other by declaring it a friend (in its entirety) of the other. However, in some cases it may be better to simply declare external functions to be friends of both instead. In this way, the classes can be treated separately and can be fully encapsulated, while the external functions can be used to treat them as a single unit wherever required.

Ultimately, friends should be used as minimally as possible in order to minimise coupling and dependency. With a well-designed interface, friends should rarely, if ever, be required, but the option is there for those cases where there is no other solution.