|Pathologically Eclectic Rubbish Lister|
Using can With Inheritanceby btrott (Parson)
|on Apr 18, 2000 at 22:07 UTC||Need Help??|
Using can With InheritanceThis started out as a Q & A on using can, so that's how it'll start.
If you asked, "How can I determine whether a particular object has a particular method?", an answer would be:
A ScenarioThis is relatively simple and straightforward. I found a rather interesting use for it the other day, though. Here was the scenario:
I had created (basically) an abstract base class for holding a tree-like data structure. The base class contained all of the methods necessary to interact with the data structure--to get data out, to put data in, etc.
But the base class didn't have any notion of how to read the data in from an external source; that was to be provided by the subclasses.
So I also defined two subclasses: one for processing XML data and one for processing a different format.
The base class also had a recurse method--supplied with a callback method, it would recurse into the data structure and call the callback for each node found in the tree. I had defined some simple callbacks: one to pretty-print the data to the screen; one to dump the data in HTML format using unordered lists; and one to write the file to disk in either XML or this other format. These callbacks were "private" functions, though, so I also provided accessor methods to return sub references to the callbacks.
A ProblemThe problem, though, was that these accessor methods were defined in my base class, and I wanted to allow for my subclasses to override the callback methods. However, I (obviously) wouldn't know whether or not they overrode the methods until run-time, so I couldn't hard-code the subclass name into my accessor functions.
A SolutionSo I used can. When I defined a new object, it was blessed into one of my subclasses; so, in my accessor methods, I realized that I could call can with the callback method name as the argument, and I'd get back the "correct" version of the callback method.
So, for example, here's a stripped-down version of my base class:
The method "_write_file" is defined in my subclasses, so here's an example one of those: