C++ Using Reference Variables as Function Parameters #197
Description
Concept: When used as parameters, reference variables allow a function to access the parameter's original argument. Changes to the parameter are also made to the argument.
A reference variable is an alias for another variable.
Reference variables are defined like regular variables, except you place an ampersand (&) in front of the number.
Example01:
#include <iostream>
using namespace std;
void addOne(int&);
int main() {
int y = 1;
cout << "In main function, y is " << y << endl;
addOne(y);
cout << "After executing `addOne(y);` y is now " << y << endl;
return 0;
}
// The variable x is called "a reference to an int."
void addOne(int& x) {
x++;
}Output:
In main function, y is 1
After executing `addOne(y);` y is now 2
The parameter x "points" to the y variable in function main.
When a program works with a reference variable, it is actually working with the variable it references, or to which it points.
Recall that function arguments are normally passed by value, which means a copy of the argument's value is passed into the parameter variable.
When a reference parameter is used, it is said that the argument is passed by reference.
Example02:
Function parameters passed by value (copy):
#include <iostream>
using namespace std;
void max_min(float, float);
int main() {
float larger, smaller;
cout << "Type in two values for max and min: ";
cin >> larger >> smaller;
max_min(larger, smaller);
cout << "The larger number is: " << larger << endl;
cout << "The smaller number is: " << smaller << endl;
return 0;
}
void max_min(float max, float min) {
float temp;
if (max < min) {
temp = max;
max = min;
min = temp;
}
cout << "max: " << max << ", min: " << min << endl;
}Output:
Type in two values for max and min: 100 200
max: 200, min: 100
The larger number is: 100
The smaller number is: 200
Function parameters passed by reference:
#include <iostream>
using namespace std;
void max_min(float&, float&);
int main() {
float larger, smaller;
cout << "Type in two values for max and min: ";
cin >> larger >> smaller;
max_min(larger, smaller);
cout << "The larger number is: " << larger << endl;
cout << "The smaller number is: " << smaller << endl;
return 0;
}
void max_min(float& max, float& min) {
float temp;
if (max < min) {
temp = max;
max = min;
min = temp;
}
}Output:
Type in two values for max and min: 100 200
The larger number is: 200
The smaller number is: 100
Example03:
#include <iostream>
using namespace std;
void winner_loser(float, float, float&, float&);
int main() {
float p1_score, p2_score, win_score, lose_score;
cout << "What is player1's score? ";
cin >> p1_score;
cout << "What is player2's score? ";
cin >> p2_score;
winner_loser(p1_score, p2_score, win_score, lose_score);
cout << "The winner's score was: " << win_score << endl;
cout << "The loser's score was: " << lose_score << endl;
return 0;
}
void winner_loser(float player1_score, float player2_score, float& winner_score, float& loser_score) {
if (player1_score > player2_score) {
winner_score = player1_score;
loser_score = player2_score;
}
else {
winner_score = player2_score;
loser_score = player1_score;
}
}Output:
What is player1's score? 100
What is player2's score? 200
The winner's score was: 200
The loser's score was: 100
Example04:
// This program asks the user to enter the diameter of a circle,
// and then calculates the circumference of the circle.
#include <iostream>
using namespace std;
// Function prototypes
double getCircumference(double);
void setRadius(double, double&);
// Global variable
const double PI = 3.14159265;
int main() {
double diameter, radius, circumference;
cout << "What is the diameter (diameter must be greater than 0)? ";
cin >> diameter;
// Get the value of the radius
setRadius(diameter, radius);
// Print the radius
cout << "The radius is " << radius << endl;
// Assign the value (circumference) to the variable circumference
circumference = getCircumference(radius);
cout << "The circumference of the circle is " << circumference << endl;
return 0;
}
// Calculates and returns the circumference of the circle.
double getCircumference(double circle_radius) {
double circle_circumference;
circle_circumference = 2 * PI * circle_radius;
return circle_circumference;
}
// Has 2 parameters: the diameter which is passed by value
// and a radius which is passed by reference.
// Based on the diameter, the function will assign a value to the variable
// which radius references. (radius is half of the diameter.)
void setRadius(double circle_diameter, double& circle_radius) {
circle_radius = circle_diameter / 2;
}Output:
What is the diameter (diameter must be greater than 0)? 10
The radius is 5
The circumference of the circle is 31.4159