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Defining a subroutine in another package

by jmay (Sexton)
on Jan 26, 2002 at 12:01 UTC ( #141745=perlquestion: print w/ replies, xml ) Need Help??
jmay has asked for the wisdom of the Perl Monks concerning the following question:

perldoc perlmod sez:

Subroutine definitions (and declarations, for that matter) need not necessarily be situated in the package whose symbol table they occupy. You can define a subroutine outside its package by explicitly qualifying the name of the subroutine: 

    package main;
    sub Some_package::foo { ... }   # &foo defined in Some_package  

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This is just a shorthand for a typeglob assignment at compile time: 

    BEGIN { *Some_package::foo = sub { ... } }  

[download]

OK, my question: Is there a clean way to add a subroutine to another package, where the "other" package name is not known at compile time?

The best I can do is: 

  eval "*${package}::foo = sub { ... }";

[download]

but I'd rather not use eval if I don't have to.

-Jason

Comment on Defining a subroutine in another package
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Re: Defining a subroutine in another package
by japhy (Canon) on Jan 26, 2002 at 12:53 UTC
    Just leave eval() out of it. You can write: *{$package . "::foo"} = sub { ... };

    _____________________________________________________
    Jeff[japhy]Pinyan: Perl, regex, and perl hacker.
    s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;

[reply]
[d/l]

      I believe this only works in perl 5.6.0 and above though.

[reply]
        It works for me on 5.004_5 and (I presume) even earlier, though you'll have to disable strict 'refs'.

        It's just a symbolic access to a typeglob, nothing special. :)

[reply]
Re: Defining a subroutine in another package
by Aristotle (Chancellor) on Jan 27, 2002 at 00:13 UTC
    I think there's a way without circumventing strict, however I only have a vague recollection of something I read in the Camel book about this, way back when. I revolves around the %main:: hash.. I'm not sure how one would go about it though. I spent a while examining the output of perl -MData::Dumper -e'print Dumper(\%main::);' but can't seem to reach a useful conclusion. What I was looking for is a way to go through a hash lookup via $main::{$package}. Anyone up to fill the void?

    Makeshifts last the longest.

[reply]
[d/l]
      Thank you for jogging my memory: 
      use strict;

my $sub = sub {
    print "Hello!\n";
};

my $var = 'Bar';
my $pak = 'Foo::';
my $ref = \%main::;

$ref->{'Foo::'}{bar} = $sub;
$main::{'Foo::'}{$var} = $sub;
$ref->{$pak}{baz} = $sub;
$main::{$pak}{boo} = $sub;

Foo::bar();
Foo::Bar();
Foo::baz();
Foo::boo()

      [download]
      Update: added variable package name examples
[reply]
[d/l]
        Ah so I can use them that way. Data::Dumper produces entries like 'UNIVERSAL::' => *{'::UNIVERSAL::'} for keys denoting a package name, which confused me - can you explain what that is? Is that a symbolic typeglob reference, or something completely different? (Incidentally, typeglobs are the only part of Perl I never even came close to grokking. I understand what they are supposed to be, but the concept is still very alien.)

        Makeshifts last the longest.

[reply]
[d/l]
        Is there a benefit to using this method over assigning the subref to a glob?

        Being right, does not endow the right to be rude; politeness costs nothing.
        Being unknowing, is not the same as being stupid.
        Expressing a contrary opinion, whether to the individual or the group, is more often a sign of deeper thought than of cantankerous belligerence.
        Do not mistake your goals as the only goals; your opinion as the only opinion; your confidence as correctness. Saying you know better is not the same as explaining you know better.

[reply]
Re: Defining a subroutine in another package
by perrin (Chancellor) on Jan 27, 2002 at 00:59 UTC
    You would probably be better off using callbacks or OO with inheritance for this.
[reply]

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