Math2220 Lecture 21

Pointer

What is pointers?

How to declare pointers?

Example 1: declare pointers 
#include <iostream>
using namespace std;

void main() {
    int *pointer, num = 5 ;

    char line[100];
    cin.getline(line,100);
}

Address-of operator

Example 2:Usage of & 
#include <iostream>
using namespace std;

void main() {
    int *pointer, num = 5;
    pointer = &num; // assign the address of num to the pointer
    cout << "The address of num is: "
         << pointer << endl;

    char line[100];
    cin.getline(line,100);
}

Output (your result may be different) 
The address of num is: 006AFDF0

Dereferencing operator

Example 3: Use dereferencing operator 
#include <iostream>
using namespace std;

void main() {
    int *pointer, num = 5;
    pointer = &num; // address of num
    cout << num << endl;
    cout << *pointer << endl; // the value the pointer points to.

    char line[100];
    cin.getline(line,100);
}
Output 
5
5

More examples

Example 4: What is the output of the following program? 
#include <iostream>
using namespace std;

void  main() {
    double *pt, x = 3.15;
    pt = &x;
    cout << x << endl;
    cout << *pt << endl;

    x = 4.2;
    cout << x << endl;
    cout << *pt << endl;

    *pt = 12.59;
    cout << x << endl;
    cout << *pt << endl;

    char line[100];
    cin.getline(line,100);
}

Sample output 
3.15
3.15
4.2
4.2
12.59
12.59

Example 5: What is the output of the following program? 
#include <iostream>
using namespace std;

void main() {
    double *pt1, *pt2, x1 = 3.15, x2 = 2.44;
    pt1 = &x1; // pt1 points to x1
    pt2 = &x2; // pt2 points to x2
    cout << x1 << " " << *pt1 << " "
         << x2 << " " << *pt2 << endl;

    pt1 = pt2; // pt1 points to what pt2 points to, i.e, x2 
    cout << x1 << " " << *pt1 << " "
         << x2 << " " << *pt2 << endl;

    x2++; // both pt1 and pt2 point to x2, the value of x2 has been changed.
    cout << x1 << " " << *pt1 << " "
         << x2 << " " << *pt2 << endl;

    pt2 = &x1; // pt2 points to x1, pt1 still points to x2
    x1 = 10.7;
    x2 = 2.98;
    cout << x1 << " " << *pt1 << " "
         << x2 << " " << *pt2 << endl;

    char line[100];
    cin.getline(line,100);
}

Sample output 
3.15 3.15 2.44 2.44
3.15 2.44 2.44 2.44
3.15 3.44 3.44 3.44
10.7 2.98 2.98 10.7

new operator

Refer to example 6, 7
Example 6: use of new operator 
#include <iostream>
using namespace std;

void main() {
    char *pt;
    pt = new char; //pt is an address of a character. It points to a char

    *pt = 'H';
    cout << *pt << endl;

    char line[100];
    cin.getline(line,100);
}

Output 
H

Example 7: What is the output of the following program 
#include <iostream>
using namespace std;

void main() {
    int *p1, *p2;

    p1 = new int; // p1 points to an integer
    *p1 = 42; // set the value of p1 points to to 42
    p2 = p1; // p2 and p1 points to the same integer

    cout << "*p1 == " << *p1 << endl;
    cout << "*p2 == " << *p2 << endl;

    *p2 = 53; // set the value of p2 points to to 53
    cout << "*p1 == " << *p1 << endl;
    cout << "*p2 == " << *p2 << endl;

    p1 = new int; // p1 points to a new integer integer
    *p1 = 88; // set the value of p1 points to to 88
    cout << "*p1 == " << *p1 << endl;
    cout << "*p2 == " << *p2 << endl;

    char line[100];
    cin.getline(line,100);
}

Sample output 
*p1 == 42
*p2 == 42
*p1 == 53
*p2 == 53
*p1 == 88
*p2 == 53