C++ Programming Text book Chapter 09 .ppt

Copyright © 2012 Pearson Education, Inc.
Chapter 9:
Pointers

9.1
Getting the Address of a Variable

Getting the Address of a
Variable
• Each variable in program is stored at a
unique address
• Use address operator & to get address of
a variable:
int num = -99;
cout << &num; // prints address
// in hexadecimal

9.2
Pointer Variables

Pointer Variables
• Pointer variable : Often just called a
pointer, it's a variable that holds an
address
• Because a pointer variable holds the
address of another piece of data, it "points"
to the data

Something Like Pointers: Arrays
• We have already worked with something similar
to pointers, when we learned to pass arrays as
arguments to functions.
• For example, suppose we use this statement to
pass the array numbers to the showValues
function:
showValues(numbers, SIZE);

Something Like Pointers : Arrays
The values parameter, in the showValues
function, points to the numbers array.
C++ automatically stores
the address of numbers in
the values parameter.

Something Like Pointers:
Reference Variables
• We have also worked with something like pointers
when we learned to use reference variables.
Suppose we have this function:
void getOrder(int &donuts)
{
cout << "How many doughnuts do you want? ";
cin >> donuts;
}
• And we call it with this code:
int jellyDonuts;
getOrder(jellyDonuts);

Something Like Pointers:
Reference Variables
The donuts parameter, in the getOrder function,
points to the jellyDonuts variable.
C++ automatically stores
the address of
jellyDonuts in the
donuts parameter.

Pointer Variables
• Pointer variables are yet another way using a
memory address to work with a piece of data.
• Pointers are more "low-level" than arrays and
reference variables.
• This means you are responsible for finding the
address you want to store in the pointer and
correctly using it.

Pointer Variables
• Definition:
int *intptr;
• Read as:
“intptr can hold the address of an int”
• Spacing in definition does not matter:
int * intptr; // same as above
int* intptr; // same as above

Pointer Variables
• Assigning an address to a pointer variable:
int *intptr;
intptr = &num;
• Memory layout:
num intptr
25 0x4a00
address of num: 0x4a00

The Indirection Operator
• The indirection operator (*) dereferences
a pointer.
• It allows you to access the item that the
pointer points to.
int x = 25;
int *intptr = &x;
cout << *intptr << endl;
This prints 25.

9-16

9.3
The Relationship Between Arrays
and Pointers

The Relationship Between
Arrays and Pointers
• Array name is starting address of array
int vals[] = {4, 7, 11};
cout << vals; // displays
// 0x4a00
cout << vals[0]; // displays 4
4 7 11
starting address of vals: 0x4a00

The Relationship Between
Arrays and Pointers
• Array name can be used as a pointer
constant:
int vals[] = {4, 7, 11};
cout << *vals; // displays 4
• Pointer can be used as an array name:
int *valptr = vals;
cout << valptr[1]; // displays 7

9-20

Pointers in Expressions
Given:
int vals[]={4,7,11}, *valptr;
valptr = vals;
What is valptr + 1? It means (address in
valptr) + (1 * size of an int)
cout << *(valptr+1); //displays 7
cout << *(valptr+2); //displays 11
Must use ( ) as shown in the expressions

Array Access
• Array elements can be accessed in many ways:
Array access method Example
array name and [] vals[2] = 17;
pointer to array and [] valptr[2] = 17;
array name and subscript
arithmetic
*(vals + 2) = 17;
pointer to array and
subscript arithmetic
*(valptr + 2) = 17;

Array Access
• Conversion: vals[i] is equivalent to
*(vals + i)
• No bounds checking performed on array
access, whether using array name or a
pointer

From Program 9-7

9.4
Pointer Arithmetic

Pointer Arithmetic
• Operations on pointer variables:
9-26
Operation Example
int vals[]={4,7,11};
int *valptr = vals;
++, -- valptr++; // points at 7
valptr--; // now points at 4
+, - (pointer and int) cout << *(valptr + 2); // 11
+=, -= (pointer
and int)
valptr = vals; // points at 4
valptr += 2; // points at 11
- (pointer from pointer) cout << valptr–val; // difference
//(number of ints) between valptr
// and val

From Program 9-9

9.5
Initializing Pointers

Initializing Pointers
• Can initialize at definition time:
int num, *numptr = &num;
int val[3], *valptr = val;
• Cannot mix data types:
double cost;
int *ptr = &cost; // won’t work
• Can test for an invalid address for ptr with:
if (!ptr) ...

9.6
Comparing Pointers

Comparing Pointers
• Relational operators (<, >=, etc.) can be
used to compare addresses in pointers
• Comparing addresses in pointers is not
the same as comparing contents pointed
at by pointers:
if (ptr1 == ptr2) // compares
//
addresses
if (*ptr1 == *ptr2) // compares
// contents
9-31

9.7
Pointers as Function Parameters

Pointers as Function
Parameters
• A pointer can be a parameter
• Works like reference variable to allow change to
argument from within function
• Requires:
1) asterisk * on parameter in prototype and heading
void getNum(int *ptr); // ptr is pointer to an int
2) asterisk * in body to dereference the pointer
cin >> *ptr;
3) address as argument to the function
getNum(&num); // pass address of num to getNum

Example
void swap(int *x, int *y)
{ int temp;
temp = *x;
*x = *y;
*y = temp;
}
int num1 = 2, num2 = -3;
swap(&num1, &num2);

(Program Continues)

Pointers to Constants
• If we want to store the address of a
constant in a pointer, then we need to
store it in a pointer-to-const.

• Example: Suppose we have the following
definitions:
const int SIZE = 6;
const double payRates[SIZE] =
{ 18.55, 17.45, 12.85,
14.97, 10.35, 18.89 };
• In this code, payRates is an array of
constant doubles.
9-38

• Suppose we wish to pass the payRates array to
a function? Here's an example of how we can do
it.
void displayPayRates(const double *rates, int size)
{
for (int count = 0; count < size; count++)
{
cout << "Pay rate for employee " << (count + 1)
<< " is $" << *(rates + count) << endl;
}
}
The parameter, rates, is a pointer to const double.

Declaration of a Pointer to
Constant

Constant Pointers
• A constant pointer is a pointer that is
initialized with an address, and cannot
point to anything else.
• Example
int value = 22;
int * const ptr = &value;

Constant Pointers

Constant Pointers to Constants
• A constant pointer to a constant is:
– a pointer that points to a constant
– a pointer that cannot point to anything except
what it is pointing to
• Example:
int value = 22;
const int * const ptr = &value;

Constant Pointers to Constants

9.8
Dynamic Memory Allocation

• Can allocate storage for a variable while
program is running
• Computer returns address of newly
allocated variable
• Uses new operator to allocate memory:
double *dptr;
dptr = new double;
• new returns address of memory location

• Can also use new to allocate array:
const int SIZE = 25;
arrayPtr = new double[SIZE];
• Can then use [] or pointer arithmetic to access array:
for(i = 0; i < SIZE; i++)
*arrayptr[i] = i * i;
or
for(i = 0; i < SIZE; i++)
*(arrayptr + i) = i * i;
• Program will terminate if not enough memory available to
allocate

Releasing Dynamic Memory
• Use delete to free dynamic memory:
delete fptr;
• Use [] to free dynamic array:
delete [] arrayptr;
• Only use delete with dynamic memory!

Program 9-14 (Continued)

Notice that in line 49 the value 0 is assigned to the sales pointer. It is a
good practice to store 0 in a pointer variable after using delete on it. First,
it prevents code from inadvertently using the pointer to access the area of
memory that was freed. Second, it prevents errors from occurring if
delete is accidentally called on the pointer again. The delete operator
is designed to have no effect when used on a null pointer.

9.9
Returning Pointers from Functions

Returning Pointers from
Functions
• Pointer can be the return type of a function:
int* newNum();
• The function must not return a pointer to a local
variable in the function.
• A function should only return a pointer:
– to data that was passed to the function as an
argument, or
– to dynamically allocated memory

From Program 9-15

C++ Programming Text book Chapter 09 .ppt

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