CSCI 1060U Lecture 09 - Pointers in C++ II.pptx

Pointer Variables

Last Lecture §  What is a pointer?

§  A pointer “points” to another variable §  It is the address/location of that variable in memory

§  We have seen pointers in our discussion of call-by-reference parameters §  A call-by-reference parameter is an example of a

pointer to a variable

© J.S. Bradbury

void bubbleSort(string& s1);

CSCI 1060U Lecture 9 Slide 1

Pointer Variables

Last Lecture §  A pointer is declared using an asterisk (*)

§  Each pointer can point to a variable of only one type §  Example: *p2 can point to a variable of type int but

not a variable of type double.

© J.S. Bradbury

double *p1; int *p2; float *p3, *p4;


The Dereferencing Operator and the Address-of Operator

Last Lecture §  The asterisk (*) before the name of a pointer

variable is referred to as the dereferencing operator §  The ampersand (&) is used in from of a non-pointer

variable (e.g., int or float variable) to get the variable’s address

§  We can use the & to allow a pointer variable to point to an already declared variable.

© J.S. Bradbury CSCI 1060U Lecture 9 Slide 3

Defined Pointer Types

Last Lecture §  Instead of always using the asterisk (*) for pointer

variables we can also define pointer types

© J.S. Bradbury

int *p1; *p1 = 32; cout << *p1 << endl;

typedef int* IntPtr; IntPtr p1; *p1 = 32; cout << *p1 << endl;

=


Dynamic Variables

Last Lecture §  Pointers are not only used to point to ordinary (also

called automatic) variables §  We can also use a pointer to create a new dynamic

variable §  This variable is created and destroyed while the

program is executing

© J.S. Bradbury

int *p1;

p1 = new int; //created

delete p1; // destroyed


Dynamic Arrays

§  We have learned how to create dynamic arrays using pointers

§  Last class we finished with an in class activity to make sure everybody understands this…

© J.S. Bradbury

IntPtr p1;

int size = 30;

p1 = new int[size];


© J.S. Bradbury

In Class Activity #1 (Last Lecture): In C++ write a program that asks the user for the number of integers to be input. The program will create a dynamic array to store the integers in and will output the number of each integer in the array. Sample Input: Enter the size: 6 Enter the integers: 1 5 3 1 5 4 Sample Output: 1 -> 2 times 3 -> 1 time 4 -> 1 time 5 -> 2 times


Pointers in C++ II

Overview §  Today we will continue to look at the concept of

pointers §  This lecture will again focus only on C++ (since Java

does not have pointers) §  Some topics covered today include:

§  More on dynamic arrays §  Multidimensional dynamic arrays


Dynamic Variables

§  Last lecture we learned that you can destroy a dynamic variable using delete

§  However when we run the program and delete a dynamic variable we can still display its value §  Why? Delete means that the memory is now free to

be used by it is not necessarily deleted

© J.S. Bradbury

int *p1; p1 = new int; //created delete p1; // destroyed

See pointers_ex4.cpp for an example.


© J.S. Bradbury

In Class Group Activity: In C++ write a program that inputs any number of positive integers from the user. The user should input a “-1” when all of the positive integers have been entered. Use a dynamic array to store the integers.


See pointers_ex5.cpp for the solution.

Dynamic Arrays

§  In C++ an array cannot be returned by a function §  One possible solution to this problem is to return a

pointer to a dynamic array.

© J.S. Bradbury

See pointers_ex6.cpp for an example.


Multidimensional Dynamic Arrays

§  We can use a pointer to an array of pointers to create multidimensional arrays.

§  For example the below code creates a 5 by 3 array.

© J.S. Bradbury

typedef int* ArrayPtr;

ArrayPtr *p1 = new ArrayPtr[5];

for (int i=0; i < 5; i++) {

p1[i] = new int[3];

}


Multidimensional Dynamic Arrays

§  When using a pointer to an array of pointers to it is important to remember to delete all of the pointers

§  For example to delete our 5 by 3 array we need to

© J.S. Bradbury

for (int i=0; i < 5; i++) {

delete[ ] p1[i];

}

delete[ ] p1;


Pointers in C++ II

Summary §  We have finished discussing the concept of pointers §  We have learned how to use pointers to create

multidimensional dynamic arrays Next time §  Introduction to object-oriented programming

§  Structures and classes in C++


Pointers in C++ II

Readings §  Walter Savitch, Absolute C++, Chapter 10

References §  …


CSCI 1060U Lecture 09 - Pointers in C++ II.pptx

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