Exception%20Handling%20Fall%202008

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Exception HandlingFall 2008

• Dr. David A. Gaitros
• dgaitros_at_admin.fsu.edu

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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

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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.

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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

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Simple Example

• include ltiostreamgt
• using namespace std
• int main()
• int cookies, people
• double cpp
• try
• cout ltlt "Enter number of people "
• cin gtgt people
• cout ltlt "Enter number of cookies "
• cin gtgt cookies
• if (cookies 0) throw people
• else if (cookies lt 0)
• throw static_castltdoublegt(people)
• cpp okies/static_castltdoublegt(people)
• cout ltlt cookies ltlt " cookies.\n"
• ltlt people ltlt " people.\n"
• ltlt "You have " ltlt cpp
• ltlt " cookies per person.\n"

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• catch(int e)
• cout ltlt e ltlt " people, and no cookies!\nGo buy
  some cookies!\n"
• catch(double t)
• cout ltlt "Second catch block type double -- do we
  reach it?\n"
• cout ltlt "End of program.\n" return 0

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try Block

• The basic handling of an exception consists of
  the try-throw-catch trio.
• Basic try block
• try
• // Some code goes here
• // Possibly throw an exception
• // More regular code can go here.

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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

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Example try-catch

• try
• if(Array_Index gt 100)
• throw Array_Index
• catch ( int e)
• cout ltlt eltlt is greater than 100ltlt
• ltlt resetting index to 100\n
• Array_Index 100

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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