Exceptions and Signals

This document needs to be updated for class based exceptions. It only describes the old instance based signals. Please read the class-documentation of the Exception class for more info.

Smalltalk provides a powerful and flexible mechanism to handle exceptions and errors in a graceful way. Errors raise a so called Signal, which can be handled by a handlerblock.
Especially with respect to instance creation, deletion and garbage collection, Smalltalk exceptions are very clean and easy to use.
All error conditions can be trapped and reacted upon by signal handlers. This includes situations which are fatal in other languages, such as stack overflow or array index bounds violations.

The most interesting classes are:

• Signal- instance based exceptions
• SignalSet- a set of signals/exceptions with single handler
• ExceptionHandlerSet- a set of signals/exceptions with mutliple handlers
• Exception - class based exceptions

Signal (instance based exceptions)

Every exception condition is assigned a signal or an exception class, which is raised when that condition is encountered. Therefore, a signal object pr an exception class can be thought of a way to identify a particular exception. Classes which raise a signal (as opposed to an exception) in one of its methods, creates these signal objects at class initialization time and keeps them in class variables for later. These classes provide access methods for these in their class protocol.

Notice that most parts of the system have been changed to use the newer class-based exception mechanism, in which the Exception class itself is used as signal. The protocols and semantics are identical, so all of the following is valid both for signal instances and Exception classes.
Typical use:

• Signal access:

      Number arithmeticSignal
  
      Number divisionByZeroSignal
  
      Object errorSignal
  

• handling signals/exceptions:

      [ ...some computation ... ] on:aSignalOrExceptionClass do:[ ... handlerBlock ...]
  
      aSignalOrExceptionClass handle:[ ... handlerBlock ...] do:[ ...some computation ... ]
  
      aSignalOrExceptionClass catch:[ ...some computation ... ]
  

More details are found in the Signal and Exception class documentation.

SignalSet

• Handling multiple Exceptions in one handler:

      [ ...some computation ... ]
  	on:ExceptionClass1,ExceptionClass1,...
  	do:[:ex | ... handlerBlock ...]
  

More details are found in the SignalSet class documentation.

ExceptionHandlerSet

They understand the same protocol as signals and exception classes.

• Handling multiple Exceptions in different handlers:

      (ExceptionHandlerSet
  	on:Ex1 do:[:ex | ... handler for Ex1 ...];
  	on:Ex2 do:[:ex | ... handler for Ex2 ...];
  	...
  	handleDo:[
  	    ...
  	    protected block
  	    ...
  	].
  

More details are found in the examples section of the ExceptionhandlerSet class documentation.

Exception

• handler setup:

      [
  	... some computation
      ] on: aSignalOrExceptionClass do:[:ex |
  	... handler action
      ]
  
    or:
  
      aSignalOrExceptionClass
  	handle:[:ex |
  	    ... handler action
  	]
  	do:[
  	    ... some computation
  	]
  

• handler actions:

      aSignalOrExceptionClass handle:[:ex |
  	...
  	ex return
      ]
  
      aSignalOrExceptionClass handle:[:ex |
  	...
  	ex restart
      ]
  
      aSignalOrExceptionClass handle:[:ex |
  	...
  	ex resume
      ]
  

• queries:

      anException parameter
  
      anException creator
  
      anException errorString
  
      anException handlerContext
  
      anException suspendedContext
  

• raising an exception/signal:

      aSignalOrExceptionClass raise
  
      aSignalOrExceptionClass raiseWith:parameter
  

More details are found in the Exception class documentation.

Copyright © 1996 Claus Gittinger Development & Consulting