Chapter 13 Exception Handling (????)

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Chapter 13 Exception Handling
(????)

• 13.1 Introduction
• 13.2 Exception Handling Mechanism
• 13.3 Throwing Mechanisms
• 13.4 Catching Mechanisms
• 13.5 Rethrowing an Exception
• 13.6 Specifying Exception

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13.1 Introduction

• A program to read in two integers and displays
  their quotient

Quotient
Run
int main() cout ltlt "Enter two integers "
int num1, num2 cin gtgt num1 gtgt num2 cout
ltltnum1ltlt"/"ltltnum2ltlt" is " ltlt (num1 / num2) ltlt
endl return 0
If you enter 0 for num2, then?
A runtime error! An exception!
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Whats Exception?

• Peculiar problems other than logic or syntax
  errors
• Run time anomalies or unusual conditions
• Usually cause the program terminate abnormally
• Including
• division by zero,
• access to an array outside of its bounds, or
• running out of memory or disk space,
• etc.

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Naive Exception Handling

• Example of Integer Division
• Adding an if statement to test the second number

if (num2 ! 0) cout ltltnum1 ltlt" / "ltltnum2ltlt"
is " ltlt (num1 / num2) ltlt endl else
cout ltlt "Divisor cannot be zero"ltltendl
QuotientWithIf
Run
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13.2 Exception Handling Mechanism

• Special Mechanism to detect, report and act
  against exceptions.
• Four tasks
• Find the problem(Hit the exception)
• Inform that an error has occurred (Throw the
  exception)
• Receive the error information (Catch the
  exception)
• Take corrective actions (Handle the exception)

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Try-throw-catch

• The try block
• To preface a block of statements which may
  generate exceptions.
• The throw statement
• To report the exception by throwing a object.
• The catch block
• To catche the exception thrown and take actions
  to handle it.

try block
Detects and throws an exception
Exception object
catch block
Catches and handles the exception
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C Exception Handling

• Using try-throw-catch

int main() cout ltlt"Enter two integers "
int num1, num2 cin gtgt num1 gtgt num2 try
if (num2 0) throw num1 cout ltlt num1 ltlt
" / " ltlt num2 ltlt " is " ltlt (num1 /
num2) ltlt endl catch (int e) cout ltlt
"Exception an integer " ltlt e ltlt "
cannot be divided by zero" ltlt endl cout ltlt
"Execution continues ..." ltlt endl
QuotientWithException
Run
Exception Handler
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Template for try-throw-catch

• try
• Code to try
• Throw an exception by throw or calling
• More code to try
• catch (type e)
• Code to process the exception

try // ... catch (type) //
The rest code will NOT be executed if the
exception occurs!
General
Parameter Omitted
Program 13.1.cpp.html
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Advantages of C Exception Handling
int quotient(int num1, int num2) if (num2 !
0) return num1 / num2 else
return -1//?
int quotient(int num1, int num2) if (num2
0) throw num1 return num1 /
num2
C Exception Handling
Naive Exception Handling

• More powerful
• - Can handle various cases
• More structural
• - Separate normal code and exception handling
  code

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When to Use Exceptions

• Yes!
• Common exceptions that may occur in multiple
  classes
• Unexpected error conditions

May be difficult to determine.
DO NOT abuse exception handling!

• NO!
• Simple errors that may occur in individual
  functions
• Simple, expected situations

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13.3 Throwing Mechanism
Throw point
Function that causes an exception

• Exception usually is thrown by a function invoked
  inside a try block.
• Throw point
• The point at which the throw is executed
• Upon exception
• The control will jump to the catch block, and
• Cannot return back the throw point.

Invoke function
try block
Invokes a function that contains an exception
Throw exception
catch block
Catches and handles the exception
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Exception from a Function

• A program to compute quotient using a function

int quotient(int num1, int num2) if (num2
0) throw num1 return num1 / num2 int
main() // try int result
quotient(num1, num2) // catch
(int e) // //
QuotientWithFunction
Run
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Exception Classes
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Using Standard Classes

• bad_alloc
• bad_cast
• runtime_error

ExceptionDemo1
Run
ExceptionDemo2
Run
ExceptionDemo3
Run
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Customed Exception Classes

• Use standard classes whenever possible
• Create yours if necessary
• An exception class is just like any C class
• It is often desirable to derive it from exception
  
• Directly or indirectly
• To utilize the common features (e.g., the what()
  function) in the exception class.

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The Triangle Exception Class
TriangleException.h
Triangle.h
TestTriangle.cpp
Run
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13.4 Catching Mechanism

• More than one exception in one try block?
• One catch block can handle only one exception

try catch (E e)

try catch (E1 e)
catch (E2 e) catch (E3
e)
Program 13.3.cpp.html
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Example of Multiple Catches

• Another type of exception in Triangle

class NonPositiveSideException public
logic_error public NonPositiveSideException(
double side) logic_error("Non-posit
ive side") this-gtside side
double getSide() return side
private double side
NewTriangle.h
NonPositiveSideException.h
MultipleCatchDemo
Run
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Notes for Multiple Catches

• If a catch block catches exception objects of a
  base class, it can catch all the exception
  objects of the derived classes of that base
  class.
• So, a catch block for a base class type should
  appear after a catch block for a derived class
  type.

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14.5 Rethrowing an Exception

• An exception can be rethrown
• By a handler
• For two reasons
• The current handler cannot process the exception
  or
• The current handler wants to let its caller be
  notified of the exception.
• The syntax

try statements catch (TheException ex)
perform operations before exits throw
Program 13.5.cpp.html
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Exception Propagation
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Exception Propagation

• An exception that cant be handled by any of a
  functions catch blocks is thrown to the caller
• The exception is propagated along the reverse
  order of the function calling

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14.6 Specifying Exception

• What try-catch block should be provided in a
  function?
• Exceptions thrown by
• The function itself
• The functions called
• What exceptions may be thrown by a function?

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Exception Specification

• Also known as throw list
• lists exceptions that a function can throw

returnType functionName(parameterList)
throw (exceptionList)
Program 13.6.cpp.html
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Two Special Lists

• No throw list
• May throw any exception!
• Empty throw list
• Cant throw any exception!

returnType functionName(parameterList)
returnType functionName(parameterList) throw ()
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Undeclared Exception

• Throwing an exception not declared in the throw
  list?
• The function unexpected will be invoked!
• A standard C function
• Normally terminates the program

Visual studio doesnt satisfy the throw list
specification well.
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Catch All Types of Exceptions

• The exception type may not be able to anticipate
• To force a catch block to catch all exceptions
  thrown

catch() //Statements for processing
//all exceptions
Program 13.4.cpp.html
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A Summary

• Various exceptions may occur
• Standard exception classes
• Customized exception classes
• try-throw-catch
• Multiple catches
• Propagation of exceptions
• Rethrow
• Exception specification