Callouts, such as numbered bullets, are an annotation mechanism. In an online system, these bullets are frequently “hot,” and clicking on them sends you to the corresponding annotation.

A programlistingco is a wrapper around an areaspec and a programlisting. An areaspec identifies the locations (coordinates) in the programlisting where the callouts occur. The programlistingco may also contain the list of annotations in a calloutlist, although the calloutlist may also occur outside the wrapper, elsewhere in the document.

It is also possible to embed co elements directly in the verbatim text, in order to avoid having to calculate the correct coordinates. If you decided to go this route, use a programlisting and a calloutlist without the programlistingco wrapper. A programlistingco must specify at least one coordinate.

For a complete description of callouts, see callout.

areaspec, calloutlist, co, coref, imageobjectco, screenco

<article xmlns='http://docbook.org/ns/docbook'>
<title>Example programlistingco</title>

<programlistingco>
<areaspec>
<areaset xml:id="ex.plco.const">
  <area xml:id="ex.plco.c1" coords='4'/>
  <area xml:id="ex.plco.c2" coords='8'/>
</areaset>
<area xml:id="ex.plco.ret" coords='12'/>
<area xml:id="ex.plco.dest" coords='12'/>
</areaspec>
<programlisting>
sub do_nothing_useful {
    my($a, $b, $c);

    $a = new A;
    $a->does_nothing_either();
    $b = new B; 
    $c = "frog";

    return ($a, $c);
}
</programlisting>
<calloutlist>
  <callout arearefs="ex.plco.const">
    <para>These are calls to the constructor
      <function>new</function> in the object classes.
    </para>
  </callout>
  <callout arearefs="ex.plco.ret">
    <para>This function returns a two-element list.
    </para>
  </callout>
  <callout arearefs="ex.plco.dest">
    <para>The <emphasis>destructor</emphasis>
      (<function>DESTROY</function>) for the object
      <literal>$b</literal> will be called automatically
      for this object since there can be no other references
      to it outside this function.
    </para>
  </callout>
</calloutlist>
</programlistingco>

</article>

sub do_nothing_useful {
    my($a, $b, $c);

    $a = new A;
    $a->does_nothing_either();
    $b = new B; 
    $c = "frog";

    return ($a, $c);
}