An example demonstrating the use of utility functions within classes in OOP. Gotten from the book C How to Program.

main.cpp

// Demonstrating utility function

#include "SalesPerson.h"

int main(){
    SalesPerson s; // create a SalesPerson object

    s.getSalesFromUser();
    s.printAnnualSales();

    return 0;
}

SalesPerson.cpp

// Implementation for class SalesPerson
#include <iostream>
using std::cout;
using std::cin;
using std::endl;
using std::fixed;

#include <iomanip>
using std::setprecision;

#include "SalesPerson.h" // Include SalesPerson definition class

// Initialize elements of array sales to 0.0
SalesPerson::SalesPerson(){
    for ( int i = 0; i < 12; i++ )
        sales[ i ] = 0.0;
}

// get sales figure from the user
void SalesPerson::getSalesFromUser(){
    double salesFigure;

    for ( int i = 1; i <= 12; i++ ){
        cout << "Enter sales account for month " << i << ": ";
	cin >> salesFigure;
	setSales( i, salesFigure );
    }
}

// set sales for a particular month in the array
void SalesPerson::setSales( int month, double amount ){
    // test for valid month and amount values
    if ( month >= 1 && month <= 12 && amount > 0 )
        sales[month - 1] = amount; // adjust for subscription 0-11
    else
        cout << "Invalid month or sales figure" << endl;
}

// print total annual sales using the private utility function
void SalesPerson::printAnnualSales(){
    cout << setprecision( 2 ) << fixed
	    << "\nThe total anual sales are: "
	    << totalAnnualSales() << endl;
}

// private utility function to total annual sales
double SalesPerson::totalAnnualSales(){
    double total = 0.0 ; // initialize total

    for ( int i = 0; i < 12; i++ ) // summarize sales result
        total += sales[ i ];

    return total;
}

SalesPerson.h

// SalesPerson class definition

#ifndef SALESP_H
#define SALESP_H

class SalesPerson{
    public:
        SalesPerson(); // constructor
        void getSalesFromUser(); //  input sales from keyboard
        void setSales( int, double ); // set sales for specific month
        void printAnnualSales(); // summarize and print sales
    private:
        double totalAnnualSales(); // prototype for utility function
        double sales[ 12 ]; // 12 monthly sales figures
};

#endif