Handling Exceptionally Sticky Problems Exceptions Handling Exceptionally Sticky Problems

Handling Errors Some things can't be done

Guarding Can protect against foreseeable errors: ALWAYS validate user input

Lack Of Context Low level code How should it respond?

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

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

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

Option 2 - Return Devise special error code to return

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

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

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

Catching A thrown exception will blow up your program…

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

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

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

Defining Exceptions Exception can be anything Custom type:

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

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?

Std::Excptions Std library defines exception class exception is parent to all Has virtual what() function http://www.cplusplus.com/reference/exception/exception/

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

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

Proper Catching Using out_of_range exception class:

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

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

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

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

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

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