Exceptions

1. Exceptions

2. Errors • Low-level errors • Divide by 0 • Dereference a null pointer • High-level, logical errors • Inserting past the end of a list • Calling list.get(int) when the list is empty • Require immediate handling

3. Cause of errors • User error • Giving a bad file name • Trying to undo when nothing has been done • Programmer error • Bugs • These errors are dealt with by an exception class.

4. Handling errors • Exceptions deal with errors • The exception class passes information about the error that just occurred • from where the error is detected • to where the error will be resolved • Exceptions allow us to separate problem detection from problem resolution

5. Handling exceptions • Throw the exception • Catch and handle the exception • Catch it, then re-throw it or throw a different exception • Ignore the exception • If there is no handler to deal with the exception, then a default terminate function is called

6. Throw • Exceptions are “thrown” • To a part of the code that can handle them • Halts current code • Control transferred to a “catch clause” • When a program runs into an error that it cannot handle • Many throw expressions take a string initializer but not all

7. Try blocks • Try blocks wrap a series of statements • Can be nested • Followed by one or more catch clauses • Try block is a local scope • Any variable declared inside is not accessible outside of the block (including catch clauses) • Exceptions are “thrown” from insidetry blocks

8. Catch clauses • Catch what is thrown • Catch clause has 3 parts: • Keyword catch • Exception specifier (the type of exception) • And a block of code • Often called “handlers” because they handle the exception • The order handlers appear is important • For a specific try block, the catch clauses are examined in order of their appearance

9. Try/catch block • Catch blocks can only be entered by catching thrown statements • Syntax try{ //program statements } catch (exception-specifier){ //handler statements }//…can have more than one catch clause

10. Rethrow • If a catch cannot completely handle an exception, the catch clause can throw the exception to another catch further up the list of function calls • Not followed by a type or expression • Syntax: • throw; • can only appear in a catch

11. Functions can tell compiler what type of throw • throw() • Tells the compiler the function does not throw an exception • void myFunction(inti) throw(); • throw(…) • Tells the compiler the function can throw an exception • void myFunction(inti) throw(…); • throw(type) • Tells the complier the function can only throw an exception of type type • void myFunction(inti) throw(int);

12. Throw • We can throw exceptions in the middle of a function //… if(!<some true condition>) throw runtime_error(“Not true.”); //if the statement was true, just continue on //…

13. Catch • Similar to throw, catch(…) is a catch-all handler • It can catch an exception of any type • It is often used with a rethrow expression • Syntax: void myFunction() { try{ //program statements }catch(…) { //work to partially handle the exception throw; } }

14. Standard exceptions class • exception • Most general kind of problem • Provides little information about the error, only that it has occurred • Takes in NO string initializer • runtime_error • Can only be detected at run time • Some more specific types of runtime_error • range_error : outside of meaningful value range • overflow_error : computation that overflowed

15. Standard Exceptions • logic_error • Detected before run time • Some more specific logic_errors • invalid_argument : inappropriate argument • length_error : attempt to create an object larger than the maximum size for that type • out_of_range : used a value outside of the valid range • Both runtime and logic errors require string initializers • Used to provide additional information about the error that occurred.

16. Standard exceptions Hierarchy

17. Stack unwinding • When looking for a catch, each function in which a catch is not found is popped off the stack • If a throw is in a try block, then the catch clause associated with the try block is examined first • If no matching catch, then this function is exited and the search continues in the function that called this one • When a function exits due to an exception, local objects are destroyed properly

18. catchable types • When an exception is thrown, it can be caught by the following handlers: • One that can catch any type • One that accepts the same type as was thrown • Or a reference to the same type as was thrown • One that accepts the base class as the type thrown • Or a reference to the same base class as was thrown • A catch handler for a base class must not precede the catch handler for the derived class

19. terminate • Exceptions cannot remain unhandled • If no matching catch is found, terminate is called • terminate is a library function which ends your program

20. Questions?

21. In tic-tac-toe, a player can only enter X’s and O’s. Write an exception to deal with a player entering something other than X’s and O’s.