1 / 16

Exceptions

Exceptions. General mechanism for handling abnormal conditions Predefined exceptions : constraint violations, I/O errors, communication errors, other illegalities User-defined exceptions for robust abstractions

barb
Download Presentation

Exceptions

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Exceptions • General mechanism for handling abnormal conditions • Predefined exceptions: constraint violations, I/O errors, communication errors, other illegalities • User-defined exceptions for robust abstractions • Predefined exception raised by the runtime system. User-defined exception can be raised (thrown) by user code. • Exception handlers specify remedial actions or proper shutdown • Exceptions can be stored and re-raised later concurrency - 1

  2. Predefined exceptions in Ada • Defined in Standard: • Constraint_Error : value out of range • Program_Error : illegality not detectable at compile-time: unelaborated package, exception during finalization... • Storage_Error : allocation cannot be satisfied (heap or stack) • Tasking _Error : communication failure • Defined in Ada.IO_Exceptions: • Data_Error, End_Error, Name_Error, Use_Error, Mode_Error, Status_Error, Device_Error concurrency - 2

  3. Handling exceptions • Any begin-end block can have an exception handler: procedure test is x : integer := 25; y : integer := 0; begin x := x / y; exception when Constraint_Error => Put_Line (“as expected”); when others => Put_Line (“out of the blue!”); end; concurrency - 3

  4. A common idiom with Integer_Text_Io; use Integer_Text_Io; function Get_Data return Integer is X : Integer; begin loop-- as long as input is not legal begin Get (X); return X; -- if got here, input is valid exception when others => Put_Line (“input must be integer, try again”); end; end loop; end; concurrency - 4

  5. Exception propagation • When an exception is raised, thecurrent sequence of statements is abandoned • If an exception handler for the current exception is present, it is executed, and the current frame is completed • Otherwise, the frame is discarded, and the enclosing dynamic scopes are examined to find a frame that contains a handler for the current exception • If none is found, the program terminates • The current frame is never resumed concurrency - 5

  6. User-defined Exceptions • Client-server contract: if inputs are proper, either the output is correct or else client is notified of failure. The inputs are the responsibility of the client (the caller). package Stacks is Stack_Empty : exception; … package body Stacks is procedure Pop (X : out Integer; From : in out Stack) is begin if Empty (From) then raise Stack_Empty; else ... concurrency - 6

  7. The scope of exceptions • Exception has the same visibility as other declared entities: to handle an exception it must be visible in the handler • An others clause can handle unamable exceptions partially when others => Put_Line (“disaster somewhere”); raise; -- propagate exception, program will terminate concurrency - 7

  8. Exception information • An exception is not a type: we cannot declare exception variables and assign to them • An exception occurrence is a value that can be stored and examined • an exception occurrence may include additional information: source location of occurrence, contents of stack, etc. • Predefined package Ada.Exceptions contains needed machinery. concurrency - 8

  9. Ada.Exceptions package Ada.Exceptions is type Exception_Id is private; type Exception_Occurrence is limited private; function Exception_Identity (X : Exception_Occurrence) return Exception_Id; function Exception_Name (X : Exception_Occurrence) return String; procedure Save_Occurrence (Target : out Exception_Occurrence; Source : Exception_Occurrence); procedure Raise_Exception (E : Exception_Id; Message : in String := “”) ... concurrency - 9

  10. Using exception information exception when Expected : Constraint_Error => Save_Occurrence (Event_Log, Expected); when Trouble : others => Put_Line (“unexpected “ & Exception_Name (Trouble) & “ raised”); Put_Line (“shutting down”); raise; ... concurrency - 10

  11. Exceptions in C++ • Same runtime model • Exceptions are classes • Handlers appear in try blocks try { some_complex_calculation (); } catch (range_error) { // range error might be raised // in some_complex_calculation cerr << “oops\n”; catch (zero_divide) { // ditto for zero_divide cerr << “why is x zero?\n”; } concurrency - 11

  12. Defining and throwing exceptions • The program throws an object. There is nothing in the declaration to indicate it will be used as an exception. struct Zero_Divide { public: int lineno; // useful information Zero_Divide () {…} // constructor }; … try { … if (x == 0) throw Zero_Divide (..); // call constructor and go concurrency - 12

  13. Exceptions and inheritance • A handler names a class, and can handle an object of a derived class as well: class Matherr {}; // a bare object, no info class Overflow: public Matherr {…}; class Underflow: public Matherr {…}; class Zero_Divide: public Matherr {…}; try { weather_prediction_model (…); // who knows what will happen } catch (Overflow) {… // e.g. change parameters in caller catch (Matherr) { … // Underflow, Zero_Divide handled here catch(Disk_Error D) {… // Local name to capture exception info catch (…); // handle anything else (ellipsis) concurrency - 13

  14. Exceptions in Java • Model and terminology similar to C++: • exception are objects that are thrown and caught • try blocks have handlers, which are examined in succession • a handler for an exception can handle any object of a derived class • Differences: • all exceptions are extension of predefined class Throwable • checked exceptions are part of method declaration • the finally clause specifies clean-up actions concurrency - 14

  15. If a method might throw an exception, callers should know about it public void replace (String name, Object newvalue) throws NoSuch { Attribute attr := find (name); if (attr == null) throw new NoSuch (name); // build before throw newvalue.update (attr); } Caller must have a handler for NoSuch, or else must be declared as throwing NoSuch itself. Compiler checks. Only required for checked exceptions (not predefined ones, which are extensions of RuntimeException and Error). concurrency - 15

  16. Mandatory cleanup actions • Some cleanups must be performed whether the method terminates normally or throws an exception. publicvoid encrypt (String file) throws StreamException { Stream input; try { input = new Stream (file); iterator Words = new iterator (input); for (word w = Words.init (); Words.more(); w = Words.next()) { RSAencode(w); // may fail somewhere } finally { if (input != null) input.close(); }; // regardless of how we exit concurrency - 16

More Related