perlref(1)

NAME
     perlref - Perl references and nested data structures

DESCRIPTION
     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.

     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.)  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.

     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>;
             }


     Using References

     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";


     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 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.

     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", 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 -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.

     Function Templates

     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.  Suppopose 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.

WARNING
     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.

SEE ALSO
     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.