1. Handling Exceptionally Sticky Problems
Exceptions
Handling Exceptionally Sticky Problems

2. Handling Errors
Some things can't be done

3. Guarding Can protect against foreseeable errors:
ALWAYS validate user input

4. Lack Of Context
Low level code
How should it respond?

5. Lack Of Context
What should we do with a bad zip code?

6. Proper Response
Proper response to low level errors depends on high level code
Is this a GUI app?
Unattended server application?

7. Option 1 - Assert
assert( expression ) If expression is not true, self-destruct with message

8. Option 2 - Return
Devise special error code to return

9. Option 2 - Return Not always an option…
Any possible int might be a valid return value

10. Extra parameters Can add extra parameters for error code…
Nothing forces client code to deal with it

11. Option 3 - Exceptions Exceptions : alternative return mechanism
Way for code to return an error
Indicate error by throwing a value:
Does not have to match return type

12. Catching
A thrown exception will blow up your program…

13. Catching …unless you catch it
try : Try this code… something bad might happen
catch : Here is how to handle any exceptions
Only run if an exception thrown in try

14. Catch Catch specifies Type of thing it catches
Only catches that type
What it will call the thing it caught
Use as variable inside catch

15. Catch Wrong Type == No Catch Can have multiple catches: catch(…)
Catches anything - But can't use it as variable

16. Defining Exceptions
Exception can be anything
Custom type:

17. Stack Unwinding
Call stack stores how we got to a line of code:

18. Stack Unwinding
Thrown exception works back down stack looking for a catch
Does function C catch?
No, how bout B?
No, how bout A?
No, how bout Main?

19. Std::Excptions Std library defines exception class
exception is parent to all
Has virtual what() function

20. Exception Subclasses Many subclasses to exception All support what()
All can be caught as exception

21. Exception Subclasses Can't add information to plain exception:
Allow for construction with string message
Help specify problem

22. Proper Catching
Using out_of_range exception class:

23. Reacting to Exceptions
Choices for dealing with exceptions
Fix the error
Log & Continue
Blow up
But get to make decision at appropriate level

24. Announcing a Throw Can announce what your function throws:
I throw nothing:
I thrown exception (or subtypes)
I throw these two types

25. What programmers want Programmers think this means:
Announce to other programmers what to expect
Check at compile time that someone will catch
Speed things up by not worrying about other types

26. What compiler does Compiler does:
Check at compile time that someone will catch Blow up program at run time if it throws something else
Speed things up by not worrying about other types Probably slow things down by doing checks at runtime

27. What Programmers Need Use comments to announce what to expect:
Announce to other programmers what to expect
Check at compile time that someone will catch
Speed things up by not worrying about other types

28. Final Thoughts When to throw What to throw
Exceptional problem you can't handle at this level
Still try to prevent errors at higher level
What to throw
Approproiate std exception (i.e. out_of_range)
std::logic_error : good catch all