Computer Notes - Techniques for Error Handling

8/3/2019 Computer Notes - Techniques for Error Handling

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Techniques for Error Handling


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Techniques for Error Handling

Abnormal termination Graceful termination

Return the illegal value

Return error code from a function

Exception handling

http://ecomputernotes.com


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Example Abnormal Termination

void GetNumbers( int &a, int &b ) {

cout > a >> b;

}

int Quotient( int a, int b ){return a / b;

}

void OutputQuotient( int a, int b, int quo ) {cout


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Example Abnormal Terminationint main(){

int sum = 0, quot;

int a, b;for (int i = 0; i < 10; i++){

GetNumbers(a,b);

quot = Quotient(a,b);sum += quot;

OutputQuotient(a,b,quot);

}cout


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OutputEnter two integers

1010

Quotient of 10 and 10 is 1Enter two integers

100

Program terminated abnormally


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Graceful Termination

Program can be designed in such away that instead of abnormal

termination, that causes the wastageof resources, program performs

clean up tasks



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Example Graceful Terminationint Quotient (int a, int b ) {if(b == 0){

cout


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1010

Quotient of 10 and 10 is 1Enter two integers

100

Denominator cant be zero


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Error Handling

The clean-up tasks are of localnature only

There remains the possibility ofinformation loss



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Example Return Illegal Valueint Quotient(int a, int b){if(b == 0)

b = 1;OutputQuotient(a, b, a/b);

return a / b ;

}int main() {

int a,b,quot; GetNumbers(a,b);

quot = Quotient(a,b);

return 0;

}



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100

Quotient of 10 and 1 is 10



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Error Handling

Programmer has avoided the systemcrash but the program is now in an

inconsistent state



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Example Return Error Code

bool Quotient ( int a, int b, int & retVal ) {if(b == 0){

return false;

}retVal = a / b;

return true;

}



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Part ofmain Functionfor(int i = 0; i < 10; i++){GetNumbers(a,b);

while ( ! Quotient(a, b, quot) ) {

cout


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OutputEnter two integers10

0

Denominator cant be zero. Give input again.

Enter two integers10

10


...//there will be exactly ten quotients



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Error Handling

Programmer sometimes has tochange the design to incorporate

error handling Programmer has to check the return

type of the function to know whetheran error has occurred



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Error Handling

Programmer of calling function canignore the return value

The result of the function mightcontain illegal value, this may cause

a system crash later



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Programs Complexity Increases

The error handling code increases

the complexity of the code

Error handling code is mixed with

program logic

The code becomes less readable

Difficult to modify



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Exampleint main() {

function1();function2();

function3();

return 0;

}http://ecomputernotes.com


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Exampleint main(){if( function1() ) {

if( function2() ) {

if( function3() ) {...

}

else cout


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Exception Handling

Exception handling is a muchelegant solution as compared to

other error handling mechanisms It enables separation of main logic

and error handling code



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Exception Handling Process Programmer writes the code that is

suspected to cause an exception intry block

Code section that encounters anerrorthrows an object that is used to

represent exception

Catch blocks follow try block to catch

the object thrown


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Syntax - Throw The keyword throw is used to throwan exception

Any expression can be used torepresent the exception that has

occurred

throw X;throw (X);



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Examplesint a;Exception obj;

throw 1; // literalthrow (a); // variable

throw obj; // objectthrow Exception();

// anonymous object

throw 1+2*9;// mathematical expression



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Throw Primitive data types may be avoided

as throw expression, as they cancause ambiguity

Define new classes to represent theexceptions that has occurred

This way there are less chances of

ambiguity



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Syntax Try and Catchint main () {try {

...}

catch ( Exception1 ) {

...

}

catch ( Exception2 obj ) {

...

}return 0;

}http://ecomputernotes.com


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Catch Blocks Catch handler must be preceded by

a try block or an other catch handler Catch handlers are only executed

when an exception has occurred Catch handlers are differentiated on

the basis of argument type



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Catch Handler

The catch blocks are tried in orderthey are written

They can be seen as switchstatement that do not need break

keyword



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Exampleclass DivideByZero {public:

DivideByZero() {}

};

int Quotient(int a, int b){

if(b == 0){

throw DivideByZero();

}

return a / b;}



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Body ofmain Functionfor(int i = 0; i < 10; i++) {

try{

GetNumbers(a,b);quot = Quotient(a,b);

OutputQuotient(a,b,quot); sum += quot;

}

catch(DivideByZero) {

i--;

cout


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10

10


Enter two integers10

0

Attempt to divide numerator with zero...

// there will be sum of exactly ten quotients


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Catch Handler

The catch handler catches the

DivideByZero object throughanonymous object

Program logic and error handling

code are separated

We can modify this to use the object

to carry information about the cause

of error

S ti f P L i d


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Separation of Program Logic and

Error Handlingint main() {try {

function1();function2();

function3();

}catch( ErrorX) { ... }

catch( ErrorY) { ... }

catch( ErrorZ) { ... }

return 0;

}


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Computer Notes - Techniques for Error Handling