How can I create a Perl variable name based on a string?

https://stackoverflow.com/questions/2275463

21-09-2019
|

Question

In Perl, is it possible to create a global variable based on a string?

E.g., if I had a function like:

sub create_glob_var {
    my ($glob_var_str) = @_;
    # something like this ( but not a hash access).
    our ${$glob_var_str};
};

and I called it like:

create_glob_var( "bar" );

How could I modify create_glob_var to actually create a global variable called $bar?

My project is using perl 5.8.5.

EDIT

The following doesn't work:

use strict;
BEGIN {
  sub create_glob_var {
    my ($glob_var_str) = @_;
    no strict 'refs';
    $$glob_var_str = undef;  # or whatever you want to set it to
  }

  create_glob_var("bah");
};

$bah = "blah";

Produces:

Variable "$bah" is not imported at /nfs/pdx/home/rbroger1/tmp2.pl line 12.
Global symbol "$bah" requires explicit package name at /nfs/pdx/home/rbroger1/tmp2.pl line 12.
Execution of /nfs/pdx/home/rbroger1/tmp2.pl aborted due to compilation errors.

NOTE I realize that using global variables causes ozone depletion and male pattern baldness. I'm trying to clean up some legacy code that is already completely infected with the use of global variables. One refactor at a time...

Solution

If you are trying to clean up old code, you can write a module which exports the required variable(s). Every time you feel the need to invoke create_glob_var, instead add a variable to this package and put that in the import list.

This will help you keep track of what is going on and how variables are being used.

package MyVars;

use strict; use warnings;

use Exporter 'import';

our($x, %y, @z);

our @EXPORT_OK = qw( $x %y @z );

The script:

#!/usr/bin/perl

use strict;use warnings;

use MyVars qw( $x %y @z );

$x = 'test';
%y = (a => 1, b => 2);
@z = qw( a b c);

use Data::Dumper;
print Dumper \($x, %y, @z);

Output:

$VAR1 = \'test';
$VAR2 = {
          'a' => 1,
          'b' => 2
        };
$VAR3 = [
          'a',
          'b',
          'c'
        ];

OTHER TIPS

sub create_glob_var {
    my ($glob_var_str) = @_;
    no strict 'refs';
    $$glob_var_str = undef;  # or whatever you want to set it to
}

The no strict 'refs' is only necessary if use strict is in effect, which it always should be.

Addendum:

If you're asking if there's a way to write a subroutine create_glob_var such that the following code will succeed:

use strict;
create_glob_var("bar");
$bar = "whatever";

...then the answer is "No." However, Perl's vars pragma will do what you want:

use strict;
use vars qw($bar);
$bar = "whatever";

But this is kind of old-style Perl coding. Nowadays, one would typically do this:

use strict;
our $bar = "blah";

our can also just declare global variables that can be freely used later:

our ($foo, @bar, %baz);
# ...
$foo = 5;
@bar = (1, 2, 3);
%baz = (this => 'that');

Try looking at this question: Does Perl have PHP-like dynamic variables?

In brief, it seems like you should be able to do $$glob_var_str = "whatever";

You would have to use an eval, but that's generally considered evil. Something like:

eval("$glob_var_str = \@_;");

EDIT

Just verified that you can only do this without the my and with no strict refs.

The vars pragma already does the heavy lifting for what you want, so put it to work:

#! /usr/bin/perl

use warnings;
use strict;
use vars;

BEGIN { vars->import(qw/ $bah /) }

$bah = "blah";
print $bah, "\n";

If you prefer to spell it create_glob_var, then use

#! /usr/bin/perl

use warnings;
use strict;
use vars;

sub create_glob_var { vars->import("\$$_[0]") }

BEGIN { create_glob_var "bah" }

$bah = "blah";
print $bah, "\n";

Either way, the output is

blah

I'm curious to know why you want to do it this way rather than declaring these variables with our. Yes, it may take a few iterations to catch them all, but these are short-term fixes anyway, right?

In general, you can use a variable as a variable name (see "Symbolic references" in perlref), but you really, really, really don't want to do that: enabling the strict 'refs' pragma disables this feature.

Rafael Garcia-Suarez showed great wisdom when he wrote, “I don't know what your original problem is, but I suggest to use a hash.”