vivification of package variables

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

i don't understand why the package variables are vivificated _before_ the require.

it looks like aaa::var defines a variable in namespace aaa. (and vivificates it if the package is not yet defined)

thanks

::::::::::::::
perlexe.pl
::::::::::::::
#!/usr/bin/perl
use strict;
use warnings;


BEGIN { 
  $perlpkg::var_our = "foo";
  $perlpkg::var_my  = "foo";
  $perlpkg::var_lcl = "foo";
}
print "\noml : $perlpkg::var_our $perlpkg::var_my $perlpkg::var_lcl\n"
+;

require "perlpkg.pm";

$perlpkg::var_our = "bar";
$perlpkg::var_my  = "bar";
$perlpkg::var_lcl = "bar";
print "oml : $perlpkg::var_our $perlpkg::var_my $perkpkg::var_lcl\n\n"
+;
::::::::::::::
perlpkg.pm
::::::::::::::
package perlpkg;

our $var_our;
my  $var_my ;
local  $var_lcl;
print "    oml : $var_our $var_my $var_lcl\n";

$var_our = "baz";
$var_my  = "baz";
$var_lcl = "baz";
print "    oml : $var_our $var_my $var_lcl\n";

1;

STDOUT :

oml : foo foo foo
    oml : foo  
    oml : baz baz baz
oml : bar bar
[download]

Comment on vivification of package variables Download Code

Replies are listed 'Best First'.
Re: vivification of package variables by GrandFather (Saint) on Sep 21, 2010 at 05:42 UTC
Despite the names, all your :: style variables are package variables which pop into existence when they are used. Consider: `use strict; use warnings; $main::var_my = "foo1"; my $var_my = "foo2"; print "Lexical: $var_my\n"; print "Package: $main::var_my\n";` [download] Prints: `Lexical: foo2 Package: foo1` [download] True laziness is hard work	[reply] [d/l] [select]
Re: vivification of package variables by ikegami (Patriarch) on Sep 21, 2010 at 05:34 UTC
Just mentioning its name is enough to vivify it. (You're going far beyond that and assigning a value to it!) Using a symbolic reference doesn't vivify it until the reference evaluated. `>perl -le"$pkg::var1; $pkg::{'var2'}; print for keys %pkg::;" var1` [download] I'm not sure I understand your disconnect. Do you think `$pkg::var = "foo"; package pkg; print $var;` [download] is different than `package pkg; $var = "foo"; print $var;` [download] They're not.	[reply] [d/l] [select]
Re: vivification of package variables by JavaFan (Canon) on Sep 21, 2010 at 09:56 UTC
From a language point of view, all namespaces always exist. In fact, all package variables exist as well - the implementation just delays creating the internal bookkeeping associated with variables until they're actually encountered. The keyword `package` doesn't create much (just an entry in the stash, if existing already); it's main effect is compile time: it just sets the default package name non-lexical, not-special names live in until the end of the current scope, or the next package statement - whatever comes first.	[reply] [d/l]