Smalltalk and Object Orientation: an Introduction - Free
Smalltalk and Object Orientation: an Introduction - Free
Smalltalk and Object Orientation: an Introduction - Free
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Notifiers c<strong>an</strong> be displayed under a number of circumst<strong>an</strong>ces. They c<strong>an</strong> be caused:<br />
• Accidentally, by sending a message to <strong>an</strong> object which has no corresponding method. This is the<br />
usual response to a program error.<br />
• Deliberately, by typing 'CTRL-C' or equivalent to break into the current execution.<br />
• Deliberately, by inserting a breakpoint. We will discuss these later.<br />
• Accidentally, by the system running out of memory.<br />
• Accidentally, by recursion occurring in the system error h<strong><strong>an</strong>d</strong>ling.<br />
5.5 Error processing<br />
H<strong><strong>an</strong>d</strong>ling run time errors in <strong>Smalltalk</strong> is slightly different to the way in which run time errors are<br />
h<strong><strong>an</strong>d</strong>led in some other l<strong>an</strong>guages. Unlike, for example, C++ on a UNIX box, you do not need to take<br />
your core dump <strong><strong>an</strong>d</strong> <strong>an</strong>alyze it using <strong>an</strong>other piece of software. Instead, the system indicates the fault<br />
<strong><strong>an</strong>d</strong> gives you a ch<strong>an</strong>ce to decide what to do next. One possibility is to interactively fix the problem.<br />
This was what was done during the exercise at the end of the last chapter.<br />
Figure 5.4: An inspector, notifier <strong><strong>an</strong>d</strong> debugger<br />
Figure 5.4 illustrates <strong>an</strong> exception notifier warning that a message ( newaAddress:for:) has been<br />
sent to <strong>an</strong> o bject which does not underst<strong><strong>an</strong>d</strong> that message. It also illustrates the debugger which would<br />
be displayed if the debug option was selected. Finally, it illustrates <strong>an</strong> inspector examining <strong>an</strong> inst<strong>an</strong>ce<br />
of class <strong>Object</strong>.<br />
5.5.1 Exception notifiers<br />
When a message is sent to <strong>an</strong> object with no corresponding method, then <strong>an</strong> exception notifier window<br />
(as illustrated in the bottom left h<strong><strong>an</strong>d</strong> corner of Figure 5.4) is displayed. This notifier is also displayed if<br />
a halt message (a breakpoint) is encountered. The exception notifier has a number of options which<br />
allow the programmer to determine what should happen next. The options are:<br />
• Debug This closes the notifier view <strong><strong>an</strong>d</strong> opens a debugger view on the error condition.<br />
• Proceed Continue as if the error had not occurred.<br />
• Terminate This terminates the current evaluation.<br />
• Copy Stack This copies the current state of the execution stack.<br />
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