How can I use a variable as a module name in Perl?

Question

I know it is possible to use a variable as a variable name for package variables in Perl. I would like to use the contents of a variable as a module name. For instance:

package Foo;
our @names =("blah1", "blah2");
1;

And in another file I want to be able be able to set the contents of a scalar to "foo" and then access the names array in Foo through that scalar.

my $packageName = "Foo";

Essentially I want to do something along the lines of:

@{$packageName}::names; #This obviously doesn't work.

I know I can use

my $names = eval '$'. $packageName . "::names"

But only if Foo::names is a scalar. Is there another way to do this without the eval statement?

Edited the question. I want to be able to access the package variable "@names" within foo — mjn12
– mjn12, Commented Apr 15, 2010 at 19:58

friedo · Accepted Answer · 2010-04-15 20:03:12Z

19

To get at package variables in package $package, you can use symbolic references:

no strict 'refs';
my $package = 'Foo';

# grab @Foo::names
my @names = @{ $package . '::names' }

A better way, which avoids symbolic references, is to expose a method within Foo that will return the array. This works because the method invocation operator (->) can take a string as the invocant.

package Foo;

our @names = ( ... );
sub get_names { 
    return @names;
}

package main;
use strict;

my $package = 'Foo';
my @names = $package->get_names;

answered Apr 15, 2010 at 20:03

friedo

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4 Comments

mjn12 Over a year ago

I think I like the alternative method you described, long as I can define get_names in Foo's base class (we'll call it Bar to be original) that way I can just define @names in every class which inherits from Bar. I'm using Moose objects if that makes any difference.

friedo Over a year ago

@Paul, parens are not required on method calls if there are no arguments. Some people prefer to always include an empty () for consistency, but I generally leave them out if I can.

Paul Nathan Over a year ago

Gotcha. I prefer the consistency personally.

brian d foy Over a year ago

You can consistentially leave off parens, too. That's consistent. :)

Ether · Accepted Answer · 2010-04-15 20:07:44Z

5

Strict checking is preventing you from using a variable (or literal string) as part of a name, but this can be disabled locally:

my @values;
{
    no strict 'refs';
    @values = @{"Mypackage"}::var;
}

edited Apr 15, 2010 at 20:07

answered Apr 15, 2010 at 19:57

Ether

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4 Comments

mjn12 Over a year ago

Essentially what I'm doing is globbing a directory for *.pm and then for each file I find I require $filename. Once those modules are loaded I then need to access the @names variable defined inside each package. I don't know the package's name until runtime, all I have is a variable which will contain the name of the package.

mjn12 Over a year ago

I'm not have a problem loading packages from a variable name - I've got that part down. Its accessing the variables within those packages. This answer would have definitely save me a lot of googling an hour ago though :)

Ether Over a year ago

@mjn: oops sorry, I'm editing my answer now to make the real issue more prominent :)

Bernhard Bodenstorfer Over a year ago

When I try this solution on Perl v5.30.1, I get an error: “

Bareword found where operator expected at test15.pl line 4, near "}::var" 	(Missing operator before ::var?) syntax error at test15.pl line 4, near "}::var"

”

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