Exception HandlingFall 2008

Dr. David A. Gaitrosdgaitros@admin.fsu.edu

Exception Handling

“A well behaved program never abnormally terminates. All errors or problems should be handled by the program”

Exception: A problem that occurs that is outside the normal expected behavior of the system. Example: Divide by zero

Exception Handling

• Most exceptions are infrequent or should not happen at all.

• The code for the exception should be separated from the main body to avoid confusing maintenance activities.

• Harkins back to the day of the “ go to “ when the execution path of computer programs was less well behaved.

Exception Handling

• Exception handling best used for situations that would normally cause the program to abnormally terminate or produce unpredictable results: – Divide by Zero– Array subscript out of bounds– Register or number overflow or

underflow– Stack overflow or underflow– Abnormal operation– Parity error

Simple Example#include <iostream>

using namespace std;

int main()

{

int cookies, people;

double cpp;

try {

cout << "Enter number of people: ";

cin >> people;

cout << "Enter number of cookies: ";

cin >> cookies;

if (cookies == 0) throw people;

else if (cookies < 0)

throw static_cast<double>(people);

cpp =okies/static_cast<double>(people);

cout << cookies << " cookies.\n"

<< people << " people.\n"

<< "You have " << cpp

<< " cookies per person.\n";

}

catch(int e)

{

cout << e << " people, and no cookies!\nGo buy some cookies!\n";

}

catch(double t)

{

cout << "Second catch block type double -- do we reach it?\n";

}

cout << "End of program.\n"; return 0; }

}

try Block

• The basic handling of an exception consists of the try-throw-catch trio.

• Basic try blocktry { // Some code goes here // Possibly throw an exception // More regular code can go here. }

try Block

• The try block is a method of grouping code together that could be associated with one or more exceptions and note affecting the rest of the program.

• If an exception is detected, the alternative code is executed while the execution of the normal code within the try block is halted.

• The code after the try block is not affected. • If an exception occurs within a try block, the

try block ends and the program attempts to match the exception with one of the catch handlers.

• If match found, code in catch is executed • Only one catch block will be executed • Execution resumes after last catch block

Example try-catch

try

{

if(Array_Index > 100)

throw Array_Index;

}

catch ( int e)

{

cout << e<< “ is greater than 100”<<

<< “resetting index to 100”\n;

Array_Index = 100;

}

Overuse of Exceptions

• Exceptions alter flow of control– Similar to old "goto" construct– "Unrestricted" flow of control

• Should be used sparingly• Good rule:– If desire a "throw": consider how to

write program without throw– Ask yourself how often you expect

to encounter this error– Errors caused by bad user input is

usually not a reason to write an exception to handle it

– If alternative reasonable do it