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PERLREF(1)			 Perl Programmers Reference Guide		       PERLREF(1)

       perlref - Perl references and nested data structures

       This is complete documentation about all aspects of references.	For a shorter, tutorial
       introduction to just the essential features, see perlreftut.

       Before release 5 of Perl it was difficult to represent complex data structures, because
       all references had to be symbolic--and even then it was difficult to refer to a variable
       instead of a symbol table entry.  Perl now not only makes it easier to use symbolic refer-
       ences to variables, but also lets you have "hard" references to any piece of data or code.
       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.  (Reference counts
       for values in self-referential or cyclic data structures may not go to zero without a lit-
       tle help; see "Two-Phased Garbage Collection" in perlobj for a detailed explanation.)  If
       that thing happens to be an object, the object is destructed.  See perlobj 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 something of a
       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, as in the following paragraph, it is usu-
       ally 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 subrou-
       tine; you have to tell it explicitly to do so, by dereferencing it.

       Making References

       References can be created 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.)  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:

	       $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.  The most you can get is a reference to a typeglob,
	   which is actually a complete symbol table entry.  But see the explanation of the
	   *foo{THING} syntax below.  However, you can still use type globs and globrefs as
	   though they were IO handles.

       2.  A reference to an anonymous array can be created using square brackets:

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

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

	   Taking a reference to an enumerated list is not the same as using square brack-
	   ets--instead it's the same as creating a list of references!

	       @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, except that the key references are to
	   copies (since the keys are just strings rather than full-fledged scalars).

       3.  A reference to an anonymous hash can be created using curly brackets:

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

	   Anonymous hash and array composers like these 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 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

	   On the other hand, if you want the other meaning, you can do this:

	       sub showem {	   { @_ } }   # ambiguous (currently ok, but may change)
	       sub showem {	  {; @_ } }   # ok
	       sub showem { { return @_ } }   # ok

	   The leading "+{" and "{;" always serve to disambiguate the expression to mean either
	   the HASH reference, or the BLOCK.

       4.  A reference to an anonymous subroutine can be created by using "sub" without a sub-

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

	   Note the semicolon.	Except for the code inside not being immediately executed, a "sub
	   {}" is not so much a declaration as it is an operator, like "do{}" or "eval{}".  (How-
	   ever, no matter how many times you execute that particular line (unless you're in an
	   "eval("...")"), $coderef will still have a reference to the same anonymous subrou-

	   Anonymous subroutines act as closures with respect to my() variables, that is, vari-
	   ables lexically visible 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 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 perlobj.

	   You might also think of closure as a way to write a subroutine template without using
	   eval().  Here's a small example of how closures work:

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

	       # Time passes...


	   This prints

	       Howdy, world!
	       Greetings, earthlings!

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

	   This applies only to 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 type 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
	   often named new() and called indirectly:

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

	   But don't have to be:

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

	       use Term::Cap;
	       $terminal = Term::Cap->Tgetent( { OSPEED => 9600 });

	       use Tk;
	       $main	= MainWindow->new();
	       $menubar = $main->Frame(-relief		    => "raised",
				       -borderwidth	    => 2)

       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 ("open" in perlfunc), sockets ("socket" in perlfunc and "socketpair"
	   in perlfunc), and directory handles ("opendir" in perlfunc).  For compatibility with
	   previous versions of Perl, *foo{FILEHANDLE} is a synonym for *foo{IO}, though it is
	   deprecated as of 5.8.0.  If deprecation warnings are in effect, it will warn of its

	   *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.

	   *foo{IO} is an alternative to the *HANDLE mechanism given in "Typeglobs and Filehan-
	   dles" in perldata for passing filehandles into or out of subroutines, or storing into
	   larger data structures.  Its disadvantage is that it won't create a new filehandle for
	   you.  Its advantage is that you have less risk of clobbering more than you want to
	   with a typeglob assignment.	(It still conflates file and directory handles, though.)
	   However, if you assign the incoming value to a scalar instead of a typeglob as we do
	   in the examples below, there's no risk of that happening.

	       splutter(*STDOUT);	   # pass the whole glob
	       splutter(*STDOUT{IO});	   # pass both file and dir handles

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

	       $rec = get_rec(*STDIN);	   # pass the whole glob
	       $rec = get_rec(*STDIN{IO}); # pass both file and dir handles

	       sub get_rec {
		   my $fh = shift;
		   return scalar <$fh>;

       Using References

       That's it for creating references.  By now you're probably dying to know how to use refer-
       ences 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 contain-
	   ing a reference of the correct type:

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

	   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:

	       $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]"

	       $array[$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.	This process is
	   called autovivification.

	   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 multidimen-
	   sional 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.

       Using a string or number as a reference produces a symbolic reference, as explained above.
       Using a reference as a number produces an integer representing its storage location in
       memory.	The only useful thing to be done with this is to compare two references numeri-
       cally to see whether they refer to the same location.

	   if ($ref1 == $ref2) {  # cheap numeric compare of references
	       print "refs 1 and 2 refer to the same thing\n";

       Using a reference as a string produces both its referent's type, including any package
       blessing as described in perlobj, as well as the numeric address expressed in hex.  The
       ref() operator returns just the type of thing the reference is pointing to, without the
       address.  See "ref" in perlfunc for details and examples of its use.

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

       A typeglob may be dereferenced the same way a reference can, because the dereference syn-
       tax always indicates the type 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:

	   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 eval-
       uated 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

       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, 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 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 (globals, even if localized) 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:

	   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

       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", even though 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.)  This construct is not considered to be a sym-
       bolic 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

	   $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

	   $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 "use warnings" pragma or 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.

       Pseudo-hashes: Using an array as a hash

       WARNING:  This section describes an experimental feature.  Details may change without
       notice in future versions.

       NOTE: The current user-visible implementation of pseudo-hashes (the weird use of the first
       array element) is deprecated starting from Perl 5.8.0 and will be removed in Perl 5.10.0,
       and the feature will be implemented differently.  Not only is the current interface rather
       ugly, but the current implementation slows down normal array and hash use quite notice-
       ably.  The 'fields' pragma interface will remain available.

       Beginning with release 5.005 of Perl, you may use an array reference in some contexts that
       would normally require a hash reference.  This allows you to access array elements using
       symbolic names, as if they were fields in a structure.

       For this to work, the array must contain extra information.  The first element of the
       array has to be a hash reference that maps field names to array indices.  Here is an exam-

	   $struct = [{foo => 1, bar => 2}, "FOO", "BAR"];

	   $struct->{foo};  # same as $struct->[1], i.e. "FOO"
	   $struct->{bar};  # same as $struct->[2], i.e. "BAR"

	   keys %$struct;   # will return ("foo", "bar") in some order
	   values %$struct; # will return ("FOO", "BAR") in same some order

	   while (my($k,$v) = each %$struct) {
	      print "$k => $v\n";

       Perl will raise an exception if you try to access nonexistent fields.  To avoid inconsis-
       tencies, always use the fields::phash() function provided by the "fields" pragma.

	   use fields;
	   $pseudohash = fields::phash(foo => "FOO", bar => "BAR");

       For better performance, Perl can also do the translation from field names to array indices
       at compile time for typed object references.  See fields.

       There are two ways to check for the existence of a key in a pseudo-hash.  The first is to
       use exists().  This checks to see if the given field has ever been set.	It acts this way
       to match the behavior of a regular hash.  For instance:

	   use fields;
	   $phash = fields::phash([qw(foo bar pants)], ['FOO']);
	   $phash->{pants} = undef;

	   print exists $phash->{foo};	  # true, 'foo' was set in the declaration
	   print exists $phash->{bar};	  # false, 'bar' has not been used.
	   print exists $phash->{pants};  # true, your 'pants' have been touched

       The second is to use exists() on the hash reference sitting in the first array element.
       This checks to see if the given key is a valid field in the pseudo-hash.

	   print exists $phash->[0]{bar};      # true, 'bar' is a valid field
	   print exists $phash->[0]{shoes};# false, 'shoes' can't be used

       delete() on a pseudo-hash element only deletes the value corresponding to the key, not the
       key itself.  To delete the key, you'll have to explicitly delete it from the first hash

	   print delete $phash->{foo};	   # prints $phash->[1], "FOO"
	   print exists $phash->{foo};	   # false
	   print exists $phash->[0]{foo};  # true, key still exists
	   print delete $phash->[0]{foo};  # now key is gone
	   print $phash->{foo}; 	   # runtime exception

       Function Templates

       As explained above, a closure is an anonymous function with access to the lexical vari-
       ables visible when that function was compiled.  It retains access to those variables even
       though it doesn't get run until later, such as in a signal handler or a Tk callback.

       Using a closure as a function template allows us to generate many functions that act simi-
       larly.  Suppose you wanted functions named after the colors that generated HTML font
       changes for the various colors:

	   print "Be ", red("careful"), "with that ", green("light");

       The red() and green() functions would be similar.  To create these, we'll assign a closure
       to a typeglob of the name of the function we're trying to build.

	   @colors = qw(red blue green yellow orange purple violet);
	   for my $name (@colors) {
	       no strict 'refs';       # allow symbol table manipulation
	       *$name = *{uc $name} = sub { "<FONT COLOR='$name'>@_</FONT>" };

       Now all those different functions appear to exist independently.  You can call red(),
       RED(), blue(), BLUE(), green(), etc.  This technique saves on both compile time and memory
       use, and is less error-prone as well, since syntax checks happen at compile time.  It's
       critical that any variables in the anonymous subroutine be lexicals in order to create a
       proper closure.	That's the reasons for the "my" on the loop iteration variable.

       This is one of the only places where giving a prototype to a closure makes much sense.  If
       you wanted to impose scalar context on the arguments of these functions (probably not a
       wise idea for this particular example), you could have written it this way instead:

	   *$name = sub ($) { "<FONT COLOR='$name'>$_[0]</FONT>" };

       However, since prototype checking happens at compile time, the assignment above happens
       too late to be of much use.  You could address this by putting the whole loop of assign-
       ments within a BEGIN block, forcing it to occur during compilation.

       Access to lexicals that change over type--like those in the "for" loop above--only works
       with closures, not general subroutines.	In the general case, then, named subroutines do
       not nest properly, although anonymous ones do.  If you are accustomed to using nested sub-
       routines in other programming languages with their own private variables, you'll have to
       work at it a bit in Perl.  The intuitive coding of this type of thing incurs mysterious
       warnings about ``will not stay shared''.  For example, this won't work:

	   sub outer {
	       my $x = $_[0] + 35;
	       sub inner { return $x * 19 }   # WRONG
	       return $x + inner();

       A work-around is the following:

	   sub outer {
	       my $x = $_[0] + 35;
	       local *inner = sub { return $x * 19 };
	       return $x + inner();

       Now inner() can only be called from within outer(), because of the temporary assignments
       of the closure (anonymous subroutine).  But when it does, it has normal access to the lex-
       ical variable $x from the scope of outer().

       This has the interesting effect of creating a function local to another function, some-
       thing not normally supported in Perl.

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

	   $x{ \$a } = $a;

       If you try to dereference the key, it won't do a hard dereference, and you won't accom-
       plish 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.

       The standard Tie::RefHash module provides a convenient workaround to this.

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

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

perl v5.8.0				    2003-02-18				       PERLREF(1)

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