|Technote PT 17||June 1989|
Although Pascal can be used to write desk accessories, drivers, XCMDs and other
types of stand-alone code, and Object Pascal is an extension of Pascal, Object
Pascal cannot be used to write anything other than an application. This
limitation is due to the fact that Object Pascal method dispatching relies on a
valid A5 pointing to a jump table. Because MacApp is written in
Object Pascal, this limitation applies to it as well.
Object methods cannot always be called directly. To explain why this is so, let's take a case from MacApp. Part of the way MacApp works includes defining TView objects that can draw themselves. Whenever an update event occurs, MacApp traverses the list of TView objects that are installed in a window and calls the Draw method for each one. However, how does Pascal know which Draw method to call? Does it call TYourView.Draw? Does it call TView.Draw? There is no way to know, at compile time, what TView objects and descendants of TView will be passed to the MacApp update routine. Therefore, there is no way to determine the appropriate Draw routine at compile time and generate a direct call to it.
Object Pascal solves this problem by maintaining data structures called Class Info Tables for each Object Class defined. These Class Info Tables not only contain information about the correct procedure to call whenever a message is sent to an object, but they also contain information used to create a new instance of that object.
The mechanism for this dispatching is quite complex and not described here. However, the main point is that the mechanism absolutely relies on special jump table entries. These jump table entries are used to dynamically map method calls to the correct procedure, using the information found in the Class Info Tables. Since desk accessories, drivers, and XCMDs, by their very nature, cannot have a jump table, you cannot use Object Pascal to create them.
Object Pascal can be used to write MPW tools, and, in fact, was used to create the MABuild and PostRez tools that come with MacApp 2.0.
For more information on how Object Pascal works, I highly recommend the article
by Ken Doyle, "Introduction to Object Pascal," anthologized in The Complete
MacTutor, Volume 2. However, keep in mind that this information is already
slightly out of date, and should not be counted on to be completely accurate at
this time. In general, however, it is a good description of what is actually
happening when a method call is made.