Introduction to Exceptions

• Chapter 14, READ IT!
• Your code can encounter errors
  • Consider a rational number class.
    • The user somehow manages to set a 0 denonominator
  • Or our queue class
    • The user trys to Front or Dequeue on an empty queue.
• In either case, you as a good programmer probably caught the error
  • You always check to see that the queue has an element before you call Dequeue.
• But what do you do as a queue implementor if they do not?
  • You can crash the program (exit, _exit, abort, ...)
  • You can try to fix it, but what if you are wrong?
    • How could you fix this situation?
  • You can print an error message, but who will see that?
  • You can return a value, but what if that is wrong?
    • What value should you return?
    • If the user did not check to see if there was a value in the queue, will they check to see if the return value is valid?
    • What is an invalid value anyway?
• Exceptions are built to handle these exceptional but not unexpected situations.
• The vector at function does this error checking
  • See error.cpp.
• Catching exceptions
  • Exceptions are caught in a try-catch block
  • reference
  • We will build this up slowly.
• try
  • This marks the beginning of a code block that could contain an error.
  • It is followed by one or more catch blocks.
• catch (...)
  • This is the most basic catch block
  • It matches all exceptions.
  • It contains code to "clean up" after the exception.
  • More later.
• see case1,cpp.
• throw
  • The second half of the equation is the throw expression
  • throw expression
  • Constructs the expression object.
  • Unwinds back through the stack until it reaches an appropriate exception handler
    • invokes all destructors for objects that go out of lifetime
    • If the exception is called anywhere but in a constructor, the corresponding destructor is called.
    • If the exception is called in a method everything is fine.
  • If no corresponding catch is found, the program is aborted.
• We will be back to this in the future.
• But look at throw.cpp.