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

NNAAMMEE
       perlref - Perl references and nested data structures

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

DDEESSCCRRIIPPTTIIOONN
       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 references 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.  (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, as in
       the following paragraph, it is usually 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.

       MMaakkiinngg RReeffeerreenncceess

       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.)  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:

               $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 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 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

           Note how 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 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 particular 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
           closures.)

           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...

               &$h("world");
               &$g("earthlings");

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

           *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 the section on Typeglobs and Filehandles in
           the perldata manpage 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 no risk of clobbering more than you want to with
           a typeglob assignment, although if you assign to a
           scalar instead of a typeglob, you're ok.

               splutter(*STDOUT);
               splutter(*STDOUT{IO});

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

               $rec = get_rec(*STDIN);
               $rec = get_rec(*STDIN{IO});

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

       UUssiinngg RReeffeerreenncceess

       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";
               &$coderef(1,2,3);
               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";
               &{$coderef}(1,2,3);
               $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] 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.  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 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 the object a
       reference points to 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:

           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";

       SSyymmbboolliicc rreeffeerreenncceess

       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 (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.

       NNoott--ssoo--ssyymmbboolliicc rreeffeerreenncceess

       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

           $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 --ww 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.

       PPsseeuuddoo--hhaasshheess:: UUssiinngg aann aarrrraayy aass aa hhaasshh

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

       Beginning with release 5.005 of Perl you can 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 example:

          $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 delete keys
       from a pseudo-hash or try to access nonexistent fields.
       For better performance, Perl can also do the translation
       from field names to array indices at compile time for
       typed object references.  See the fields manpage.

       FFuunnccttiioonn TTeemmppllaatteess

       As explained above, a closure is an anonymous function
       with access to the lexical variables 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 similarly.  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 very 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
       assignments 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 subroutines 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 kind 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 lexical variable $x from the scope of
       outer().

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

WWAARRNNIINNGG
       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.

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

SSEEEE AALLSSOO
       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 perltoot manpage, the perlobj manpage, and the
       perlbot manpage for how to use them to create objects.

27/Mar/1999            perl 5.005, patch 03                     1