1. Exceptions C++ Interlude 3
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

2. Contents Background Assertions Throwing Exceptions Handling Exceptions
Programmer-Defined Exception Classes
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

3. Definition An exception is an object We handle the exception Example:
Signals the rest of the program
Something unexpected has happened.
We handle the exception
When we detect and react to it.
Example:
Need a consistent mechanism to let client know a method could not perform task
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

4. Problem to Solve
Consider the video game that we are working on for our friend
Next task – create function that searches for given string in a number of boxes
First try at the function findBox
Meets the basic requirements: Searches array of boxes and returns box containing target string.
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

5. Function findBox First try at the function findBox
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

6. But may not be best solution
Function findBox
But may not be best solution
Revised findBox function with assertions
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

7. Throwing Exceptions
Alternate way of communicating or returning information to function’s client is throw an exception
Bypasses normal execution,
Control immediately returns to client.
Exception can contain information about error or unusual condition
Helps the client resolve the issue and possibly try the function again
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

8. Throwing Exceptions Hierarchy of C++ exception classes
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

9. Throwing Exceptions Revised findBox function throws exception
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

10. When to Throw Exception
If you can resolve unusual situation in a reasonable manner
Do so without exception
Several resolutions to abnormal occurrence, you want client to choose
Throw an exception.
Detect programmer used method incorrectly
Throw runtime exception.
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

11. Handling Exceptions Code that consists of two pieces.
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

12. Handling Exceptions
Statements in try block execute as if block not present
If no exception, catch block ignored
If exception occurs of type specified in catch block
Remaining part of try block ignored
Program jumps to statements in catch block
Program then continues to statements after the catch block
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

13. Handling Exceptions Trying the function findBox
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

14. Handling Exceptions Trying the function findBox
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

15. Multiple catch Blocks
Catch blocks must be ordered so most specific exception classes caught before more general exception classes
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

16. Uncaught Exceptions A program with an uncaught exception
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

17. Uncaught Exceptions A program with an uncaught exception
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

18. Uncaught Exceptions Flow of control for an uncaught exception
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

19. Uncaught Exceptions Flow of control for an uncaught exception
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

20. Programmer-Defined Exception Classes
Exception class typically consists of a constructor that has a string parameter.
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

21. To Throw An Exception
Determine what errors to check, where they can occur
Detect error, usually with if statement
If error occurs, use throw statement
Pass string description parameter
Add throw clause to function header indicating exceptions
Derive custom exception class to better identify conditions (optional)
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012

22. To Handle An Exception
Place statement that might throw an exception in try block
After try block, place catch blocks for each type of exception that can be thrown by statements in try block.
Place the catch blocks in order from most specific exception classes to more general
Data Structures and Problem Solving with C++: Walls and Mirrors, Frank Carrano, © 2012