Unit 14: Introduction to Exception Handling

1
Unit 14 Introduction to Exception Handling

• How to Handle Errors
• Nested if statements
• Use exceptions
• Exceptions
• try/catch block
• Example

2
How to Handle Errors (1) Nested if statements

• Preventing potential run-time errors
• Bad parameters (e.g., null string)
• Un-initialized variables and pointers
• Division by zero
• Resource limits, etc.
• Result
• Large percentage of deeply nested if code used to
  prevent errors from occurring.

3
How to Handle Errors (1) Nested if Problems

• Clutter normal program logic and error handling
  logic not separated
• if (DM ! NULL)
• if (DM-gtValidData()) DM-gtqCust-gtPost()
• else
• ShowMessage("Data module not instantiated")
• Details of the Error May be Lost
• if (TimeToMin(StartTime) lt 0)
• return -1

4
BCB Exceptions

• CBuilder throws an exception when an error
  occurs by default, VCL-based applications shows
  the exception
• BCB displays an exception that is difficult for
  users to understand.
• Solution catch the exception and show a
  user-friendly message instead.
• Reference BCB Help Contents Programming with
  CBuilder Exception Handling

5
BCB Exceptions Example

• E.g., EAccessViolation occurs if data module not
  instantiated

6
How to Handle Errors (2) Using Exceptions

• Separates the handling of errors at the end of
  the normal program logic.
• try/catch block
• The block of code within the try block is subject
  to exception handling
• The closing catch identifies the class of
  exception to be handled
• To catch multiple exception classes, you must
  catch the most specific one first (check the
  Exception hierarchy in BCB help)

7
How to Handle Errors (2) Using Exceptions
Example

• try
• if (DM-gtValidData()) DM-gtqCust-gtPost()
• catch (EAccessViolation AccessViolation)
• ShowMessage("Data module not instantiated")
• catch (Exception E) // catch all other
  exceptions
• ShowException(this, E) // BCB default

8
Your Turn

• An error occurs when the file is not found.
• How do you handle this exception?