Object Oriented Programming in C++ Chapter 7 Dynamic Binding

class Employee { public: Employee( String nm, int ag ) ; Employee( ) ; void print( void ) const ; private: String name; int age; };

Implementation of Employee’s methods Employee ::Employee( String nm, int ag ) : age( ag ), name( nm ) { } Employee ::Employee( ): age(0), name("") { } void Employee ::print( void ) const { cout << "Name " << name << " Age " << age; }

Manager inherits from Employee class Manager : public Employee { public: Manager( String nm, int ag, int lev ); Manager( ); void print( void ) const private: int level; }; print redefined

Manager’s member functions Manager ::Manager( char* nm, int ag, int lev ) : Employee( nm, ag ), level( lev ) { } Manager :: Manager( ) : Employee( ), level( 0 ) { } void Manager :: print( void ) const { Employee::print(); cout << " Level " << level; }

Using Employee and Manager Employee emp1( “Shem ", 42 ); Employee emp2( “Ham ", 32 ); Manager mgr1( “Yefet", 50, 3 ); Manager mgr2( “Zefet ", 46, 2 ); emp1.print();// Employee’s print function emp2.print();//“““ mgr1.print();// Manager’s print function mgr2.print();//“““ Name Shem Age 42 Name Ham Age 32 Name Yefet Age 50 Level 3 Name Zefet Age 46 Level 2

Storing pointers to objects, then printing Employee* employees[4]; // array of pointers to Employee objects employees[0] = &emp1; // Store address of emp1 in first array element employees[1] = &mgr1; // store address of mgr1 in second element employees[2] = &emp2; // etc. employees[3] = &mgr2; // etc. for ( int i = 0; i < 4; i++ ) { employees[i]->print(); // call print member function of each element cout << endl; }

Oops… Name Shem Age 42 Name Yefet Age 50 Name Ham Age 32 Name Zefet Age 46 Not what we had expected The Employee version of print() has been invoked for objects of both Employee and Manager This was determined at compile-time - Static Binding by reference to the array element type (Employee*) We want the Manager version of print() to be called for Manager objects

Solution Make Employee’s print method virtual Ensures that the version of print() invoked is determined at run-time by reference to the actual object referred to by each pointer stored in the array - Dynamic Binding In Employee class declaration: virtual void print( void ) const { cout << "Name " << name << " Age " << age; } l No change required to class Manager

Output from loop with virtual print() Name Shem Age 42 Name Yefet Age 50 Level 3 Name Ham Age 32 Name Zefet Age 46 Level 2 OK now. Base class function is virtual - ensures Dynamic Binding

Virtual functions Default is static binding - faster Dynamic binding only if specifically requested

Abstract base classes classes which we never intend to instantiate cannot create objects of this class can declare pointers to this class vs concrete classes too generic to define real objects used only as a base class for inheritance required to have a derived class made by defining 1 or more virtual functions (methods) as pure if derived class does not override all pvf, it becomes an abstract class as well! A pure virtual function - virtual void print( void ) const = 0; // no parentheses

Employee as an Abstract class class Employee { public: Employee( String nm, int ag ) : age( ag ), name( nm ) { } Employee( ): age(0), name("") { } virtual void print( void ) const = 0; protected: String name; int age; };

Class Manager class Manager : public Employee { public: Manager( String nm, int ag, int lev ) : Employee( nm, ag ), level( lev ) { } Manager( ) : Employee(), level( 0 ){ } // redefinition of pure virtual function void print( void ) const { cout << "Name " << name << " Age " << age << " Level " << level; } private: int level; };

Class Worker class Worker : public Employee { public: Worker ( String nm, int ag ) : Employee( nm, ag ) { } Worker ( ) : Employee() { } // redefinition of pure virtual function void print( void ) const { cout << "Name " << name << " Age " << age; } };

2 derived classes in Employee* array Worker wkr1( “Shem ", 42 ); Worker wkr2( “Ham ", 32 ); Manager mgr1( “Yefet", 50, 3 ); Manager mgr2( “Zefet ", 46, 2 ); Employee* employees[4]; employees[0] = &wkr1; employees[1] = &mgr1; employees[2] = &wkr2; employees[3] = &mgr2; for ( int i = 0; i < 4; i++ ) { employees[i]->print(); cout << endl; } Name Shem Age 42 Name Yefet Age 50 Level 3 Name Ham Age 32 Name Zefet Age 46 Level 2