Exceptions - Exception Handling

A Program mainly has two type of errors: logical errors and syntactic errors.

The logical errors occur due to poor understanding of problem and syntactic errors arise due to poor understanding of language. These errors can be detected by debugging and testing procedures.

There are some problems called Exceptions that are run time anomalies or unusual conditions that a program may encounter while executing. 

These anomalies can be division by zero, access to an array outside of its bounds or running out of memory or disk space. When a program encounters an exceptional condition it is important to identify it and dealt with it effectively.

An exception is an object that is sent from the part of the program where an error occurs to that part of program which is going to control the error.

Exception Handling

The purpose of exception handling mechanism is to detect and report an exceptional circumstance so that appropriate action can be taken.

The mechanism for exception handling is

1. Find the problem (hit the exception).

2. Inform that an error has occurred (throw the exception).

3. Receive the error information (Catch the exception).

4. Take corrective actions (Handle the exception).

The error handling code mainly consists of two segments:

1. To detect error and throw exceptions and

2. To catch the exceptions and to take appropriate actions.

C++ exception handling mechanism is basically built upon three keywords:

try, throw and catch.

The keyword try is used to preface a block of statements which may generate exceptions. This block of statement is called try block. When an exception is detected it is thrown using throw statement in the try block.

A catch block defined by the keyword catch ‘catches’ the exception thrown by the throw statement in the try block and handles it appropriately. The catch block that catches an exception must immediately follow the try block that throws the exception.

The general form for this is

……………….// program code

………………..

try

{

…………

………… //block of statements which detects and throw an exceptions

throw exception;

…………….

…………….

}

catch(type arg) //catches exceptions

{

…………… // Block of statements that handles the exceptions

………………

…………….

}

………….// remaining program code

…………..

When the try block throws an exception, the program control leaves the try block and enters the catch statement of the catch block. If the type of object thrown matches the argument type in the catch statement, then the catch block is executed for handling the exception. If they do not match, the program is aborted with the help of abort() function which is executed implicitly by the compiler. When no exception is detected, the control goes to the statement immediately after the catch block i.e catch block is skipped.

Example: Division by zero exception handling

#include<iostream.h>

void main()

{

int a,b;

cout<<”enter the values of a and b”;

cin>>a;

cin>>b;

int x = a- b;

try

{

if(x!=0)

{

cout<<”result(a/x) = “<<a/x<<”\n”;

}

else

{

throw(x);

}

}

catch(int i)

{

cout<<”exception caught : x = “<<x<<”\n”;

}

cout<<”end”;

}

The point at which the throw is executed is called throw point. Once an exception is thrown to catch block, control cannot return to the throw point.