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pure virtual function and abstract classes.ppt
- 2. The Islamia University of Bahawalpur
- 3. Department Of Computer Science Presented By: WAHEED ANWER
- 4. Virtual Functions Define a virtual function in the base class. The word virtual appears only in the base class Virtual function in base class stays virtual in all the derived classes It can be overridden in the derived classes But, a derived class is not required to re- implement a virtual function. If it does not, the base class version is used
- 5. Static Binding Static binding is the compile-time determination of which function to call for a particular object based on the type of the formal parameter When pass-by-value is used, static binding occurs
- 6. Dynamic Binding Is the run-time determination of which function to call for a particular object of a derived class based on the type of the argument Declaring a member function to be virtual instructs the compiler to generate code that guarantees dynamic binding Dynamic binding requires pass-by- reference
- 7. Pure Virtual Function A pure virtual function is a virtual member function of a base class that must be overridden. A pure virtual function is a virtual member function declared in a manner similar to the following: virtual void showInfo(void) = 0;
- 8. Abstract Class A class becomes an abstract base class when it contains one or more pure virtual functions. An abstract base class is not instantiated, but other classes are derived from it.
- 9. Abstract Classes & Pure Virtual Functions class Shape //Abstract { public : //Pure virtual Function virtual void draw() = 0; } A class with one or more pure virtual functions is an Abstract Class Objects of abstract class cant be created Shape s; // error : variable of an abstract class Some classes exist logically but not physically. Example : Shape Shape s; Shape makes sense only as a base of some classes derived from it. Serves as a category Hence instantiation of such a class must be prevented
- 10. Example 10 Shape virtual void draw() Circle public void draw() Triangle public void draw()
- 11. 11 A pure virtual function not defined in the derived class remains a pure virtual function. Hence derived class also becomes abstract class Circle : public Shape { //No draw() - Abstract public : void print(){ cout << I am a circle << endl; } class Rectangle : public Shape { public : void draw(){ // Override Shape::draw() cout << Drawing Rectangle << endl; } Rectangle r; // Valid Circle c; // error : variable of an abstract class
- 12. Pure Virtual Functions: Summary Pure virtual functions are useful because they make explicit the abstractness of a class Tell both the user and the compiler how it was intended to be used Note : It is a good idea to keep the common code as close as possible to the root of you hierarchy 12