manpagez: man pages & more
man perlref(1)
Home | html | info | man
PERLREF(1pm)           Perl Programmers Reference Guide           PERLREF(1pm)




NAME

       perlref - Perl references and nested data structures


NOTE

       This is complete documentation about all aspects of references.  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
       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
       "Circular References" for a detailed explanation.)  If that thing
       happens to be an object, the object is destructed.  See perlobj for
       more about objects.  (In a sense, everything in Perl is an object, but
       we usually reserve the word for references to objects that have been
       officially "blessed" into a class package.)

       Symbolic references are names of variables or other objects, just as a
       symbolic link in a Unix filesystem contains merely the name of a file.
       The *glob notation is something of a symbolic reference.  (Symbolic
       references are sometimes called "soft references", but please don't
       call them that; references are confusing enough without useless
       synonyms.)

       In contrast, hard references are more like hard links in a Unix file
       system: They are used to access an underlying object without concern
       for what its (other) name is.  When the word "reference" is used
       without an adjective, as in the following paragraph, it is usually
       talking about a hard reference.

       References are easy to use in Perl.  There is just one overriding
       principle: in general, 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.

       Backslash Operator

       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.

       Square Brackets

       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).

       Curly Brackets

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

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

       Anonymous hash and array composers like these can be intermixed freely
       to produce as complicated a structure as you want.  The
       multidimensional syntax described below works for these too.  The
       values above are literals, but variables and expressions would work
       just as well, because assignment operators in Perl (even within local()
       or my()) are executable statements, not compile-time declarations.

       Because curly brackets (braces) are used for several other things
       including BLOCKs, you may occasionally have to disambiguate braces at
       the beginning of a statement by putting a "+" or a "return" in front so
       that Perl realizes the opening brace isn't starting a BLOCK.  The
       economy and mnemonic value of using curlies is deemed worth this
       occasional extra hassle.

       For example, if you wanted a function to make a new hash and return a
       reference to it, you have these options:

           sub hashem {        { @_ } }   # silently wrong
           sub hashem {       +{ @_ } }   # ok
           sub hashem { return { @_ } }   # ok

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

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

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

       Anonymous Subroutines

       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 context.

       In human terms, it's a funny way of passing arguments to a subroutine
       when you define it as well as when you call it.  It's useful for
       setting up little bits of code to run later, such as callbacks.  You
       can even do object-oriented stuff with it, though Perl already provides
       a different mechanism to do that--see perlobj.

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

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

           # Time passes...

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

       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.

       Constructors

       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()".  You can call them indirectly:

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

       But that can produce ambiguous syntax in certain cases, so it's often
       better to use the direct method invocation approach:

           $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)

       Autovivification

       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.

       Typeglob Slots

       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};
           $formatref = *foo{FORMAT};
           $globname  = *foo{NAME};    # "foo"
           $pkgname   = *foo{PACKAGE}; # "main"

       Most of these are self-explanatory, but *foo{IO} deserves special
       attention.  It returns the IO handle, used for file handles ("open" in
       perlfunc), sockets ("socket" in perlfunc and "socketpair" in perlfunc),
       and directory handles ("opendir" in perlfunc).  For compatibility with
       previous versions of Perl, *foo{FILEHANDLE} is a synonym for *foo{IO},
       though it is discouraged, to encourage a consistent use of one name:
       IO.  On perls between v5.8 and v5.22, it will issue a deprecation
       warning, but this deprecation has since been rescinded.

       *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{NAME} and *foo{PACKAGE} are the exception, in that they return
       strings, rather than references.  These return the package and name of
       the typeglob itself, rather than one that has been assigned to it.  So,
       after "*foo=*Foo::bar", *foo will become "*Foo::bar" when used as a
       string, but *foo{PACKAGE} and *foo{NAME} will continue to produce
       "main" and "foo", respectively.

       *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.

       Simple Scalar

       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;

       Block

       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.

       Arrow Notation

       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.

       Objects

       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.

       Miscellaneous Usage

       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 perlobj, as well as the
       numeric address expressed in hex.  The ref() operator returns just the
       type of thing the reference is pointing to, without the address.  See
       "ref" in perlfunc for details and examples of its use.

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

       A typeglob may be dereferenced the same way a reference can, because
       the dereference 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 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";

       Similarly, an expression that returns a reference to a scalar can be
       dereferenced via "${...}". Thus, the above expression may be written
       as:

           print "That yields ${\($n + 5)} widgets\n";

   Circular References
       It is possible to create a "circular reference" in Perl, which can lead
       to memory leaks. A circular reference occurs when two references
       contain a reference to each other, like this:

           my $foo = {};
           my $bar = { foo => $foo };
           $foo->{bar} = $bar;

       You can also create a circular reference with a single variable:

           my $foo;
           $foo = \$foo;

       In this case, the reference count for the variables will never reach 0,
       and the references will never be garbage-collected. This can lead to
       memory leaks.

       Because objects in Perl are implemented as references, it's possible to
       have circular references with objects as well. Imagine a TreeNode class
       where each node references its parent and child nodes. Any node with a
       parent will be part of a circular reference.

       You can break circular references by creating a "weak reference". A
       weak reference does not increment the reference count for a variable,
       which means that the object can go out of scope and be destroyed. You
       can weaken a reference with the "weaken" function exported by the
       Scalar::Util module.

       Here's how we can make the first example safer:

           use Scalar::Util 'weaken';

           my $foo = {};
           my $bar = { foo => $foo };
           $foo->{bar} = $bar;

           weaken $foo->{bar};

       The reference from $foo to $bar has been weakened. When the $bar
       variable goes out of scope, it will be garbage-collected. The next time
       you look at the value of the "$foo->{bar}" key, it will be "undef".

       This action at a distance can be confusing, so you should be careful
       with your use of weaken. You should weaken the reference in the
       variable that will go out of scope first. That way, the longer-lived
       variable will contain the expected reference until it goes out of
       scope.

   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()
           $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
       Brackets around a symbolic reference can simply serve to isolate an
       identifier or variable name from the rest of an expression, just as
       they always have within a string.  For example,

           $push = "pop on ";
           print "${push}over";

       has always meant to print "pop on over", even though push is a reserved
       word.  This is generalized to work the same without the enclosing
       double quotes, so that

           print ${push} . "over";

       and even

           print ${ push } . "over";

       will have the same effect.  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, the same rule applies to any bareword that is used for
       subscripting a hash.  So now, instead of writing

           $hash{ "aaa" }{ "bbb" }{ "ccc" }

       you can write just

           $hash{ 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

           $hash{ shift }

       you can force interpretation as a reserved word by adding anything that
       makes it more than a bareword:

           $hash{ shift() }
           $hash{ +shift }
           $hash{ 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
       Pseudo-hashes have been removed from Perl.  The 'fields' pragma remains
       available.

   Function Templates
       As explained above, an anonymous function with access to the lexical
       variables visible when that function was compiled, creates a closure.
       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 compilation.

       Access to lexicals that change over time--like those in the "for" loop
       above, basically aliases to elements from the surrounding lexical
       scopes-- only works with anonymous subs, not with named subroutines.
       Generally said, named subroutines do not nest properly and should only
       be declared in the main package scope.

       This is because named subroutines are created at compile time so their
       lexical variables get assigned to the parent lexicals from the first
       execution of the parent block. If a parent scope is entered a second
       time, its lexicals are created again, while the nested subs still
       reference the old ones.

       Anonymous subroutines get to capture each time you execute the "sub"
       operator, as they are created on the fly. 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" due to the reasons explained above.  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 anonymous subroutine. But when it does, it
       has normal access to the lexical variable $x from the scope of outer()
       at the time outer is invoked.

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

   Postfix Dereference Syntax
       Beginning in v5.20.0, a postfix syntax for using references is
       available.  It behaves as described in "Using References", but instead
       of a prefixed sigil, a postfixed sigil-and-star is used.

       For example:

           $r = \@a;
           @b = $r->@*; # equivalent to @$r or @{ $r }

           $r = [ 1, [ 2, 3 ], 4 ];
           $r->[1]->@*;  # equivalent to @{ $r->[1] }

       In Perl 5.20 and 5.22, this syntax must be enabled with "use feature
       'postderef'". As of Perl 5.24, no feature declarations are required to
       make it available.

       Postfix dereference should work in all circumstances where block
       (circumfix) dereference worked, and should be entirely equivalent.
       This syntax allows dereferencing to be written and read entirely left-
       to-right.  The following equivalencies are defined:

         $sref->$*;  # same as  ${ $sref }
         $aref->@*;  # same as  @{ $aref }
         $aref->$#*; # same as $#{ $aref }
         $href->%*;  # same as  %{ $href }
         $cref->&*;  # same as  &{ $cref }
         $gref->**;  # same as  *{ $gref }

       Note especially that "$cref->&*" is not equivalent to "$cref->()", and
       can serve different purposes.

       Glob elements can be extracted through the postfix dereferencing
       feature:

         $gref->*{SCALAR}; # same as *{ $gref }{SCALAR}

       Postfix array and scalar dereferencing can be used in interpolating
       strings (double quotes or the "qq" operator), but only if the
       "postderef_qq" feature is enabled.

   Postfix Reference Slicing
       Value slices of arrays and hashes may also be taken with postfix
       dereferencing notation, with the following equivalencies:

         $aref->@[ ... ];  # same as @$aref[ ... ]
         $href->@{ ... };  # same as @$href{ ... }

       Postfix key/value pair slicing, added in 5.20.0 and documented in the
       Key/Value Hash Slices section of perldata, also behaves as expected:

         $aref->%[ ... ];  # same as %$aref[ ... ]
         $href->%{ ... };  # same as %$href{ ... }

       As with postfix array, postfix value slice dereferencing can be used in
       interpolating strings (double quotes or the "qq" operator), but only if
       the "postderef_qq" feature is enabled.

   Assigning to References
       Beginning in v5.22.0, the referencing operator can be assigned to.  It
       performs an aliasing operation, so that the variable name referenced on
       the left-hand side becomes an alias for the thing referenced on the
       right-hand side:

           \$a = \$b; # $a and $b now point to the same scalar
           \&foo = \&bar; # foo() now means bar()

       This syntax must be enabled with "use feature 'refaliasing'".  It is
       experimental, and will warn by default unless "no warnings
       'experimental::refaliasing'" is in effect.

       These forms may be assigned to, and cause the right-hand side to be
       evaluated in scalar context:

           \$scalar
           \@array
           \%hash
           \&sub
           \my $scalar
           \my @array
           \my %hash
           \state $scalar # or @array, etc.
           \our $scalar   # etc.
           \local $scalar # etc.
           \local our $scalar # etc.
           \$some_array[$index]
           \$some_hash{$key}
           \local $some_array[$index]
           \local $some_hash{$key}
           condition ? \$this : \$that[0] # etc.

       Slicing operations and parentheses cause the right-hand side to be
       evaluated in list context:

           \@array[5..7]
           (\@array[5..7])
           \(@array[5..7])
           \@hash{'foo','bar'}
           (\@hash{'foo','bar'})
           \(@hash{'foo','bar'})
           (\$scalar)
           \($scalar)
           \(my $scalar)
           \my($scalar)
           (\@array)
           (\%hash)
           (\&sub)
           \(&sub)
           \($foo, @bar, %baz)
           (\$foo, \@bar, \%baz)

       Each element on the right-hand side must be a reference to a datum of
       the right type.  Parentheses immediately surrounding an array (and
       possibly also "my"/"state"/"our"/"local") will make each element of the
       array an alias to the corresponding scalar referenced on the right-hand
       side:

           \(@a) = \(@b); # @a and @b now have the same elements
           \my(@a) = \(@b); # likewise
           \(my @a) = \(@b); # likewise
           push @a, 3; # but now @a has an extra element that @b lacks
           \(@a) = (\$a, \$b, \$c); # @a now contains $a, $b, and $c

       Combining that form with "local" and putting parentheses immediately
       around a hash are forbidden (because it is not clear what they should
       do):

           \local(@array) = foo(); # WRONG
           \(%hash)       = bar(); # WRONG

       Assignment to references and non-references may be combined in lists
       and conditional ternary expressions, as long as the values on the
       right-hand side are the right type for each element on the left, though
       this may make for obfuscated code:

           (my $tom, \my $dick, \my @harry) = (\1, \2, [1..3]);
           # $tom is now \1
           # $dick is now 2 (read-only)
           # @harry is (1,2,3)

           my $type = ref $thingy;
           ($type ? $type eq 'ARRAY' ? \@foo : \$bar : $baz) = $thingy;

       The "foreach" loop can also take a reference constructor for its loop
       variable, though the syntax is limited to one of the following, with an
       optional "my", "state", or "our" after the backslash:

           \$s
           \@a
           \%h
           \&c

       No parentheses are permitted.  This feature is particularly useful for
       arrays-of-arrays, or arrays-of-hashes:

           foreach \my @a (@array_of_arrays) {
               frobnicate($a[0], $a[-1]);
           }

           foreach \my %h (@array_of_hashes) {
               $h{gelastic}++ if $h{type} eq 'funny';
           }

       CAVEAT: Aliasing does not work correctly with closures.  If you try to
       alias lexical variables from an inner subroutine or "eval", the
       aliasing will only be visible within that inner sub, and will not
       affect the outer subroutine where the variables are declared.  This
       bizarre behavior is subject to change.

   Declaring a Reference to a Variable
       Beginning in v5.26.0, the referencing operator can come after "my",
       "state", "our", or "local".  This syntax must be enabled with "use
       feature 'declared_refs'".  It is experimental, and will warn by default
       unless "no warnings 'experimental::refaliasing'" is in effect.

       This feature makes these:

           my \$x;
           our \$y;

       equivalent to:

           \my $x;
           \our $x;

       It is intended mainly for use in assignments to references (see
       "Assigning to References", above).  It also allows the backslash to be
       used on just some items in a list of declared variables:

           my ($foo, \@bar, \%baz); # equivalent to:  my $foo, \my(@bar, %baz);


WARNING: Don't use references as hash keys

       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
       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(1) and perllol(1) for how to use references to create
       complex data structures, and perlootut(1) and perlobj(1) for how to use
       them to create objects.



perl v5.34.0                      2020-10-04                      PERLREF(1pm)

perl 5.34.0 - Generated Sun Feb 27 11:39:42 CST 2022
© manpagez.com 2000-2022
Individual documents may contain additional copyright information.