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Virtual function and abstract class
- 1. VIRTUAL FUNCTIONS AND ABSTRACT CLASS (WITH SAMPLE CODE )
- 2. VIRTUAL FUNCTION • A virtual function is a member function which is declared within a base class and is re-defined (Overridden) by a derived class. • When you refer to a derived class object using a pointer or a reference to the base class, you can call a virtual function for that object and execute the derived class’s version of the function.
- 3. Virtual functions ensure that the correct function is called for an object, regardless of the type of reference (or pointer) used for function call. They are mainly used to achieve Runtime polymorphism Functions are declared with a virtual keyword in base class. The resolving of function call is done at Run- time. VIRTUAL FUNCTION
- 4.  Virtual functions cannot be static and also cannot be a friend function of another class.  Virtual functions should be accessed using pointer or reference of base class type to achieve run time polymorphism.  The prototype of virtual functions should be same in base as well as derived class.  They are always defined in base class and overridden in derived class. It is not mandatory for derived class to override (or re-define the virtual function), in that case base class version of function is used.  A class may have virtual destructor but it cannot have a virtual constructor. RULES FOR VIRTUAL FUNCTIONS
- 7. NOTE: If we have created a virtual function in the base class and it is being overridden in the derived class then we don’t need virtual keyword in the derived class, functions are automatically considered as virtual functions in the derived class.
- 8. Working of virtual functions (concept of VTABLE and VPTR) If a class contains a virtual function then compiler itself does two things:  If object of that class is created then a virtual pointer(VPTR) is inserted as a data member of the class to point to VTABLE of that class. For each new object created, a new virtual pointer is inserted as a data member of that class.  Irrespective of object is created or not, a static array of function pointer called VTABLE where each cell contains the address of each virtual function contained in that class.
- 13. ABSTRACT CLASSES Abstract Class is a class which contains at least one Pure Virtual function in it. Abstract classes are used to provide an Interface for its sub classes. Classes inheriting an Abstract Class must provide definition to the pure virtual function, otherwise they will also become abstract class.
- 14. CHARACTERISTICS OF ABSTRACT CLASS • Abstract class cannot be instantiated, but pointers and references of Abstract class type can be created. • Abstract class can have normal functions and variables along with a pure virtual function. • Abstract classes are mainly used for Up casting, so that its derived classes can use its interface. • Classes inheriting an Abstract Class must implement all pure virtual functions, or else they will become Abstract too.
- 15. PURE VIRTUAL FUNCTIONS IN C++  Pure virtual Functions are virtual functions with no definition.  They start with virtual keyword and ends with = 0. Here is the syntax for a pure virtual function. virtual void f() = 0;
- 17. Why can't we create Object of an Abstract Class? When we create a pure virtual function in Abstract class, we reserve a slot for a function in the VTABLE, but doesn't put any address in that slot. Hence the VTABLE will be incomplete. As the VTABLE for Abstract class is incomplete, hence the compiler will not let the creation of object for such class and will display an error message whenever you try to do so.
- 18. PURE VIRTUAL DEFINITIONS Pure Virtual functions can be given a small definition in the Abstract class, which you want all the derived classes to have. Still you cannot create object of Abstract class. Also, the Pure Virtual function must be defined outside the class definition. If you will define it inside the class definition, complier will give an error. Inline pure virtual definition is Illegal.
- 21. THANK YOU