Exception Handling

1
Exception Handling

• Lesson 11

Note CIS 601 notes were originally developed by
H. Zhu for NJIT DL Program. The notes were
subsequently revised by M. Deek
2
Content

• Exception and Exception Handling
• Structure of exception handling in C
• Exception in Templates
• Group of exceptions
• Scope

3
Choices upon an error

• Terminate immediately
• Ignore the error
• Set an error flag, check by the caller

4
What is an Exception?

• An error generated by the program at run-time
  which would normally terminate the execution of
  the program.
• Such as, memory exhaustion, subscript range
  errors, or division by zero.

5
What is Exception Handling?

• A mechanism that allows a program to detect and
  possibly recover from errors during execution.
• The alternative is to terminate the program.

6
Common Exceptions

• out-of-bound array subscript
• arithmetic overflow
• divide by zero
• out of memory reference
• invalid function parameters
• etc.

7
Benefits of Exception Handling

• improved readability
• easier to modify code
• more robust code

8
Without Exception Handling

• include ltiostream.hgt
• include ltstdlib.hgt
• double hmean(double a, double b)
• int main(void)
• double x, y, z
• cout ltlt "Enter two numbers "
• while (cin gtgt x gtgt y)
• z hmean(x,y)
• cout ltlt "Harmonic mean of " ltlt x ltlt " and " ltlt
  y ltlt " is " ltlt z ltlt "\n"
• cout ltlt "Enter next set of numbers ltq to quitgt
  "

9
Without Exception Handling

• cout ltlt "Bye!\n"
• return 0
• double hmean(double a, double b)
• if (a -b)
• cout ltlt "untenable arguments to hmean()"ltltendl
• abort()
• return 2.0 a b / (a b)
• //ex11-1.cpp

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With Exception Handling

• include ltiostream.hgt
• include ltstdlib.hgt
• double hmean(double a, double b)
• int main(void)
• double x, y, z
• cout ltlt "Enter two numbers "
• while (cin gtgt x gtgt y)
• try
• z hmean(x,y)
• // end of try block

12
With Exception Handling

• catch (const char s) // start of exception
  handler
• cout ltlt s ltlt "\n"
• cout ltlt "Enter a new pair of numbers "
• continue
• // end of handler
• cout ltlt "Harmonic mean of " ltlt x ltlt " and " ltlt
  y
• ltlt " is " ltlt z ltlt "\n"
• cout ltlt "Enter next set of numbers ltq to quitgt
  "
• cout ltlt "Bye!\n"
• return 0

13
With Exception Handling

• double hmean(double a, double b)
• if (a -b)
• throw "bad hmean() arguments a -b not
  allowed"
• return 2.0 a b / (a b)
• //ex11-2.cpp

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Structure of Exception Handling

• try
• statement-list
• catch(exception)
• statement-list
• catch(exception)
• statement-list

16
Procedure for exception handling

• try func()//1.The program call in try
• catch(Class anObj)
• //3.Transfer the execution to the program
• func() throw exObj
• //2.The function throws an exception and transfer
  the //execution to the catch block,and assign the
  exObj //to anObj

17
Naming of an Exception

• include ltiostream.hgt
• class ZeroDivide
• const char errmsg
• public
• ZeroDivide()
• errmsg("Error dividing by zero")
• void print() cout ltlt errmsg
• float divide(int n1, int n2)
• void main()
• int a, b
• cout ltlt"Input two integers"

18
Naming

• cin gtgt a gtgt b
• try float c divide(a,b)
• cout ltlt "a / b " ltlt c ltlt '\n'
• catch (ZeroDivide err)
• err.print()
• cout ltlt\nltlt "end of program\n"
• float divide(int n1, int n2)
• if (n2 0) throw ZeroDivide()
• return (float) n1 / n2
• //ex11-3.cpp

Results
Input two integers34 0 Error dividing by
zero end of program
19
Example

• include ltiostream.hgt
• const unsigned ArraySize 50
• int arrayArraySize
• class RangeError
• void InsertValue(unsigned i, int value)
• if (igtArraySize) throw RangeError()
• arrayi value
• void TestTheArray()
• unsigned j
• int anInt
• cout ltlt"Enter a subscript and a value"

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Example

• cin gtgt j gtgt anInt
• InsertValue(j, anInt)
• void main()
• try
• TestTheArray()
• catch (const RangeError )
• cout ltlt"Range Error in main()!"ltlt endl
• //ex11-4.cpp

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Passing Data with Exceptions

• include ltiostream.hgt
• class array
• int v, lower, upper
• public
• class SubscriptError
• int index
• public
• SubscriptError(int i) index(i)
• int get() return index
• array(int l, int u)lower(l), upper(u)
• v new int upper-lower 1
• int operator(int)
• array()delete v

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• int arrayoperator(int i)
• if (igtlower iltupper)
• return (vi-lower)
• throw SubscriptError(i)
• return v
• void fun(array a)
• try a100
• catch (arraySubscriptError s)
• cerr ltlt"illegal subscript" ltlts.get() ltlt '\n'
• void main()
• array ar(20, 40)
• fun (ar)
• //ex-5.cpp

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More than One Exception

• const MAX 500
• class array
• int v, lower, upper
• public
• //
• class SizeError
• array(int l, int u)lower(l), upper(u)
• int size upper-lower1
• if (sizelt0 sizegtMAX)
• throw SizeError()
• else v new intsize

If we add another exception SizeError, how is it
done?
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More than One Exception

• void main()
• int l, u
• try
• cout ltlt "Input two bounds for the array"
• cin gtgt lgtgtu
• array ar(l, u)
• fun (ar)
• catch (arraySizeError )
• cerr ltlt"illegal size" ltltu - l 1 ltlt '\n'
• //ex11-6.cpp

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Unwinding the Exception Stack

• Stack unwinding
• When a program throw an exception, the destructor
  is called for each object that was constructed
  when the program entered the try block.

29
Unwinding the Exception Stack

• When an exception is thrown during the
  construction of a composite object, destructors
  will be invoked only for those data members that
  have already been constructed.

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Unwinding

• include ltiostream.hgt
• include ltstring.hgt
• class Test
• class VString
• public
• VString()
• VString(char s)
• VString()cout ltlt"VString Des!"ltltendl
• class Transcript
• public
• Transcript()
• throw Test()
• Transcript()cout ltlt"Transcript Des!"ltltendl

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Unwinding

• class Student
• VString lastName
• Transcript records
• public
• Student()
• Student()cout ltlt"Student Des!"ltltendl
• void main()
• try
• VString collegeName("NJIT")
• Student S
• catch (...) cout ltlt "exception!"ltltendl
• //ex11-7.cpp

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Multiple Handlers

• Most programs performing exception handling have
  to handle more than one type of exception. A
  single try block can be followed by multiple
  handlers(catch), each configured to match a
  different exception type.

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Example

• include ltstdio.hgt
• include ltiostream.hgt
• include ltfstream.hgt
• class RangeError //Exception
• class InputFileError //Exception
• class LongArray
• public
• LongArray( unsigned sz 0 )
• LongArray()
• long Get( unsigned i ) const
• private
• long data // array of long integers
• unsigned size // allocation size
• //cis601source/chap8/except/except2.cpp

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Example

• LongArrayLongArray( unsigned sz )
• size sz
• data new longsize
• LongArrayLongArray()
• delete data
• long LongArrayGet( unsigned i ) const
• if( i gt size )
• throw RangeError()
• return datai

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Example

• unsigned ArraySize 50
• void ReadFile( ifstream infile, LongArray L )
• infile.open( "INDATA.TXT" )
• if( !infile )
• throw InputFileError()
• void GetArrayElement( const LongArray L )
• unsigned i
• cout ltlt "Enter subscript "
• cin gtgt i
• long n L.Get( i )

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Example

• int main()
• try
• ifstream infile
• LongArray L( ArraySize )
• ReadFile( infile, L )
• GetArrayElement( L )
• cout ltlt "Program completed normally.\n"
• catch( const RangeError R )
• cout ltlt "Subscript out of range\n"
• catch( const InputFileError F )
• cout ltlt "Unable to open input file\n"
• catch( ... ) cout ltlt "Unknown exception
  thrown\n"
• return 0
• //except2.cpp

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Results

• Enter subscript 23
• Program completed normally.
• Enter subscript 56
• Subscript out of range

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Ex RangeError Class

• include ltiostream.hgt
• include ltstdlib.hgt
• include ltfstream.hgt
• include "fstring.h"
• include "range.h"
• class LongArray
• public
• LongArray( unsigned sz 0 )size sz data
  new long sz
• LongArray() delete data
• unsigned GetSize() const
• long Get( unsigned i ) const
• void Put( unsigned i, long item )
• private
• long data // array of long integers
• unsigned size // allocation size
• //cis601source/chap8/except/except3.cpp

40
Range.h

• include ltiostream.hgt
• include ltstring.hgt
• const unsigned FileNameSize 50
• // Make this gt longest filename on target
  system.
• class RangeError
• public
• RangeError( const char fname,
• unsigned line,
• unsigned subscr )
• strncpy(fileName, fname, FileNameSize)
• lineNumber line
• value subscr

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Range.h

• friend ostream operator ltlt( ostream os,
• const RangeError R )
• os ltlt "\nRangeError exception thrown "
• ltlt R.fileName
• ltlt ", line " ltlt R.lineNumber
• ltlt " value " ltlt R.value
• ltlt endl
• return os
• private
• char fileNameFileNameSize1
• unsigned lineNumber
• unsigned value

42
RangeError
The name of the current source file. __FILE__
expands to a string surrounded by double
quotation marks.

• inline unsigned LongArrayGetSize() const
• return size
• inline void LongArrayPut( unsigned i, long item
  )
• if( i gt size )
• throw RangeError( __FILE__ ,__LINE__, i )
• datai item
• inline long LongArrayGet( unsigned i ) const
• if( i gt size )
• throw RangeError( __FILE__ ,__LINE__, i )
• return datai

The line number in the current source file. The
line number is a decimal constant.
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RangeError

• unsigned GetArraySize()
• unsigned n
• cout ltlt "Number of array elements? "
• cin gtgt n
• return n
• void FillArray( LongArray L )
• int i
• try for( i 0 i lt L.GetSize() i )
• L.Put( i, rand() )
• catch( const RangeError R )
• cout ltlt R
• throw R

44
RangeError

• void GetArrayElement( const LongArray L )
• int ok 0
• while( !ok )
• unsigned i
• cout ltlt "Enter an array subscript (0-"ltlt (
  L.GetSize()-1 ) ltlt ") "
• cin gtgt i
• long n
• try n L.Get( i )
• ok 1
• cout ltlt "Element contains " ltlt n ltlt endl
  
• catch( const RangeError R )
• cout ltlt R
• cout ltlt "Caught at " ltlt __FILE__ltlt ", line
  " ltlt __LINE__ ltlt endl
• throw

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RangeError

• int main()
• try
• LongArray L( GetArraySize() )
• FillArray( L )
• GetArrayElement( L )
• catch( ... )
• cout ltlt "Exception caught in main().\n"
• return 1
• return 0
• //except3.cpp

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Exception with no Catch

• If no catch matches the exception generated by
  the try block, the search continues with the next
  enclosing try block.
• If no catch is found, then error!

48

• void fun1(array a)
• ...
• try
• fun2(a) ...
• catch (arraySubscriptError) ...
• ...

49

• void fun2(array a)
• ...
• try
• ... // use array a
• catch (arraySizeError) ...
• ...

50
Exception within an Exception

• If the same exception type is raised while
  handling the exception of a try block, then the
  new exception is handled by the outer try block.

51
No Infinite Loop

• include ltiostream.hgt
• class Over
• void fun()cout ltlt "fun"ltltendl
• throw Over()
• void main()
• try
• fun()
• catch (Over)
• cout ltlt"Over1"ltltendl
• throw Over()
• //ex11-8.cpp

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Nesting Exception Handlers

• Exception handlers may be nested.
• Nesting is rarely needed or useful.
• Nesting can make the code difficult to read.

54

• include ltiostream.hgt
• class Over
• void fun()cout ltlt "fun"ltltendl
• throw Over()
• void main()
• try fun()
• catch (Over)
• cout ltlt"Over1"ltltendl
• try fun()
• catch (Over)
• cout ltlt"Over2"ltltendl
• //ex11-9.cpp

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Exceptions in Templates

• Each class generated can have its own exception
  class, or
• we can have one exception that is common to all
  the classes generated from the template.

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• templateltclass TPgt
• class array
• TP v
• int lower, int upper
• public
• class Exception
• ...

58

• void fun(arrayltintgt intArr, arrayltfloatgt
  fltArr)
• try
• ...
• catch (arrayltintgtException) ...
• catch (arrayltfloatgtException) ...

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A common Exception for a Class Template

• class Exception
• templateltclass TPgt class array
• ...

60

• void fun(arrayltintgt intArr, arrayltfloatgt
  fltArr)
• try ...
• catch (Exception) ...

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Grouping of Exceptions

• When a set of exceptions are considered to be
  logically related, it is often best to organize
  them into a family of exceptions.

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• enum Exception Ex1, Ex2, Ex3, ...
• try ...
• catch (Exception e)
• switch (e)
• case Ex1 ....
• case Ex2 ....
• ...
• ...

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Using Inheritance

• class Exception
• class Ex1 public Exception
• class Ex2 public Exception
• class Ex3 public Exception
• ...

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• try
• ...
• catch (Ex1)
• ...
• catch (Exception)
• ...
• //ex11-11.cpp

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Derived Exceptions
include ltiostream.hgt const MAXINT 200 const
MININT 0 class Matherr public virtual void
debug_print() class int_overflow public
Matherr public char op int opr1, opr2
int_overflow( char p, int a, int b) op p
opr1 a opr2 b virtual void
debug_print() cerr ltlt op ltlt '(' ltlt opr1
ltlt ',' ltlt opr2 ltlt')' ltltendl
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int add(int x, int y) if( x gt 0 y gt 0 x
gt MAXINT - y x lt 0 y lt 0 x lt MININT -
y) throw int_overflow("", x, y) return x
y void f( ) try add(1, 2) add(MAXINT,
-2) add(MAXINT, 2) catch (Matherr m)
m.debug_print( )
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Result
void main( ) try f() catch ( Matherr)
cout ltlt"catch another Matherr"ltltendl //e
x11-10.cpp
(200,2)
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Re-throwing an Exception

• In some cases when an exception handler is unable
  to deal with the exception it can re-throw the
  exception to allow another handler to deal with
  it.
• The original exception is re-thrown.

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• void fun()
• try ...
• catch (Exception)
• if ( ... )
• ...
• else
• throw

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A Catch for All Exceptions

• void fun()
• try g()
• catch (...)
• // do some handling
• throw

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The Order of Handlers

• A handler matches if
• it directly refers to the exception thrown.
• it is of a base class of that exception.

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Exception Interface Specification

• int fun( ) throw (Ex1, Ex2, Ex3)
• ...
• Specifies that fun may throw exceptions Ex1, Ex2,
  and Ex3 and any other exceptions derived from
  them.

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Interface Specification

• A function that can throw any exception
• void fun()
• A function that do not throw any exception
• void fun() throw

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Unexpected Exceptions

• If a function throws an exception not listed or
  covered by its interface, the function unexpected
  is called, which results with a call to
  terminate().
• The meaning of unexpected can be redefined using
  the function set_unexpected().

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• void fun() throw(Ex1, Ex2) ...
• is equivalent to
• void fun()
• try ...
• catch (Ex1) throw
• catch (Ex2) throw
• catch (...) unexpected()

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What Should an Exception Handler Do?

• fix the problem and try again
• Re-throw the exception
• Re-throw another exception
• return the needed result

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Scoping Rules

• variables declared within a try block are not
  accessible outside the block.
• variables declared within one catch block may not
  be accessed from other catch blocks.
• variables declared at the beginning of a function
  may be accessed throughout the try and catch
  blocks.

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Resource Acquisition

• When a function uses a resource, it is important
  for the resource to be properly released at the
  end, even when errors are encountered during the
  use of the resource.

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Example

• void fun(char s)
• FILE fp fopen(s, r)
• ...
• fclose(f)

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One Approach

• File fp fopen(s, r)
• try ...
• catch (...)
• fclose (fp)
• throw
• fclose(fp)

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A Better Approach

• class File
• FILE fp
• public
• File(char name, char access)
• fp open(name, access)
• File()
• fclose(fp)
• ...

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Exception Handling vs. Control Structures

• Exception handling is a less structured mechanism
  than language control structures.
• Exception handling is usually less efficient than
  control mechanisms.
• When used for dealing with situations other than
  errors, it could lead to less understandable code.

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Readings

• Reading
• 8.3,8.4