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perlref



NOTE

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


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 vari­
       ables, but also lets you have "hard" references to any
       piece of data or code.  Any scalar may hold a hard refer­
       ence.  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
       "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 usu­
       ally 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 con­
       tains 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 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.

               $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 ref­
           erence 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 refer­
           ences to the contents of @foo, not a reference to @foo
           itself.  Likewise for %foo, except that the key refer­
           ences 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 struc­
           ture 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

               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 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 exe­
           cute 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 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 per­
           lobj.

           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"; };

           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 pro­
           grammers 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 asso­
           ciation.  They do so by starting with an ordinary ref­
           erence, 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};

           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 Filehandles" 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 references to get back to your
       long-lost data.  There are several basic methods.

       1.  Anywhere you'd put an identifier (or chain of identi­
           fiers) as part of a variable or subroutine name, you
           can replace the identifier with a simple scalar vari­
           able 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
           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 sim­
           ple 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 ele­
           ments arise often enough that it gets cumbersome to
           use method 2.  As a form of syntactic sugar, the exam­
           ples 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 derefer­
           ence.  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
           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.

       Using a string or number as a reference produces a sym­
       bolic 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 per­
       lobj, as well as the numeric address expressed in hex.
       The ref() operator returns just the type of thing the ref­
       erence 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 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
       string:

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

       The way it works is that when the "@{...}" is seen in the
       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
           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
       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 dep­
       recated starting from Perl 5.8.0 and will be removed in
       Perl 5.10.0, and the feature will be implemented differ­
       ently.  Not only is the current interface rather ugly, but
       the current implementation slows down normal array and
       hash use quite noticeably.  The 'fields' pragma interface

           $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 nonexis­
       tent fields.  To avoid inconsistencies, 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 sit­
       ting 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 element.

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

       The red() and green() functions would be similar.  To cre­
       ate 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 indepen­
       dently.  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 (proba­
       bly 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 dur­
       ing compilation.

       Access to lexicals that change over type--like those in
       the "for" loop above--only works with closures, not gen­
       eral subroutines.  In the general case, then, named sub­
       routines 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
       work:

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

       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 sup­
       ported 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 attempt­
       ing.  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 instruc­
       tive.  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 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 v5.8.1                 2003-09-02                 PERLREF(1)
  
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