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perlref (1)
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         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 the perlreftut manpage.


         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.  (Reference counts
         for values in self-referential or cyclic data structures may
         not go to zero without a little help; see the Two-Phased
         Garbage Collection entry 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
         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
         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
         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.) 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".)
             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
             The leading `+{' and `{;' always serve to disambiguate
             the expression to mean either the HASH reference, or the
         4.  A reference to an anonymous subroutine can be created by
             using `sub' without a subname:
                 $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{}'.  (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 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
             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 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
                 $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 Typeglobs and Filehandles entry 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 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 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";
                 $$hashref{"KEY"} = "VALUE";
                 print $globref "output\n";
             It's important to understand that we are specifically
             not dereferencing `$arrayref[0]' or `$hashref{"KEY"}'
             there.  The dereference of the scalar variable happens
             before it does any key lookups.  Anything more
             complicated than a simple scalar variable must use
             methods 2 or 3 below.  However, a "simple scalar"
             includes an identifier that itself uses method 1
             recursively.  Therefore, the following prints "howdy".
                 $refrefref = \\\"howdy";
                 print $$$$refrefref;
         2.  Anywhere you'd put an identifier (or chain of
             identifiers) as part of a variable or subroutine name,
             you can replace the identifier with a BLOCK returning a
             reference of the correct type.  In other words, the
             previous examples could be written like this:
                 $bar = ${$scalarref};
                 push(@{$arrayref}, $filename);
                 ${$arrayref}[0] = "January";
                 ${$hashref}{"KEY"} = "VALUE";
                 $globref->print("output\n");  # iff IO::Handle is loaded
             Admittedly, it's a little silly to use the curlies in
             this case, but the BLOCK can contain any arbitrary
             expression, in particular, subscripted expressions:
                 &{ $dispatch{$index} }(1,2,3);      # call correct routine
             Because of being able to omit the curlies for the simple
             case of `$$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]' 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
         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 numerically 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 the
         perlobj manpage, 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 the ref
         entry in the perlfunc manpage 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 the perlobj manpage.
         A typeglob may be dereferenced the same way a reference can,
         because the dereference syntax 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
             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, 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
             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
         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
         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
             $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.
         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 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 access
         nonexistent fields.  To avoid inconsistencies, always use
         the fields::phash() function provided by the `fields'
             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 the fields manpage.
         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 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 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
         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 type of thing incurs mysterious warnings
         about ``will not stay shared''.  For example, this won't
             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


         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.


         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 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.

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