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

Hi Great Monks,

Say i have two modules named A.pm and B.pm each of them containing a subroutine named 'func'. If i use both of these modules in a program and if i call &func(), which subroutine will be executed. In the mean time, im using Exporter module in both the modules A and B.

Please regret me incase if this question seems to be insane.

As always, million thanks in advance.

Regards,
Murugesan Kandasamy

Replies are listed 'Best First'.
Re: Function Identification.
by holli (Abbot) on Jul 26, 2005 at 11:57 UTC
    If i use both of these modules in a program and if i call &func(), which subroutine will be executed.
    Without having tested it, I'd say the one that loaded (used) last. To make it clear, simply call the functions with their fully qualified name: &A::func and &B::func respectively.


    holli, /regexed monk/
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Re: Function Identification.
by jkeenan1 (Deacon) on Jul 26, 2005 at 12:38 UTC
    A side note:

    Even in constructing little test cases, B::ware of writing packages that begin with 'B', as they will interfere with the B:: modules included with core. Even the gods get tripped up with this one; cf. Is Conway's NEXT.pm broken?.

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Re: Function Identification.
by mattk (Pilgrim) on Jul 26, 2005 at 12:48 UTC
    Subroutine calls will be inherited differently depending on whether you're calling them as functions, or as object methods. We can test this with two different modules: 
    package Foo;
use base 'Exporter';
@EXPORT = "func";

sub func { return "Foo" }

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    package Bar;
use base 'Exporter';
@EXPORT = "func";

sub func { return "Bar" }

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    Exporter overwrites the current package's symbol table entry for func() when you use a module. So, as holli has pointed out, if two seperate modules export the same function, the most recent one will be the one that is called: when you use Foo, func() is created locally as a reference to Foo::func(), and then when Bar is imported it replaces the existing reference with one to Bar::func(): 
    [matt@blue 478146] cat func.pl 
package func;
use Foo;
use Bar;
print func();
[matt@blue 478146] perl -l func.pl 
Bar

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    OO Perl's inheritance chain works differently - it's done at run-time, left-to-right, depth first (left-to-right is referring to the contents of @ISA). So, if you're using OO modules, you will get the opposite result - a call to func() will go to the *first* module Perl finds, that provides the requested method (function): 
    [matt@blue 478146] cat meth.pl 
package meth;

use base 'Foo';
use base 'Bar';

sub new { bless {}, __PACKAGE__ }
$o = new meth;
print $o->func;
[matt@blue 478146] perl -l meth.pl 
Foo

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