CS210 Lab: Stacks--Exception Handling

Overview:

• What is an Exception?
• How do you handle an Exception?
• Throw Statement
• Try-Catch Statement
• Example Code of Try-Catch and Throw
• Lab Specific Hints

What is an Exception?

• An Exception is an unusual event, often an error, that requires special processing.
• In this lab, the Exception is that the stack is full.

How do you handle an Exception?

• a try clause encloses the code segment in which an exception might occur.
• throw signals the fact that an exception has occurred and control should pass to the exception handler.
• catch processes the thrown exception. Each catch clause is an exception handler.

Throw Statement

	throw Expession;

	throw FullStack();

	throw EmptyStack();

class FullStack
// Exception class thrown by Push when stack is full.
{};

class EmptyStack
// Exception class thrown by Pop and Top when stack is emtpy.
{};

Try-Catch Statement

The general syntax of a try-catch statement is as follows:

try
  Block
catch ( FormalParameter ) 
  Block
catch ( FormalParameter ) 
  Block
	.
	.
	.

When a statement or group of statements might result in an exception, we enclose them in a try clause. For each type of exception that can be produced by the statements in the try clause, we write a catch clause (exception handler).

Example Code of Try-Catch and Throw

#include <iostream>
#include <string>

using namespace std;

int Dividing( int, int);

class ZeroDiv // Exception class
{};

int main()
{
	int num; // Numerator
	int num2; //Denominator
        float quo;
	cout << "Enter numerator: ";
  	cin >> num;
 	cout<< "Enter Denominator: ";
  	cin >> num2;	
	try
	{
	   if (num2==0)
	   throw ZeroDiv();
           quo = num/num2;
           cout<< "Their quotient is : " <<quo<<endl;
        }
	catch (ZeroDiv)
	{
		cout << "Can't divide by zero" << endl;
	}
	return 0;
}

Lab Specific Hints

1. Add a try clause
2. Add a catch clause for FullStack, you don't need to add throw FullStack() since it is given in StackType.cpp

• Special Note: The hardest part is probably deciding which statements will be enclosed in the try clause. Things to note are:
  • If no exceptions are thrown, then the catch clauses are skipped and control goes to the statement following the entire try-catch statement.
  • However, if an exception is thrown by a statement in the try clause, control immediately transfers to the appropriate exception handler (catch clause). That means that, if there are statements in the try clause following the one that caused the exception, they are skipped.