Using can With Inheritance

This 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:

Use the can method on your object. From perlobj:

    can(METHOD)

    can checks to see if its object has a method called METHOD. If it 
+does
    then a reference to the sub is returned; if it does not then undef
+ is
    returned.
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A Scenario

This 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 Problem

The 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 Solution

So 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:

    package A;

    # stuff taken out

    sub get_write_file_func {
        my $self = shift;
        return $self->can("_write_file");
    }
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The method "_write_file" is defined in my subclasses, so here's an example one of those:

    package A::B;
    use A;

    use vars qw/@ISA/;
    @ISA = qw/A/;

    # stuff taken out

    sub _write_file {
        my($path, $val, $start) = @_;
        # ...
    }
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