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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

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Exceptions

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  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

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