Chapter 11, Problem 11.3E

Exercises11.3 (Composition as an Alternative to Inheritance) Many programs written with inheritance can be written with composition instead, and vice versa. Rewrite class asePlusCommissionEmp1oyee of the CommissionEmployee-BasePlusCommissionEmployee hierarchy to use composition rather than inheritance. After you do this, assess the relative merits of the two approaches for designing classes CommissionEmployee and BasePlusCommissionEmployee, as well as for object-oriented pro- grams in general. Which approach is more natural? Why?

Program Plan Intro

Program Plan:

We will implement the BasePlusCommissionEmp class using composition instead of inheritance and invoke different functions in the test program subsequently.

Explanation of Solution

Explanation:

Program Description:

The program demonstrates composition as an alternate way of implementing functionality in Object oriented programming. Some of the pros and cons are self evident in the program, yet we’ll discuss the merits and demerits of using composition over inheritance herewith. As obvious, composition increases duplicacy of code as seen in BasePlusCommissionEmp class where a large part of attributes and functions of the Employee class have to be repeated in the BasePlusCommissionEmp class. Also, the test code or the actual application using these objects becomes more complicated because the use of Data structures like Vectors to store all similar objects together and invoke common functionality in a single loop gets limited. If there are a large number of objects with similar functionality and a little variances, the redundant code soon becomes prone to defect and maintenance nightmares. On the same hand, composition provides more control at the compile time by limiting common access modifier errors and methos overriding errors during development time. has-a relationship is suited mostly where there is limited or no commonality in attributes and functionality of the objects being modelled. Its always better to create an is-a classheirarchywhenthe objects being modelled are having a lot of common attributes and method, resulting in a generic common subset (the base class) and other derived class specializing form it. Inheritance makes the code cleaner to write, read and maintain.

Program:

// BasePlusCommissionEmp class definition .
#ifndef BP_COMMISSION_H
#define BP_COMMISSION_H
#include<string>// C++ standard string class
usingnamespace std;
classBasePlusCommissionEmp
{
public:
BasePlusCommissionEmp(conststring&, conststring&, conststring&,
		double = 0.0,double = 0.0, double = 0.0 );
//For generic attributes of Employee
	voidsetFirstName( conststring& ); // set first name
	stringgetFirstName() const; // return first name
	voidsetLastName( conststring& ); // set last name
	stringgetLastName() const; // return last name
	voidsetSocialSecurityNumber( conststring& ); // set SSN
	stringgetSocialSecurityNumber() const; // return SSN

// additional functions for attributes of CommisionEmployee
	voidsetGrossSales( double ); // set gross sales amount
	doublegetGrossSales() const; // return gross sales amount
	voidsetCommissionRate( double ); // set commission rate
	doublegetCommissionRate() const; // return commission rate

//additional functions for baseSalary
	voidsetBaseSalary( double ); // set base salary
	doublegetBaseSalary() const; // return base salary

// Generic functions of Employee
	doubleearnings() const;
	voidprint() const;

private:
	//Generic attributes of Employee
	stringfirstName; // composition: member object
	stringlastName; // composition: member object
	stringsocialSecurityNumber; //composition: member object
	//attributes of CommisionEmployee
	doublegrossSales; // gross weekly sales
	doublecommissionRate; // commission percentage

	//attribute for BaseSalary
	doublebaseSalary; // base salary
}; // end class BasePlusCommissionEmp
#endif
BasePlusCommisionEmp.cpp
/* BasePlusCommissionEmp.cpp using composition Created on: 31-Jul-2018 :rajesh@acroknacks.com */
#include<string>// C++ standard string class
#include"BasePlusCommissionEmp.h"
#include<iostream>
usingnamespace std;

BasePlusCommissionEmp::BasePlusCommissionEmp(conststring&fname, conststring&lname, conststring&ssn1,
		doublebaseSalary, doublegrossSales , doublecomRate )
:firstName (fname), lastName ( lname),socialSecurityNumber (ssn1 )
{
	setBaseSalary(baseSalary ); // validate and store base salary
	setGrossSales(grossSales);//validate and store gross sales
	setCommissionRate(comRate);//validate and store commision rate

}// end constructor

/&Functions Below  are specific to This class */
// set gross sales amount
voidBasePlusCommissionEmp::setGrossSales( double sales )
	{
	if ( sales **gt;= 0.0 )
		grossSales = sales;
	else
		throwinvalid_argument( "Gross sales must be >= 0.0" );
} // end function setGrossSales

// return gross sales amount
doubleBasePlusCommissionEmp::getGrossSales() const
{
	returngrossSales;
} // end function getGrossSales

// set commission rate
voidBasePlusCommissionEmp::setCommissionRate( double rate )
{

	if ( rate > 0.0 && rate < 1.0 )
		commissionRate = rate;
	else
		throwinvalid_argument( "Commission rate must be > 0.0 and < 1.0" );
} // end function setCommissionRate
doubleBasePlusCommissionEmp::getCommissionRate() const
{
	returncommissionRate;
} // end function getCommissionRate

voidBasePlusCommissionEmp::setBaseSalary( double salary )
{
	if ( salary >= 0.0 )
	baseSalary = salary;
	else
	throwinvalid_argument( "Salary must be >= 0.0" );
} // end function setBaseSalary
// return base salary
doubleBasePlusCommissionEmp::getBaseSalary() const
{
	returnbaseSalary;
} // end function getBaseSalary

//compute earnings
doubleBasePlusCommissionEmp::earnings() const
{
	return ( (getCommissionRate() * getGrossSales()) + getBaseSalary()) ;
} // end function earnings
// print CommissionEmployee object
voidBasePlusCommissionEmp::print() const
{
	cout<<"\nBasePlusCommission employee: ";
	cout<<lastName<<", "<<firstName<<endl;
	cout<<"SSN : "<<socialSecurityNumber<<endl;
	cout<<"\n gross sales: $ "<<getGrossSales()
	<<"\n Base Salary: $ "<<getBaseSalary()
	<<"\n commission rate: "<<getCommissionRate() ;
} // end function print

/&Generic Employee functions **/
// set first name
voidBasePlusCommissionEmp::setFirstName( conststring**first )
{
	firstName = first; // should validate
} // end function setFirstName
// return first name
stringBasePlusCommissionEmp::getFirstName() const
{
	returnfirstName;
} // end function getFirstName
// set last name
voidBasePlusCommissionEmp::setLastName( conststring&last )
{
	lastName = last; // should validate
} // end function setLastName
// return last name
stringBasePlusCommissionEmp::getLastName() const
{
	returnlastName;
} // end function getLastName
// set social security number
voidBasePlusCommissionEmp::setSocialSecurityNumber( conststring&ssn )
{
	socialSecurityNumber = ssn; // should validate
} // end function setSocialSecurityNumber
// return social security number
stringBasePlusCommissionEmp::getSocialSecurityNumber() const
{
	returnsocialSecurityNumber;
} // end function getSocialSecurityNumber

Test Program
// Testing class BasePlusCommissionEmp.
#include<iostream>
#include<iomanip>
#include"BasePlusCommissionEmp.h"// BasePlusCommissionEmp class definition
usingnamespace std;
intmain()
{
// instantiate a BasePlusCommissionEmp object
	BasePlusCommissionEmpemployee("Sue", "Jones", "222-22-2222",1500,10000,0.16 );
	// get commission employee data
	cout<<"Employee information obtained by get functions: \n"
			<<"\nFirst name is "<<employee.getFirstName()
			<<"\nLast name is "<<employee.getLastName()
			<<"\nSocial security number is "
			<<employee.getSocialSecurityNumber()
			<<"\nBase Salary is $"<<employee.getBaseSalary()
			<<"\nGross sales is $"<<employee.getGrossSales()
			<<"\nCommission rate is $"<<employee.getCommissionRate() <<endl;

	cout<<"Earnings based on current Data : $"<<employee.earnings();
	//Modify Sales data
		employee.setGrossSales( 8000 ); // set gross sales
		employee.setCommissionRate( .1 ); // set commission rate
cout<<"\nUpdated employee information output by print function: \n"
<<endl;
employee.print(); // display the new employee information
// display the employee's earnings
cout<<"\n\n Updated Employee's earnings: $"<<employee.earnings() <<endl;
} // end main

Sample Output

Employee information obtained by get functions:

First name is Sue
Last name is Jones
Social security number is 222-22-2222
Base Salary is $1500
Gross sales is $10000
Commission rate is $0.16

Earnings based on current Data : $3100

Updated employee information output by print function:

BasePlusCommission employee: Jones, Sue

SSN : 222-22-2222

gross sales: $ 8000

Base Salary: $ 1500

commission rate: 0.1

Updated Employee's earnings: $2300