perlfaq7.pod

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If you are experiencing variable suicide, that C<my $f> in the subroutine
doesn't pick up a fresh copy of the C<$f> whose value is <foo>. The output
shows that inside the subroutine the value of C<$f> leaks through when it
shouldn't, as in this output:

	foobar
	foobarbar
	foobarbarbar
	Finally foo

The $f that has "bar" added to it three times should be a new C<$f>
C<my $f> should create a new lexical variable each time through the loop.
The expected output is:

	foobar
	foobar
	foobar
	Finally foo

=head2 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 L<perlsub/"Pass by Reference"> for this particular
question, and L<perlref> for information on references.

See "Passing Regexes", later in L<perlfaq7>, for information on
passing regular expressions.

=over 4

=item 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] }   );

=item 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 $line = <$passed_fh>;
		}

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

=item Passing Regexes

To pass regexes around, you'll need to be using a release of Perl
sufficiently recent as to support the C<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 C<qr//>:

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

Notice how C<qr//> allows flags at the end.  That pattern was compiled
at compile time, although it was executed later.  The nifty C<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 C<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,
I<Mastering Regular Expressions>, by Jeffrey Friedl.  Page 273's
Build_MatchMany_Function() is particularly interesting.  A complete
citation of this book is given in L<perlfaq2>.

=item 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).

=back

=head2 How do I create a static variable?

(contributed by brian d foy)

Perl doesn't have "static" variables, which can only be accessed from
the function in which they are declared. You can get the same effect
with lexical variables, though.

You can fake a static variable by using a lexical variable which goes
out of scope. In this example, you define the subroutine C<counter>, and
it uses the lexical variable C<$count>. Since you wrap this in a BEGIN
block, C<$count> is defined at compile-time, but also goes out of
scope at the end of the BEGIN block. The BEGIN block also ensures that
the subroutine and the value it uses is defined at compile-time so the
subroutine is ready to use just like any other subroutine, and you can
put this code in the same place as other subroutines in the program
text (i.e. at the end of the code, typically). The subroutine
C<counter> still has a reference to the data, and is the only way you
can access the value (and each time you do, you increment the value).
The data in chunk of memory defined by C<$count> is private to
C<counter>.

	BEGIN {
		my $count = 1;
		sub counter { $count++ }
	}
	
	my $start = counter();
	
	.... # code that calls counter();
	
	my $end = counter();

In the previous example, you created a function-private variable
because only one function remembered its reference. You could define
multiple functions while the variable is in scope, and each function
can share the "private" variable. It's not really "static" because you
can access it outside the function while the lexical variable is in
scope, and even create references to it. In this example,
C<increment_count> and C<return_count> share the variable. One
function adds to the value and the other simply returns the value.
They can both access C<$count>, and since it has gone out of scope,
there is no other way to access it.

	BEGIN {
		my $count = 1;
		sub increment_count { $count++ }
		sub return_count    { $count }
	}

To declare a file-private variable, you still use a lexical variable.
A file is also a scope, so a lexical variable defined in the file
cannot be seen from any other file.

See L<perlsub/"Persistent Private Variables"> for more information.
The discussion of closures in L<perlref> may help you even though we
did not use anonymous subroutines in this answer. See
L<perlsub/"Persistent Private Variables"> for details.

=head2 What's the difference between dynamic and lexical (static) scoping?  Between local() and my()?

C<local($x)> saves away the old value of the global variable C<$x>
and assigns a new value for the duration of the subroutine I<which is
visible in other functions called from that subroutine>.  This is done
at run-time, so is called dynamic scoping.  local() always affects global
variables, also called package variables or dynamic variables.

C<my($x)> creates a new variable that is only visible in the current
subroutine.  This is done at compile-time, so it is called lexical or
static scoping.  my() always affects private variables, also called
lexical variables or (improperly) static(ly scoped) variables.

For instance:

	sub visible {
		print "var has value $var\n";
		}
	
	sub dynamic {
		local $var = 'local';	# new temporary value for the still-global
		visible();              #   variable called $var
		}
	
	sub lexical {
		my $var = 'private';    # new private variable, $var
		visible();              # (invisible outside of sub scope)
		}
	
	$var = 'global';
	
	visible();      		# prints global
	dynamic();      		# prints local
	lexical();      		# prints global

Notice how at no point does the value "private" get printed.  That's
because $var only has that value within the block of the lexical()
function, and it is hidden from called subroutine.

In summary, local() doesn't make what you think of as private, local
variables.  It gives a global variable a temporary value.  my() is
what you're looking for if you want private variables.

See L<perlsub/"Private Variables via my()"> and
L<perlsub/"Temporary Values via local()"> for excruciating details.

=head2 How can I access a dynamic variable while a similarly named lexical is in scope?

If you know your package, you can just mention it explicitly, as in
$Some_Pack::var. Note that the notation $::var is B<not> the dynamic $var
in the current package, but rather the one in the "main" package, as
though you had written $main::var.

	use vars '$var';
	local $var = "global";
	my    $var = "lexical";

	print "lexical is $var\n";
	print "global  is $main::var\n";

Alternatively you can use the compiler directive our() to bring a
dynamic variable into the current lexical scope.

	require 5.006; # our() did not exist before 5.6
	use vars '$var';

	local $var = "global";
	my $var    = "lexical";

	print "lexical is $var\n";

	{
		our $var;
		print "global  is $var\n";
	}

=head2 What's the difference between deep and shallow binding?

In deep binding, lexical variables mentioned in anonymous subroutines
are the same ones that were in scope when the subroutine was created.
In shallow binding, they are whichever variables with the same names
happen to be in scope when the subroutine is called.  Perl always uses
deep binding of lexical variables (i.e., those created with my()).
However, dynamic variables (aka global, local, or package variables)
are effectively shallowly bound.  Consider this just one more reason
not to use them.  See the answer to L<"What's a closure?">.

=head2 Why doesn't "my($foo) = E<lt>FILEE<gt>;" work right?

C<my()> and C<local()> give list context to the right hand side
of C<=>.  The <FH> read operation, like so many of Perl's
functions and operators, can tell which context it was called in and
behaves appropriately.  In general, the scalar() function can help.
This function does nothing to the data itself (contrary to popular myth)
but rather tells its argument to behave in whatever its scalar fashion is.
If that function doesn't have a defined scalar behavior, this of course
doesn't help you (such as with sort()).

To enforce scalar context in this particular case, however, you need
merely omit the parentheses:

	local($foo) = <FILE>;	    # WRONG
	local($foo) = scalar(<FILE>);   # ok
	local $foo  = <FILE>;	    # right

You should probably be using lexical variables anyway, although the
issue is the same here:

	my($foo) = <FILE>;	# WRONG
	my $foo  = <FILE>;	# right

=head2 How do I redefine a builtin function, operator, or method?

Why do you want to do that? :-)

If you want to override a predefined function, such as open(),
then you'll have to import the new definition from a different
module.  See L<perlsub/"Overriding Built-in Functions">.  There's
also an example in L<perltoot/"Class::Template">.

If you want to overload a Perl operator, such as C<+> or C<**>,
then you'll want to use the C<use overload> pragma, documented
in L<overload>.

If you're talking about obscuring method calls in parent classes,
see L<perltoot/"Overridden Methods">.

=head2 What's the difference between calling a function as &foo and foo()?

When you call a function as C<&foo>, you allow that function access to
your current @_ values, and you bypass prototypes.
The function doesn't get an empty @_--it gets yours!  While not
strictly speaking a bug (it's documented that way in L<perlsub>), it
would be hard to consider this a feature in most cases.

When you call your function as C<&foo()>, then you I<do> get a new @_,
but prototyping is still circumvented.

Normally, you want to call a function using C<foo()>.  You may only
omit the parentheses if the function is already known to the compiler
because it already saw the definition (C<use> but not C<require>),
or via a forward reference or C<use subs> declaration.  Even in this
case, you get a clean @_ without any of the old values leaking through
where they don't belong.

=head2 How do I create a switch or case statement?