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PERLREF(1)							    PERLREF(1)

NAME    [Toc]    [Back]

     perlref - Perl references and nested data structures

DESCRIPTION    [Toc]    [Back]

     Before release 5 of Perl it was difficult to represent complex data
     structures, because all references	had to be symbolic, and	even that was
     difficult to do when you wanted to	refer to a variable rather than	a
     symbol table entry.  Perl not only	makes it easier	to use symbolic
     references	to variables, but lets you have	"hard" references to any piece
     of	data.  Any scalar may hold a hard reference.  Because arrays and
     hashes contain scalars, you can now easily	build arrays of	arrays,	arrays
     of	hashes,	hashes of arrays, arrays of hashes of functions, and so	on.

     Hard references are smart--they keep track	of reference counts for	you,
     automatically freeing the thing referred to when its reference count goes
     to	zero.  (Note: The reference counts for values in self-referential or
     cyclic data structures may	not go to zero without a little	help; see the
     section on	Two-Phased Garbage Collection in the perlobj manpage for a
     detailed explanation.  If that thing happens to be	an object, the object
     is	destructed.  See the perlobj manpage for more about objects.  (In a
     sense, everything in Perl is an object, but we usually reserve the	word
     for references to objects that have been officially "blessed" into	a
     class package.)

     Symbolic references are names of variables	or other objects, just as a
     symbolic link in a	Unix filesystem	contains merely	the name of a file.
     The *glob notation	is a kind of symbolic reference.  (Symbolic references
     are sometimes called "soft	references", but please	don't call them	that;
     references	are confusing enough without useless synonyms.)

     In	contrast, hard references are more like	hard links in a	Unix file
     system: They are used to access an	underlying object without concern for
     what its (other) name is.	When the word "reference" is used without an
     adjective,	like in	the following paragraph, it usually is talking about a
     hard reference.

     References	are easy to use	in Perl.  There	is just	one overriding
     principle:	Perl does no implicit referencing or dereferencing.  When a
     scalar is holding a reference, it always behaves as a simple scalar.  It
     doesn't magically start being an array or hash or subroutine; you have to
     tell it explicitly	to do so, by dereferencing it.

     References	can be constructed in several ways.

     1.	 By using the backslash	operator on a variable,	subroutine, or value.
	 (This works much like the & (address-of) operator in C.)  Note	that
	 this typically	creates	ANOTHER	reference to a variable, because
	 there's already a reference to	the variable in	the symbol table.  But
	 the symbol table reference might go away, and you'll still have the
	 reference that	the backslash returned.	 Here are some examples:

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PERLREF(1)							    PERLREF(1)

	     $scalarref	= \$foo;
	     $arrayref	= \@ARGV;
	     $hashref	= \%ENV;
	     $coderef	= \&handler;
	     $globref	= \*foo;

	 It isn't possible to create a true reference to an IO handle
	 (filehandle or	dirhandle) using the backslash operator.  See the
	 explanation of	the *foo{THING}	syntax below.  (However, you're	apt to
	 find Perl code	out there using	globrefs as though they	were IO
	 handles, which	is grandfathered into continued	functioning.)

     2.	 A reference to	an anonymous array can be constructed using square

	     $arrayref = [1, 2,	['a', 'b', 'c']];

	 Here we've constructed	a reference to an anonymous array of three
	 elements whose	final element is itself	a reference to another
	 anonymous array of three elements.  (The multidimensional syntax
	 described later can be	used to	access this.  For example, after the
	 above,	$arrayref->[2][1] would	have the value "b".)

	 Note that taking a reference to an enumerated list is not the same as
	 using square brackets--instead	it's the same as creating a list of

	     @list = (\$a, \@b,	\%c);
	     @list = \($a, @b, %c);	 # same	thing!

	 As a special case, \(@foo) returns a list of references to the
	 contents of @foo, not a reference to @foo itself.  Likewise for %foo.

     3.	 A reference to	an anonymous hash can be constructed using curly

	     $hashref =	{
		 'Adam'	 => 'Eve',
		 'Clyde' => 'Bonnie',

	 Anonymous hash	and array constructors can be intermixed freely	to
	 produce as complicated	a structure as you want.  The multidimensional
	 syntax	described below	works for these	too.  The values above are
	 literals, but variables and expressions would work just as well,
	 because assignment operators in Perl (even within local() or my())
	 are executable	statements, not	compile-time declarations.

	 Because curly brackets	(braces) are used for several other things
	 including BLOCKs, you may occasionally	have to	disambiguate braces at
	 the beginning of a statement by putting a + or	a return in front so
	 that Perl realizes the	opening	brace isn't starting a BLOCK.  The

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PERLREF(1)							    PERLREF(1)

	 economy and mnemonic value of using curlies is	deemed worth this
	 occasional extra hassle.

	 For example, if you wanted a function to make a new hash and return a
	 reference to it, you have these options:

	     sub hashem	{	 { @_ }	}   # silently wrong
	     sub hashem	{	+{ @_ }	}   # ok
	     sub hashem	{ return { @_ }	}   # ok

     4.	 A reference to	an anonymous subroutine	can be constructed by using
	 sub without a subname:

	     $coderef =	sub { print "Boink!\n" };

	 Note the presence of the semicolon.  Except for the fact that the
	 code inside isn't executed immediately, a sub {} is not so much a
	 declaration as	it is an operator, like	do{} or	eval{}.	 (However, no
	 matter	how many times you execute that	line (unless you're in an
	 eval("...")), $coderef	will still have	a reference to the SAME
	 anonymous subroutine.)

	 Anonymous subroutines act as closures with respect to my() variables,
	 that is, variables visible lexically within the current scope.
	 Closure is a notion out of the	Lisp world that	says if	you define an
	 anonymous function in a particular lexical context, it	pretends to
	 run in	that context even when it's called outside of the context.

	 In human terms, it's a	funny way of passing arguments to a subroutine
	 when you define it as well as when you	call it.  It's useful for
	 setting up little bits	of code	to run later, such as callbacks.  You
	 can even do object-oriented stuff with	it, though Perl	already
	 provides a different mechanism	to do that--see	the perlobj manpage.

	 You can also think of closure as a way	to write a subroutine template
	 without using eval.  (In fact,	in version 5.000, eval was the only
	 way to	get closures.  You may wish to use "require 5.001" if you use

	 Here's	a small	example	of how closures	works:

	     sub newprint {
		 my $x = shift;
		 return	sub { my $y = shift; print "$x,	$y!\n";	};
	     $h	= newprint("Howdy");
	     $g	= newprint("Greetings");

	     # Time passes...

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PERLREF(1)							    PERLREF(1)


	 This prints

	     Howdy, world!
	     Greetings,	earthlings!

	 Note particularly that	$x continues to	refer to the value passed into
	 newprint() despite the	fact that the "my $x" has seemingly gone out
	 of scope by the time the anonymous subroutine runs.  That's what
	 closure is all	about.

	 This applies to only lexical variables, by the	way.  Dynamic
	 variables continue to work as they have always	worked.	 Closure is
	 not something that most Perl programmers need trouble themselves
	 about to begin	with.

     5.	 References are	often returned by special subroutines called
	 constructors.	Perl objects are just references to a special kind of
	 object	that happens to	know which package it's	associated with.
	 Constructors are just special subroutines that	know how to create
	 that association.  They do so by starting with	an ordinary reference,
	 and it	remains	an ordinary reference even while it's also being an
	 object.  Constructors are customarily named new(), but	don't have to

	     $objref = new Doggie (Tail	=> 'short', Ears => 'long');

     6.	 References of the appropriate type can	spring into existence if you
	 dereference them in a context that assumes they exist.	 Because we
	 haven't talked	about dereferencing yet, we can't show you any
	 examples yet.

     7.	 A reference can be created by using a special syntax, lovingly	known
	 as the	*foo{THING} syntax.  *foo{THING} returns a reference to	the
	 THING slot in *foo (which is the symbol table entry which holds
	 everything known as foo).

	     $scalarref	= *foo{SCALAR};
	     $arrayref	= *ARGV{ARRAY};
	     $hashref	= *ENV{HASH};
	     $coderef	= *handler{CODE};
	     $ioref	= *STDIN{IO};
	     $globref	= *foo{GLOB};

	 All of	these are self-explanatory except for *foo{IO}.	 It returns
	 the IO	handle,	used for file handles (the open	entry in the perlfunc
	 manpage), sockets (the	socket entry in	the perlfunc manpage and the
	 socketpair entry in the perlfunc manpage), and	directory handles (the
	 opendir entry in the perlfunc manpage).  For compatibility with

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PERLREF(1)							    PERLREF(1)

	 previous versions of Perl, *foo{FILEHANDLE} is	a synonym for

	 *foo{THING} returns undef if that particular THING hasn't been	used
	 yet, except in	the case of scalars.  *foo{SCALAR} returns a reference
	 to an anonymous scalar	if $foo	hasn't been used yet.  This might
	 change	in a future release.

	 The use of *foo{IO} is	the best way to	pass bareword filehandles into
	 or out	of subroutines,	or to store them in larger data	structures.

	     sub splutter {
		 my $fh	= shift;
		 print $fh "her	um well	a hmmm\n";

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

	 Beware, though, that you can't	do this	with a routine which is	going
	 to open the filehandle	for you, because *HANDLE{IO} will be undef if
	 HANDLE	hasn't been used yet.  Use \*HANDLE for	that sort of thing

	 Using \*HANDLE	(or *HANDLE) is	another	way to use and store nonbareword
 filehandles (before perl version 5.002 it was	the only way).
	 The two methods are largely interchangeable, you can do

	     $rec = get_rec(\*STDIN);

	 with the above	subroutine definitions.

     That's it for creating references.	 By now	you're probably	dying to know
     how to use	references to get back to your long-lost data.	There are
     several basic methods.

     1.	 Anywhere you'd	put an identifier (or chain of identifiers) as part of
	 a variable or subroutine name,	you can	replace	the identifier with a
	 simple	scalar variable	containing a reference of the correct type:

	     $bar = $$scalarref;
	     push(@$arrayref, $filename);
	     $$arrayref[0] = "January";
	     $$hashref{"KEY"} =	"VALUE";
	     print $globref "output\n";

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PERLREF(1)							    PERLREF(1)

	 It's important	to understand that we are specifically NOT
	 dereferencing $arrayref[0] or $hashref{"KEY"} there.  The dereference
	 of the	scalar variable	happens	BEFORE it does any key lookups.
	 Anything more complicated than	a simple scalar	variable must use
	 methods 2 or 3	below.	However, a "simple scalar" includes an
	 identifier that itself	uses method 1 recursively.  Therefore, the
	 following prints "howdy".

	     $refrefref	= \\\"howdy";
	     print $$$$refrefref;

     2.	 Anywhere you'd	put an identifier (or chain of identifiers) as part of
	 a variable or subroutine name,	you can	replace	the identifier with a
	 BLOCK returning a reference of	the correct type.  In other words, the
	 previous examples could be written like this:

	     $bar = ${$scalarref};
	     push(@{$arrayref},	$filename);
	     ${$arrayref}[0] = "January";
	     ${$hashref}{"KEY"}	= "VALUE";
	     $globref->print("output\n");  # iff IO::Handle is loaded

	 Admittedly, it's a little silly to use	the curlies in this case, but
	 the BLOCK can contain any arbitrary expression, in particular,
	 subscripted expressions:

	     &{	$dispatch{$index} }(1,2,3);	 # call	correct	routine

	 Because of being able to omit the curlies for the simple case of $$x,
	 people	often make the mistake of viewing the dereferencing symbols as
	 proper	operators, and wonder about their precedence.  If they were,
	 though, you could use parentheses instead of braces.  That's not the
	 case.	Consider the difference	below; case 0 is a short-hand version
	 of case 1, NOT	case 2:

	     $$hashref{"KEY"}	= "VALUE";	 # CASE	0
	     ${$hashref}{"KEY"}	= "VALUE";	 # CASE	1
	     ${$hashref{"KEY"}}	= "VALUE";	 # CASE	2
	     ${$hashref->{"KEY"}} = "VALUE";	 # CASE	3

	 Case 2	is also	deceptive in that you're accessing a variable called
	 %hashref, not dereferencing through $hashref to the hash it's
	 presumably referencing.  That would be	case 3.

     3.	 Subroutine calls and lookups of individual array elements arise often
	 enough	that it	gets cumbersome	to use method 2.  As a form of
	 syntactic sugar, the examples for method 2 may	be written:

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PERLREF(1)							    PERLREF(1)

	     $arrayref->[0] = "January";   # Array element
	     $hashref->{"KEY"} = "VALUE";  # Hash element
	     $coderef->(1,2,3);		   # Subroutine	call

	 The left side of the arrow can	be any expression returning a
	 reference, including a	previous dereference.  Note that $array[$x] is
	 NOT the same thing as $array->[$x] here:

	     $array[$x]->{"foo"}->[0] =	"January";

	 This is one of	the cases we mentioned earlier in which	references
	 could spring into existence when in an	lvalue context.	 Before	this
	 statement, $array[$x] may have	been undefined.	 If so,	it's
	 automatically defined with a hash reference so	that we	can look up
	 {"foo"} in it.	 Likewise $array[$x]->{"foo"} will automatically get
	 defined with an array reference so that we can	look up	[0] in it.

	 One more thing	here.  The arrow is optional BETWEEN brackets
	 subscripts, so	you can	shrink the above down to

	     $array[$x]{"foo"}[0] = "January";

	 Which,	in the degenerate case of using	only ordinary arrays, gives
	 you multidimensional arrays just like C's:

	     $score[$x][$y][$z]	+= 42;

	 Well, okay, not entirely like C's arrays, actually.  C	doesn't	know
	 how to	grow its arrays	on demand.  Perl does.

     4.	 If a reference	happens	to be a	reference to an	object,	then there are
	 probably methods to access the	things referred	to, and	you should
	 probably stick	to those methods unless	you're in the class package
	 that defines the object's methods.  In	other words, be	nice, and
	 don't violate the object's encapsulation without a very good reason.
	 Perl does not enforce encapsulation.  We are not totalitarians	here.
	 We do expect some basic civility though.

     The ref() operator	may be used to determine what type of thing the
     reference is pointing to.	See the	perlfunc manpage.

     The bless() operator may be used to associate a reference with a package
     functioning as an object class.  See the perlobj manpage.

     A typeglob	may be dereferenced the	same way a reference can, because the
     dereference syntax	always indicates the kind of reference desired.	 So
     ${*foo} and ${\$foo} both indicate	the same scalar	variable.

     Here's a trick for	interpolating a	subroutine call	into a string:

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PERLREF(1)							    PERLREF(1)

	 print "My sub returned	@{[mysub(1,2,3)]} that time.\n";

     The way it	works is that when the @{...} is seen in the double-quoted
     string, it's evaluated as a block.	 The block creates a reference to an
     anonymous array containing	the results of the call	to mysub(1,2,3).  So
     the whole block returns a reference to an array, which is then
     dereferenced by @{...} and	stuck into the double-quoted string. This
     chicanery is also useful for arbitrary expressions:

	 print "That yields @{[$n + 5]}	widgets\n";

     Symbolic references    [Toc]    [Back]

     We	said that references spring into existence as necessary	if they	are
     undefined,	but we didn't say what happens if a value used as a reference
     is	already	defined, but ISN'T a hard reference.  If you use it as a
     reference in this case, it'll be treated as a symbolic reference.	That
     is, the value of the scalar is taken to be	the NAME of a variable,	rather
     than a direct link	to a (possibly)	anonymous value.

     People frequently expect it to work like this.  So	it does.

	 $name = "foo";
	 $$name	= 1;		     # Sets $foo
	 ${$name} = 2;		     # Sets $foo
	 ${$name x 2} =	3;	     # Sets $foofoo
	 $name->[0] = 4;	     # Sets $foo[0]
	 @$name	= ();		     # Clears @foo
	 &$name();		     # Calls &foo() (as	in Perl	4)
	 $pack = "THAT";
	 ${"${pack}::$name"} = 5;    # Sets $THAT::foo without eval

     This is very powerful, and	slightly dangerous, in that it's possible to
     intend (with the utmost sincerity)	to use a hard reference, and
     accidentally use a	symbolic reference instead.  To	protect	against	that,
     you can say

	 use strict 'refs';

     and then only hard	references will	be allowed for the rest	of the
     enclosing block.  An inner	block may countermand that with

	 no strict 'refs';

     Only package variables are	visible	to symbolic references.	 Lexical
     variables (declared with my()) aren't in a	symbol table, and thus are
     invisible to this mechanism.  For example:

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PERLREF(1)							    PERLREF(1)

	 local($value) = 10;
	 $ref =	\$value;
	     my	$value = 20;
	     print $$ref;

     This will still print 10, not 20.	Remember that local() affects package
     variables,	which are all "global" to the package.

     Not-so-symbolic references    [Toc]    [Back]

     A new feature contributing	to readability in perl version 5.001 is	that
     the brackets around a symbolic reference behave more like quotes, just as
     they always have within a string.	That is,

	 $push = "pop on ";
	 print "${push}over";

     has always	meant to print "pop on over", despite the fact that push is a
     reserved word.  This has been generalized to work the same	outside	of
     quotes, so	that

	 print ${push} . "over";

     and even

	 print ${ push } . "over";

     will have the same	effect.	 (This would have been a syntax	error in Perl
     5.000, though Perl	4 allowed it in	the spaceless form.)  Note that	this
     construct is not considered to be a symbolic reference when you're	using
     strict refs:

	 use strict 'refs';
	 ${ bareword };	     # Okay, means $bareword.
	 ${ "bareword" };    # Error, symbolic reference.

     Similarly,	because	of all the subscripting	that is	done using single
     words, we've applied the same rule	to any bareword	that is	used for
     subscripting a hash.  So now, instead of writing

	 $array{ "aaa" }{ "bbb"	}{ "ccc" }

     you can write just

	 $array{ aaa }{	bbb }{ ccc }

     and not worry about whether the subscripts	are reserved words.  In	the
     rare event	that you do wish to do something like

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PERLREF(1)							    PERLREF(1)

	 $array{ shift }

     you can force interpretation as a reserved	word by	adding anything	that
     makes it more than	a bareword:

	 $array{ shift() }
	 $array{ +shift	}
	 $array{ shift @_ }

     The -w switch will	warn you if it interprets a reserved word as a string.
     But it will no longer warn	you about using	lowercase words, because the
     string is effectively quoted.

WARNING    [Toc]    [Back]

     You may not (usefully) use	a reference as the key to a hash.  It will be
     converted into a string:

	 $x{ \$a } = $a;

     If	you try	to dereference the key,	it won't do a hard dereference,	and
     you won't accomplish what you're attempting.  You might want to do
     something more like

	 $r = \@a;
	 $x{ $r	} = $r;

     And then at least you can use the values(), which will be real refs,
     instead of	the keys(), which won't.

SEE ALSO    [Toc]    [Back]

     Besides the obvious documents, source code	can be instructive.  Some
     rather pathological examples of the use of	references can be found	in the
     t/op/ref.t	regression test	in the Perl source directory.

     See also the perldsc manpage and the perllol manpage for how to use
     references	to create complex data structures, and the perlobj manpage for
     how to use	them to	create objects.

								       Page 10

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								       PPPPaaaaggggeeee 11111111
[ Back ]
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