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How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regexp}?

by faq_monk (Initiate)
on Oct 08, 1999 at 00:27 UTC ( #692=perlfaq nodetype: print w/ replies, xml ) Need Help??

Current Perl documentation can be found at perldoc.perl.org.

Here is our local, out-dated (pre-5.6) version:

With the exception of regexps, you need to pass references to these objects. See Pass by Reference for this particular question, and the perlref manpage for information on references.

Passing Variables and Functions

Regular variables and functions are quite easy: just pass in a reference to an existing or anonymous variable or function:

    func( \$some_scalar );

    func( \$some_array  );
    func( [ 1 .. 10 ]   );

    func( \%some_hash   );
    func( { this => 10, that => 20 }   );

    func( \&some_func   );
    func( sub { $_[0] ** $_[1] }   );
Passing Filehandles

To pass filehandles to subroutines, use the *FH or \*FH notations. These are ``typeglobs'' - see Typeglobs and Filehandles and especially Pass by Reference for more information.

Here's an excerpt:

If you're passing around filehandles, you could usually just use the bare typeglob, like *STDOUT, but typeglobs references would be better because they'll still work properly under use strict 'refs'. For example:

    splutter(\*STDOUT);
    sub splutter {
        my $fh = shift;
        print $fh "her um well a hmmm\n";
    }

    $rec = get_rec(\*STDIN);
    sub get_rec {
        my $fh = shift;
        return scalar <$fh>;
    }

If you're planning on generating new filehandles, you could do this:

    sub openit {
        my $name = shift;
        local *FH;
        return open (FH, $path) ? *FH : undef;
    }
    $fh = openit('< /etc/motd');
    print <$fh>;
Passing Regexps

To pass regexps around, you'll need to either use one of the highly experimental regular expression modules from CPAN (Nick Ing-Simmons's Regexp or Ilya Zakharevich's Devel::Regexp), pass around strings and use an exception-trapping eval, or else be be very, very clever. Here's an example of how to pass in a string to be regexp compared:

    sub compare($$) {
        my ($val1, $regexp) = @_;
        my $retval = eval { $val =~ /$regexp/ };
        die if $@;
        return $retval;
    }

    $match = compare("old McDonald", q/d.*D/);

Make sure you never say something like this:

    return eval "\$val =~ /$regexp/";   # WRONG

or someone can sneak shell escapes into the regexp due to the double interpolation of the eval and the double-quoted string. For example:

    $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger';

    eval "\$string =~ /$pattern_of_evil/";

Those preferring to be very, very clever might see the O'Reilly book, Mastering Regular Expressions, by Jeffrey Friedl. Page 273's Build_MatchMany_Function() is particularly interesting. A complete citation of this book is given in the perlfaq2 manpage.

Passing Methods

To pass an object method into a subroutine, you can do this:

    call_a_lot(10, $some_obj, "methname")
    sub call_a_lot {
        my ($count, $widget, $trick) = @_;
        for (my $i = 0; $i < $count; $i++) {
            $widget->$trick();
        }
    }

Or you can use a closure to bundle up the object and its method call and arguments:

    my $whatnot =  sub { $some_obj->obfuscate(@args) };
    func($whatnot);
    sub func {
        my $code = shift;
        &$code();
    }

You could also investigate the can() method in the UNIVERSAL class (part of the standard perl distribution).

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