Exception handling • Exception – infrequent, abnormal situation in program logic at program execution – not necessarily an error • Reasons for exceptions: – hardware events • arithmetic overflow, parity errors – operating system events • out of memory exception – programming language properties • dynamic range checks – application program properties • subprogram abuse, data structure overflow/underflow TUT Pervasive Computing Principles of programming languages 1 Maarit Harsu / Matti Rintala / Henri Hansen
protected block exception handler Exception handling raise/throw an exception • Necessary actions to continue program execution after detecting an exception – reporting about the exception – controlled program termination • Exception recovery (different choices) – new trial • e.g. after restoring data structures – alternative action – memory allocation – ask for instructions TUT Pervasive Computing Principles of programming languages 2 Maarit Harsu / Matti Rintala / Henri Hansen
Tools for reporting exceptions • Using parameters (or global variables): procedure Search ( K: Key; var D: Info; var Found: Boolean ); begin ... end ; ... Search ( k, d, f ); if not f then ... exception handling ... • Using a special exceptional value: procedure Search ( K: Key; var D: Info ); begin ... end ; ... Search ( k, d ); if d = NotFound then ... exception handling ... TUT Pervasive Computing Principles of programming languages 3 Maarit Harsu / Matti Rintala / Henri Hansen
Comparison to Go func Open ( name string ) ( file *File, err error ) f, err := os.Open ( ”filename.txt” ) if err != nil { log.Fatal ( err ) // prints error message and stops } // do something with the open *File f TUT Pervasive Computing Principles of programming languages Maarit Harsu / Matti Rintala / Henri Hansen
Tools for giving instructions • Passing a label as a parameter: procedure Search ( K: Key; var D: Info; label NotFound ); begin ... if ...not found ... then goto NotFound; end ; ... Search ( k, d, L ); // and jump over L L: ... exception handling ... • Passing a handler routine as a parameter: procedure Search ( K: Key; var D: Info; procedure NotFound ); begin ... if ...not found ... then NotFound; end ; procedure P; begin ... exception handling ... end ; ... TUT Pervasive Computing Principles of programming languages 5 Search ( k, d, P ); Maarit Harsu / Matti Rintala / Henri Hansen
Benefits of language-provided exception handling • Specific concepts for exception handling – high-level abstraction • Exceptional situations distinguished from the normal program logic – readability, understandability, (orthogonality) • Efficiency? – does not burden normal program execution • Handling mechanism also for system exceptions TUT Pervasive Computing Principles of programming languages 6 Maarit Harsu / Matti Rintala / Henri Hansen
Design issues for exception handling • How to detect an exception? How to handle hardware exceptions? • How, when and where to report about the exception? How to select a handler? • How to bind an exception and its handler together? – how is the handler related to the code sequence where its exceptions occur? • What to do after the exception is handled? TUT Pervasive Computing Principles of programming languages 7 Maarit Harsu / Matti Rintala / Henri Hansen
Ada • Exceptions are built-in types declare empty_queue: exception ; • Handlers are associated statically with large program units – blocks, subprograms, packages, tasks – handler is bound dynamically (after subprogram calling order) • Multi-level handling – moving backwards in the chain of called subprograms until the suitable handler is found • Exception can also be raised in an exception handler TUT Pervasive Computing Principles of programming languages 8 Maarit Harsu / Matti Rintala / Henri Hansen
Example on Ada package Stack is procedure Push ( X: Integer ); exception handling function Pop return Integer; Underflow, Overflow: exception ; Location for handler: end Stack; 1. Block 2. Subprogram body package body Stack is 3. Package body max: constant := 10; 4. Task body S: array ( 1.. max ) of Integer; top: Integer range 0 .. max; declare procedure Push ( X: Integer ) is use Stack; begin begin if top = max then ... raise Overflow; Push ( 3 ); end if ; ... -- actual code for Push exception end Push; when Underflow | Overflow => function Pop return Integer is Put ( ”Error in stack handling” ); ... raise Underflow; ... end ; end Stack; TUT Pervasive Computing Principles of programming languages 9 Maarit Harsu / Matti Rintala / Henri Hansen
Binding handler to an exception (Ada) • Moving exception (until suitable handler is found) – from subprogram body • the same exceptions is raised again in the call of the subprogram – from block • the same exception is raised again after the block – from package body • the same exception is raised again after the body – from task body • not moved, task is terminated – from declarations • elaboration of declarations is terminated TUT Pervasive Computing Principles of programming languages 10 Maarit Harsu / Matti Rintala / Henri Hansen
procedure P is Error: exception ; Control flow procedure R; procedure Q is in different begin R; situations ... -- raising Error ( 2 ) exception (Ada) when Error => ... ; end Q; procedure R is begin ... – raising Error ( 3 ) end R; begin ... -- raising Error ( 1 ) Q; ... exception when Error => ... ; TUT Pervasive Computing Principles of programming languages 11 end P; Maarit Harsu / Matti Rintala / Henri Hansen
Predefined exceptions (Ada) • Constraint_Error: – restriction (e.g. range restriction) is broken • Storage_Error: – memory exhaustion • Tasking_Error: – error in tasks • Program_Error: – other error occurs TUT Pervasive Computing Principles of programming languages 12 Maarit Harsu / Matti Rintala / Henri Hansen
do_task is local times_tried: INTEGER Exception do if times_tried = 0 then handling implementation1 elsif times_tried = 1 then in Eiffel implementation2 end rescue times_tried := times_tried + 1; if times_tried < 2 then ... repairing data structures ... retry end end ; TUT Pervasive Computing Principles of programming languages 13 Maarit Harsu / Matti Rintala / Henri Hansen
C++ • Exception terminology: – try -block • includes the error-prone code (protected block) – throw -command • throws (raises) an exception – catch -block • handles the exception • Exception – can be a data element or object of any type – does not necessarily have a name – exception class TUT Pervasive Computing Principles of programming languages 14 Maarit Harsu / Matti Rintala / Henri Hansen
int space_left ( int index ) { if ( index >= MAX ) throw ( index ); C++ else return 1; } int main ( void ) { int array [ MAX ]; int i = 0; try { while ( space_left ( i ) && ( cin >> array [ i ] ) ) i++; } catch ( int i ) { cout << ”Index ” << i << ”out of range\n”; } return 0; } TUT Pervasive Computing Principles of programming languages 15 Maarit Harsu / Matti Rintala / Henri Hansen
int const SIZE = 10; C++ int table [ SIZE ]; (exception class) int InsideTable ( int i ) { if ( i >= SIZE ) throw OverIndex ( i ); else return 1; } void main ( ) { int ind; class OverIndex { try { private : ind = 0; int ind; while ( InsideTable ( ind ) ) public : cin >> table [ ind++ ]; OverIndex ( int i ) { ind = i; } } void report ( ) { catch ( OverIndex error ) { cout << ”Bad index: ” << ind; error.report ( ); } } }; } TUT Pervasive Computing Principles of programming languages 16 Maarit Harsu / Matti Rintala / Henri Hansen
C++ standard exception hierarchy out_of_range bad_alloc length_error exception bad_typeid domain_error logic_error invalid_argument bad_exception range_error bad_cast overflow_error runtime_error underflow_error TUT Pervasive Computing Principles of programming languages 17 Maarit Harsu / Matti Rintala / Henri Hansen
Java • Resembles C++ exception handling • Exeptions are instances of Throwable – predefined exception classes: Error and Exception and their subclasses • Exceptions must be given in the header of an operation • finally -clause (after try and catch blocks) – is executed always (regardless of exceptions) try { ... } catch { ... } // more handlers TUT Pervasive Computing Principles of programming languages 18 finally { ... } Maarit Harsu / Matti Rintala / Henri Hansen
Haskell • Lazy evaluation -> execution order ”undefined” -> ??? • Exceptions are problematic – when to throw, when to catch • Haskell allows exceptions only in IO functions (where order is predefined) • Other mechanisms must be used in the purely functional side TUT Pervasive Computing Principles of programming languages 19 Maarit Harsu / Matti Rintala / Henri Hansen
Haskell • Function error prints an error message and terminates the program (its value is undefined) Div1 :: Int -> Int -> Int Div1 _ 0 = error ”Division by zero” Div1 x y = div x y • No error handling or recovery possible! TUT Pervasive Computing Principles of programming languages 20 Maarit Harsu / Matti Rintala / Henri Hansen
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