© Janice Regan, CMPT 128, February. 2007 0 CMPT 128: Introduction to Computing Science for Engineering Students Pointers

© Janice Regan, CMPT 128, February. 2007 1

CMPT 128: Introduction to Computing Science for Engineering Students

Pointers

© Janice Regan, CMPT 128, 2007-2013 2

What is a variable? A variable is stored in a particular location

in memory A variable is given an identifier (name)

when it is declared. We refer to the variable using that identifier

A variable has a type int myvariable1; //type int float myarray[10]; //type float[10]


Types of Constants and Variables A Data Type is

A set of values A set of operations that can be done on those values A crucial concept on modern programming

The data type of a variable or constant determines how the variable’s value will be represented in memory how many bytes are used to represent the variable

Data types may represent Numbers, addresses Characters or strings Other objects


What is a pointer? Each variable is stored at some memory address,

Count successive locations in memory Assume the first memory location counted has

address 0, the second address 1, and so on Each location in memory has an address, that

address can be represented as an integer A pointer (or reference) is a special type of variable that

holds a memory address A pointer containing the address of a variable ‘points to‘

or ‘references’ that variable


Types for pointers Include a set of values

legal memory addresses Include a set of operations on those values

+, -, --, ++ (meanings of operator somewhat different from simple arithmetic definitions)

Include a method to represent the values within the computer: Addresses are represented like integers.

IMPORTANT: Addresses are not integers, they have different properties and applications than integers.


Data Types, pointers and integers Data type int includes

a set of objects, the integers (… -10, -9,-8, … ,123, 124, …)

Operations that can be done on those objects (+, -, *, /, % …)

Data type ‘pointer to an integer’ includes A set of objects, all legal addresses for integers A set of operations +, - ,++, --

Data type ‘pointer to an double’ includes A set of objects, all legal addresses for doubles A set of operations +, - ,++, --


Declaring pointer variables Pointer variables can point to only one type of

variable

int *v1p; // pointer to an integer

double *v2p; // pointer to a double

char *v3p, *xp; // 2 pointers to char, need * for each

myStruct *v5p; //pointer to a structure of type myStruct


Pointers and Style It is useful to easily be able to see which variables in

your function are pointers C++ does not enforce any particular structure on your

variable and pointer identifiers To distinguish pointers from other variables a number of

conventions are used in different coding standards In this course the coding standard suggested is to

assure that all pointer identifiers end in p to indicate they are pointers.

double myVariable, *myVariablep;

9

Meaning of ++, --, +, - There are several types of pointers A pointer to an integer points at an integer

++ means add the length of an int to the address A pointer to a long long int

-- means subtract the length of a long long int from the address

For pointer x to a double x = x+5 means add the 5 times the length of a

double to the pointer x

© Janice Regan, CMPT 128, 2007-2013


Pointers

76.2

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11

a

-44.567

1003

1005

1001

1002

1005

1003

1004

address

Value of variable

Identifier of variable

v1

v2

v5

v3

v4

Pointer Identifier

v3p

v5p

Value of pointer variable

Assuming length of each variable is 1 for

simplicity


Where do pointers point When you declare a pointer

int *v1p, v1; Pointer variable v1p points to a random location

in memory. Why? v1p has not been initialized, it contains

whatever was in the memory location associated with v1p before it was associated with v1p

This can cause serious problems, accessing and changing values that are not even associated with your program.


Initializing pointers After you declare a pointer

int *v2p;

It is good programming practice to initialize the pointer to NULL

v2p = NULL;

The value NULL is defined in <iostream>


Initializing pointers int v1=12, *v1p;

v1p = NULL;

?

1004

v1p

addressPointer Identifier


12

Variable Identifier

Variable Value

v1

NULL

1004

v1p



12

Variable Identifier

Variable Value

v1


& Operator (address of) To find the address that a particular variable

begins at you can use the unary operator &int v1, *v1p;

v1p = &v1; The operator & applied to the variable v1 gives

the address of v1. The value of the expression &v1 is the address of v1.

In the case illustrated above &v1 is assigned to the “pointer to integer” variable v1p (using = operator).


Pointers in assignment statements

int v1=12, *v1p;

v1p = &v1;

?

1004

v1p



12

Variable Identifier

Variable Value

v1

1004

1004

v1p



12

Variable Identifier

Variable Value

v1


Initializing pointers After you declare a pointer

int *v1p, v1, *v2p, *v3p;

It is good programming practice to initialize the pointer to NULL or some other particular value. A common non NULL value is the address of some other already declared (non pointer)

variable.

v1p = &v1;


* Operator: dereferencing The dereferencing operator can only be applied to a

pointer variable *myp dereferences pointer myp,

To dereference means to extracts the value of the variable pointed to by the pointer myp

The expression *myp (other than in a declaration) has a value that is the value of the variable being pointed to

double *myp=NULL, myv=29;

myp = &myv; //make myp point at myv

cout << *myp; //extract value of myv (pointed to by myp) and print it


Dereferencing pointers double *myp=NULL, myv=29;

myp = &myv;

NULL

1004

myp



29

Variable Identifier

Variable Value

myv

1004

1004

v1p



29

Variable Identifier

Variable Value

v1


Dereferencing pointers myp = &myv;

cout << *myp; Looks for the address in myp Goes to that address and interprets the value in the

location beginning at that address as an integer Prints the integer value to the screen

1004

1004

myp



29

Variable Identifier

Variable Value

myv


Dereferencing exampledouble f2, *f2p;

f2p = &f2;

f2 = 23;

cout << *f2p << “ “ << f2:

There are two ways to refer to the value of variable f2 f2 *f2p

The result printed by the code is

23 23


Dereferencing pointers double f2, *f2p;

f2p = &f2;

?

1004

f2p



?

Variable Identifier

Variable Value

f2

1004

1004

f2p



?

Variable Identifier

Variable Value

f2


Dereferencing pointers f2 = 23;

cout << *f2p << “ “ << f2: Looks for the address in f2p Goes to that address extracts the value beginning at that address Interprets the extracted value as an integer Prints the integer value to the screen Prints the value of variable f2 to the screen

10041004

f2p



23

Variable Identifier

Variable Value

f2


Another dereferencing Example Consider: start and *startp refer to the same variable

start = 33;

startp = &start; cout << start << endl;

*startp = 77; cout << start << “ “ << *startp << endl;

start = 34;

cout << start << “ “<< *startp<< endl; Produces output:

3377 7734 34


Pointers in assignment statements One pointer can be assigned, using an assignment

statement, to another pointer

firstp = secondp; After this statement firstp points to the same variable that

secondp pointed to before the statement

The contents of the variable a pointer points to can be replaced with the contents of a variable pointed to by another pointer

*firstp = *secondp After this statement the contents of the variable pointed to by

firstp is the same as the contents of the variable pointed to by secondp


Pointers in assignment statements Before executing statement

After executing statement firstp = secondp;

76.2

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100410031004

firstp

secondp

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firstp

secondp





Pointers in assignment statements Before executing statement

After executing statement *firstp = *secondp;

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100410031004

firstp

secondp

23

23

1003

1004

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1004


firstp

secondp




Documents

© Janice Regan, CMPT 128, February. 2007 0 CMPT 128: Introduction to Computing Science for Engineering Students Pointers