Strings: C-strings vs. Strings as Objects - PowerPoint Presentation

Slide1

Strings: C-strings vs. Strings as Objects

Andy Wang

Object Oriented Programming in C++

COP 3330Slide2

C-style Strings

C-string is a null-terminated array of type char

No built-in string type in C

Must be char arrays

NOT every char array is a C-string, only null-terminated onesSlide3

Library Features

C++ <

cstring

> library provides functions to work with C-style strings

strlen

() // string length

s

trcpy

(),

strncpy

() // string copy

s

trcat

(),

strncat

() // string concatenation

s

trcmp

(),

strncmp

() // string comparison

s

trstr

() // string search

s

trtok

() // string tokenizationSlide4

Some Implementation Details

i

nt

strlen

(

const

char *s) {

for (

int

j = 0; s[j] != ‘\0’;

j++

);

return j;

}

c

har *

strcpy

(char *s1,

const

char *s2) {

int

j = 0;

for (; s2[j] != ‘\0’;

j++

) s1[j] = s2[j];

s1[j] = s2[j];

return s1;

}Slide5

Some Implementation Details

int

strcmp

(

const

char *s1,

const

char *s2) {

int

j = 0;

for (; ((s1[

i

] != ‘\0’) && (s1[

i

] == s2[

i

]));

j++

);

return (s1[j

] -

s2[j]);

}

c

har *

strcat

(char *s1,

const

char *s2) {

int

i

=

strlen

(s1), j = 0;

for (; s2[j] != ‘\0;

i

++,

j++

) s1[

i

] = s2[j];

s1[

i

] = s2[j];

return s1;

}Slide6

C++ <iostream

> Library

Provides I/O handling functions for C-style strings

c

har

str

[40];

c

out

<< str1; // insertion

c

in

>> str1; // extraction

c

in.get

(str1, 40, ‘,’); // reads to delimiter (comma)

c

in.getline

(str1, 40); // reads to delimiter (default is

endl

)

// discards delimiterSlide7

The DOWN Side of C-strings

Fixed length

String name acts as a pointer

Needs

to be passed in and out of functions by reference

Must be careful with array boundaries

C-string boundaries

are not

automatically detected, even for library functionsSlide8

More Hurdles for Common Ops

c

har greeting[40];

// can’t do greeting = “Hello World”;

instead

s

trcpy

(greeting, “Hello World”);

// can’t do if (str1 == str2); instead

i

f (

strcmp

(str1, str2) == 0)

A C coder needs to dynamically resize c-stringsSlide9

C-String Pitfalls

C-string library functions often assume the use of ‘\0’ to terminate a string

What prints here?

c

har vowel[5] = {‘A’, ‘E’, ‘I’, ‘O’, ‘U’};

c

out

<< vowels; // is this a c-string?

Here is an attempt to copy one

C-string

to another

char greeting[25] = “Take me to your leader”;

c

har welcome[10] = “Hello”;

s

trcpy

(welcome, greeting); // anything to worry about?Slide10

C-String Pitfalls

Suppose we have enough space

char buffer[40] = “Dog” // length 3, capacity 39

char word2[] = “food”; // length 4, capacity 4

strcat

(buffer, word2); // buffer = “Dogfood”

strcat

(buffer, “ breath”); // buffer

// = “Dogfood breath”

strcat

(buffer, buffer); // should be enough room?Slide11

Example

Non null terminating string

http://www.cs.fsu.edu/~

myers/cop3330/examples/strings/pitfalls/output.cppSlide12

output.cpp

#include <

iostream

>

using namespace

std

;

int

main

() {

char temperatures[3] = {'K', 'F', 'C'};

char vowels[5] = {'A', 'E', 'I', 'O', 'U'};

char directions[4] = {'N', 'S', 'E', 'W'};

char digits[10] = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9'};

Slide13

output.cpp

cout

<< "None of these are 'c-strings'. What if we print with << ?\n";

cout

<< "temperatures = " << temperatures << '\n';

cout

<< "digits = " << digits << '\n';

cout

<< "vowels = " << vowels << '\n';

cout

<< "directions = " << directions << '\n

';

}Slide14

Example

Self overwrite

http://www.cs.fsu.edu/~

myers/cop3330/examples/strings/pitfalls/copy.cppSlide15

copy.cpp

#include <

iostream

>

#include <

cstring

>

using namespace

std

;

int

main

() {

char greeting[25] = "Take me to your leader";

char welcome[10] = "Hello";

cout

<< "greeting[] = " << greeting << '\n';

cout

<< "welcome[] = " << welcome << '\n

';

Slide16

copy.cpp

strcpy

(welcome

, greeting

);//

anything to worry about?

cout

<< "greeting[] = " << greeting << '\n';

cout

<< "welcome[] = " << welcome << '\n';

// WILL this behave the same on all systems?

}Slide17

Example

Self concatenation

http://www.cs.fsu.edu/~myers/cop3330/examples/strings/pitfalls/concat.cppSlide18

concat.cpp

#include <

iostream

>

#include <

cstring

>

using namespace

std

;

int

main

() {

char buffer[40] = "Dog"; // length 3, capacity 39

char word2[] = "food"; // length 4, capacity 4

cout

<< "buffer = " << buffer << '\n';

cout

<< "word2 = " << word2 << '\n';

Slide19

concat.cpp

strcat

(buffer

, word2); // buffer is now "Dogfood"

cout

<< "buffer = " << buffer << '\n

';

strcat

(buffer, " breath");

// buffer is now

// "

Dogfood breath"

cout

<< "buffer = " << buffer << '\n

';

strcat

(buffer, buffer); // plenty of

room?

cout

<< "buffer = " << buffer << '\n

';

return 0;

}Slide20

A String Wish List

How should ideal strings behave?

Flexibility in storage capacity

Simple assignment, comparison, concatenation

Options to pass string objects by value, reference or

const

referenceSlide21

Building a S

tring class

Use dynamic allocation and resizing to get flexible capacity

Do dynamic memory management inside the class

Use operator overloads

Insertion, extraction operators for easy

IOs

Comparison operator to ease sorting

Operator+ for concatenationSlide22

Building a S

tring class

Build copy constructor and assignment operator for assignment and

pass-by-value

capabilities

Build conversion construction to convert c-style strings to string objects

Could include conversion constructors for converting other types to strings, tooSlide23

Example: BString

class

http://www.cs.fsu.edu/~myers/cop3330/examples/strings/bstring/Slide24

bstring.h

#include <

iostream

>

using namespace

std

;

class

BString

{

friend

ostream

&operator

<<(

ostream

&

os

,

const

BString

&s

);

friend

istream

&operator

>>(

istream

&is

,

BString

&s

);

friend

istream

&

getline

(

istream

&is

,

BString

&s

,

char

delim

='\n

');Slide25

bstring.h

friend

bool operator

<(

const

BString

&s1

,

const

BString

&s2

);

friend bool operator

>(

const

BString

&s1

,

const

BString

&s2

);

friend bool operator

<=(

const

BString

&s1

,

const

BString

&s2

);

friend bool operator

>=(

const

BString

&s1

,

const

BString

&s2

);

Slide26

bstring.h

friend

bool operator

==(

const

BString

&s1

,

const

BString

&s2

);

friend

bool operator

!=(

const

BString

&s1

,

const

BString

&s2);

friend

BString

operator

+(

const

BString

&s1

,

const

BString

&s2);

public:

BString

(); // create an empty string

BString

(

const

char* c);

//

conversion from

C-string

Slide27

bstring.h

~

BString

();

//

destructor, since dynamic

management

BString

(

const

BString

&s);

//

copy constructor

BString

&

operator=(

const

BString

&s);

BString

&

operator+=(

const

BString

&s);

int

length()

const

; // return length of

string

private:

char *

str

; // pointer to my dynamic array of chars

int

size; // size of the string

// allocation will always be

size+1

};Slide28

bstring.cpp

#include <

cstring

>

#include "

bstring.h

"

o

stream

&operator

<<(

ostream

&

os

,

const

BString

&s) { }

i

stream

&operator

>>(

istream

&is

,

BString

&s) { }

i

stream

&

getline

(

istream

& is,

BString

&s,

char

delim

) {}

bool operator

<(

const

BString

&s1

,

const

BString

&s2){ }

bool operator

>(

const

BString

&s1

,

const

BString

&s2){ }Slide29

bstring.cpp

bool

operator

<=(

const

BString

&s1

,

const

BString

&s2

)

{ }

bool operator

>=(

const

BString

&s1

,

const

BString

&s2) { }

bool operator

==(

const

BString

&s1

,

const

BString

&s2) { }

bool operator

!=(

const

BString

&s1

,

const

BString

&s2) { }Slide30

bstring.cpp

BString

operator

+(

const

BString

&s1

,

const

BString

&s2) { }

BString

::

BString

() { size = 0;

str

= 0; }

BString

::

BString

(

const

char* c

) {

size =

strlen

(c);

str

= new char[size+1

];

strcpy

(

str

, c); // can use

cstring

functions,

//

as long as allocation is enough for '\0'

}Slide31

bstring.cpp

BString

::~

BString

() { }

BString

::

BString

(

const

BString

&s) { }

BString

&

BString

::operator=(

const

BString

&s) { }

BString

&

BString

::operator+=(

const

BString

&s) { }

int

BString

::length()

const

{ }Slide32

main.cpp

#include <

iostream

>

#include "

bstring.h

"

using namespace

std

;

int

main

() {

BString

s1; // should be empty string

BString

s2 = "Hello"; // should be "Hello"

cout

<< "s1 = " << s1 << '\n';

cout

<< "s2 = " << s2 << '\

n

';

}Slide33

Exercise

Fill in functions that are yet to be defined

Add test calls to the driver program