FAQ 7.14 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?



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

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

See ``Passing Regexes'', below, for information on passing regular
expressions.

Passing Variables and Functions
Regular variables and functions are quite easy to pass: 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
As of Perl 5.6, you can represent filehandles with scalar variables
which you treat as any other scalar.

open my $fh, $filename or die "Cannot open $filename! $!";
func( $fh );

sub func {
my $passed_fh = shift;

my $line = <$fh>;
}

Before Perl 5.6, you had to use the *FH or "\*FH" notations. These
are "typeglobs"--see "Typeglobs and Filehandles" in perldata and
especially "Pass by Reference" in perlsub for more information.

Passing Regexes
To pass regexes around, you'll need to be using a release of Perl
sufficiently recent as to support the "qr//" construct, pass around
strings and use an exception-trapping eval, or else be very, very
clever.

Here's an example of how to pass in a string to be regex compared
using "qr//":

sub compare($$) {
my ($val1, $regex) = @_;
my $retval = $val1 =~ /$regex/;
return $retval;
}
$match = compare("old McDonald", qr/d.*D/i);

Notice how "qr//" allows flags at the end. That pattern was compiled
at compile time, although it was executed later. The nifty "qr//"
notation wasn't introduced until the 5.005 release. Before that, you
had to approach this problem much less intuitively. For example,
here it is again if you don't have "qr//":

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

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

Make sure you never say something like this:

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

or someone can sneak shell escapes into the regex 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 perlfaq2.

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, 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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