The Perl Basics You Need To Know Page 2

So the is shared between the My::HTML package
and script.pl. It will work vice versa as well, if you create the
object in My::HTML but use it in script.pl. You have
true sharing, since if you change in script.pl, it
will be changed in My::HTML as well.

What if you need to share between more than two packages?
For example you want My::Doc to share as well.

You leave My::HTML untouched, and modify script.pl to
include:

 use My::Doc qw();


 Then you add the same Exporter code that I used in

My::HTML, into My::Doc, so that it also exports

. 

 One possible pitfall is when you want to use My::Doc in both

My::HTML and script.pl. Only if you add 
  use My::Doc qw();


 into My::HTML will  be shared. Otherwise

My::Doc will not share  any more. To make things

clear here is the code: 
  script.pl:
  ----------------
  use vars qw();
  use CGI;
  use lib qw(.); 
  use My::HTML qw(); # My/HTML.pm is in the same dir as script.pl
  use My::Doc  qw(); # Ditto
   = new CGI;
  
  My::HTML::printmyheader();


  My/HTML.pm
  ----------------
  package My::HTML;
  use strict;
  
  BEGIN {
    use Exporter ();
  
    @My::HTML::ISA         = qw(Exporter);
    @My::HTML::EXPORT      = qw();
    @My::HTML::EXPORT_OK   = qw();
  
  }
  
  use vars     qw();
  use My::Doc  qw();
  
  sub printmyheader{
    # Whatever you want to do with ... e.g.
    print ->header();
  
    My::Doc::printtitle('Guide');
  }
  1;


  My/Doc.pm
  ----------------
  package My::Doc;
  use strict;
  
  BEGIN {
    use Exporter ();
  
    @My::Doc::ISA         = qw(Exporter);
    @My::Doc::EXPORT      = qw();
    @My::Doc::EXPORT_OK   = qw();
  
  }
  
  use vars qw();
  
  sub printtitle{
    my  = shift || 'None';
    
    print ->h1();
  }
  1;
 
Using the Perl Aliasing Feature

to Share Global Variables

 As the title says you can import a variable into a script or module without

using Exporter.pm. I have found it useful to keep all the

configuration variables in one module My::Config. But then I have

to export all the variables in order to use them in other modules, which is bad

for two reasons: polluting other packages' name spaces with extra tags which

increases the memory requirements; and adding the overhead of keeping track of

what variables should be exported from the configuration module and what

imported, for some particular package. I solve this problem by keeping all the

variables in one hash %c and exporting that. Here is an example of

My::Config: 

  package My::Config;
  use strict;
  use vars qw(%c);
  %c = (
    # All the configs go here
    scalar_var => 5,
  
    array_var  => [qw(foo bar)],
  
    hash_var   => {
                   foo => 'Foo',
                   bar => 'BARRR',
                  },
  );
  1;