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<TITLE>How do I design my own classes?</TITLE>
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<H1>How do I design my <EM>own</EM> classes?</H1>
<P>In addition to the methods listed above, all of the classes used
in <A HREF=sgmlspm.html><TT>SGMLS.pm</TT></A> have an <TT>ext</TT> method which returns a
reference to an initially-empty hash table. You are free to use this
hash table to store <EM>anything</EM> you want -- it
should be especially useful if you are building your own, derived
classes from the ones provided here. The following example builds a
derived class <TT>My_Element</TT> from the <A HREF=sgmlselement.html><TT>SGMLS_Element</TT></A>
class, adding methods to set and get the current font:</P>
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<PRE>use SGMLS;
package My_Element;
@ISA = qw(SGMLS_Element);
sub new {
my ($class,$element,$font) = @_;
$element->ext->{'font'} = $font;
return bless $element;
}
sub get_font {
my ($self) = @_;
return $self->ext->{'font'};
}
sub set_font {
my ($self,$font) = @_;
$self->ext->{'font'} = $font;
}</PRE>
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<P>Note that the derived class does not need to have any knowledge
about the underlying structure of the <A HREF=sgmlselement.html><TT>SGMLS_Element</TT></A>
class, and need only avoid shadowing any of the methods currently
existing there.</P>
<P>If you decide to create a derived class from the <A HREF=sgmls.html><TT>SGMLS</TT></A>, please note that in
addition to the methods listed above, that class uses internal methods
named <TT>element</TT>, <TT>line</TT>, and
<TT>file</TT>, similar to the same methods in <A HREF=sgmlsevent.html><TT>SGMLS_Event</TT></A> --
it is essential that you not shadow these method names.</P>
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<ADDRESS>David Megginson <A HREF="mailto:dmeggins@aix1.uottawa.ca"><dmeggins@aix1.uottawa.ca></A></ADDRESS>
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