Exception-Handling Overview

• Consider pseudocode
  Perform a task
  If the preceding task did not execute correctly
  Perform error processing
  
  Perform next task
  If the preceding task did not execute correctly
  Perform error processing
  
• Mixing logic and error handling
  • Can make program difficult to read/debug
  • Exception handling removes error correction from main line of program
• Exception handling
  • For synchronous errors (divide by zero, null pointer)
    • Cannot handle asynchronous errors (independent of program)
    • Disk I/O, mouse, keyboard, network messages
  • Easy to handle errors
• Terminology
  • Function that has error throws an exception
  • Exception handler (if it exists) can deal with problem; Catches and handles exception
  • If no exception handler, uncaught exception; Could terminate program
• C++ code

  try{ 
     code that may raise exception
     } 
  catch(exceptionType){ 
                      code to handle exception
                      }
  

• try block encloses code that may raise exception
• One or more catch blocks follow
  • Catch and handle exception, if appropriate
  • Take parameter; if named, can access exception object
• Throw point
  • Location in try block where exception occurred
  • If exception handled
    • Program skips remainder of try block
    • Resumes after catch blocks
  • If not handled
    • Function terminates
    • Looks for enclosing catch block (stack unwinding, 13.8)
• If no exception
  • Program skips catch blocks