Lecture12-Friends Eception Handling

7/21/2019 Lecture12-Friends Eception Handling

1/27


2/27

Computer Programming II 2

OutlinesOutlines

FriendshipFriendship

Friend Function Friend Class

Exception HandlingException Handling Exception

Exception Handler

try, throw and catch block

Exception Propagation

Multiple catch Blocks

Exception Matching

Advantages o Exception Handling
http://fit.mmu.edu.my/http://fit.mmu.edu.my/


3/27



Friendship allows a class to selectively grant

other function or class the access to its private

and protected members.

A friend function is a function that can access

private and protected members of a class, eventhough the function itself is not a memberof the

class.

A friend class is a class that can access privateand protected members of another class
http://fit.mmu.edu.my/


4/27



Friendship is granted. So for a function A or

class B to access the private and protected

members of class C, C must grant A and B the

friendship

Friendship is not automatically bi-directional.hen A ma!es B a friend, B does not

automatically ma!e A a friend


5/27


// Problem

class Human {

string secret;

public:

Human (string secret)

: secret(secret) {}

};

voidtellSecret (Human h){

cout


6/27


// riend &unction solution

class Human {

string secret;

&riendvoidtellSecret (Human);

public:


: secret(secret) {}

};

voidtellSecret (Human h){ cout


7/27


// Problem

class Human{

string secret;

public:


: secret(secret) {}

};

class Parrot{public:

void tellSecret(Human h) {

cout


8/27


// riend class solution

class Human{ &riend class Parrot;

string secret;

public:


: secret(secret) {}

};class Parrot{

public:

void tellSecret (Human h) {

cout


9/27


Exception HandlingException Handling


10/27


"#ception"#ception

When a program is executed unexpected situation ma!

occur. "uch a situation is called an exception#n other $ord: Exceptionis a runtime error caused b!

some abnormal conditions

Example:

division b! %erofailure of ne*operator to obtain a re&uested amount of

memor!

Exception handleris code that handles the exception'runtime error( $hen it occurs


11/27


Exception Example: )ivision *! +ero

double divide (double +, double -) {

return + / -; // divide b- i& -

}

int main() {

double +, -;

cin + -;

cout


12/27


Exception Example: )ivision *! +ero


return + / -; // divide b- i& -

}

int main() {

double +, -;

cin + -;

i&(- ) cout


13/27


C,, implements exception handling using tr- thro*

and catchbloc

tr- bloc:-rite the code that might generate runtime error within

the tr-bloc!.

Format: tr- {

// 0ode that ma- generate

// e+ceptions}

Exception andling


14/27


thro*statement:-)se !eyword thro*in tr-bloc! to signal that abnormal

condition or error has occurred.-&f the thro*statement is e#ecuted, the codes in the try

bloc! that appear after the thro*statement won*t be

e#ecuted.

Format: tr- {

...

thro*


15/27


catchbloc:-rite the code that catches the thrown e#ception in

catchbloc!. +his is the exception handler.-)nhandled)ncaught thrown e#ception will terminate the

program.

Format: tr- {

...

thro*


16/27



i& (- )

thro* -; return + / -;

}

int main() {

double +, -;

cin + -;

tr- { double result divide (+, -);

cout


17/27



i& (- )


}

int main() {

double +, -;

cin + -;


cout


18/27



i& (- )


}

int main() {

double +, -;

cin + -;


cout


19/27


int main() {

double +, -;

cin + -;

tr- {

i& (- )

thro* -; double result + / -;

cout


20/27


Exception 4ropagation

#f the function7method containing the thro*statement

does not catch the exception the exception $ill be

propagated up to the callerof the function until itreaches a tr-bloc or themain function.

#n the former case the tr-/catchbloc of the caller

handles the exception if the exception t!pe matchesone of the catchbloc. Other$ise the exception $ill be

propagated up again.

#f the exception has reached themainfunction and is

not handled the program $ill be terminated.


21/27


Example: Exception 4ropagation

double divide% (double +, double -) {

i& (- ) thro* -;

return + / -;

}

double divide4 (double +, double -) {

return divide% (+, -);

}


return divide4 (+, -);

}

int main() {

...

tr- {

double result divide (+, -);

cout


22/27


Some times, we might have many different e#ceptions

for a small bloc! of code

. e should write as many catchbloc!s.

. +his also means that we should have as

many thro*statements.

/. B)+'usually0, only one tr-bloc!.

But, which catchbloc! will be instigatedinvo!ed%1epend on the type of parameter

8ultiple catch*locs


23/27


tr-{

// 0ode that could generate an e+ception

}catch(


24/27


int main () {

&unc (4);

&unc (%); &unc ();

return ;

}

"utput:

0atch an int argument

0atch a char argument

n is not 4, % or

void &unc (int n) {

tr- {

i& (n 4) thro* 44; // int i& (n %) thro* a; // char

cout


25/27


Exception 8atching

+o catch every possible e#ception type, use ellipsis 393

4imitationsofcatch (...)5ou can*t tell what typeof e#ception occurred(o argument to reference

Should always be placed as the last catch

tr- {

}

catch (...) { // catches all e+ceptions

}


26/27


Exception 8atchingint main () {

&unc (4);

&unc (%); &unc ();

&unc ();

return ;

}

"utput:

0atch an int argument

0atch a char argument

5ot int nor char

n is not 4, % or

void &unc (int n) {

tr- {

i& (n 4) thro* 44; // int i& (n %) thro* a; // char

i& (n ) thro* .6; // double

cout


27/27


dvantages of Exception andling

;sing tr- thro* and catchblocs to handleexception offer the follo$ing advantages:

ou ma! thro$ an exception in afunction7method and handle it some$here else

Documents

Lecture12-Friends Eception Handling