perlref


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
       "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 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: 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
           *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" 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

           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)

       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};
               $formatref = *foo{FORMAT};

           All of these are self-explanatory except for *foo{IO}.  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 deprecated as of 5.8.0.  If deprecation warnings are
           in effect, it will warn of its use.

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

   Using References
       That's it for creating references.  By now you're probably dying to
       know how to use references to get back to your long-lost data.  There
       are several basic methods.

       1.  Anywhere you'd put an identifier (or chain of identifiers) as part
           of a variable or subroutine name, you can replace the identifier
           with a simple scalar variable containing a reference of the correct
           type:

               $bar = $$scalarref;
               push(@$arrayref, $filename);
               $$arrayref[0] = "January";
               $$hashref{"KEY"} = "VALUE";
               &$coderef(1,2,3);
               print $globref "output\n";

           It's important to understand that we are specifically not
           dereferencing $arrayref[0] or $hashref{"KEY"} there.  The
           dereference of the scalar variable happens before it does any key
           lookups.  Anything more complicated than a simple scalar variable
           must use methods 2 or 3 below.  However, a "simple scalar" includes
           an identifier that itself uses method 1 recursively.  Therefore,
           the following prints "howdy".

               $refrefref = \\\"howdy";
               print $$$$refrefref;

       2.  Anywhere you'd put an identifier (or chain of identifiers) as part
           of a variable or subroutine name, you can replace the identifier
           with a BLOCK returning a reference of the correct type.  In other
           words, the previous examples could be written like this:

               $bar = ${$scalarref};
               push(@{$arrayref}, $filename);
               ${$arrayref}[0] = "January";
               ${$hashref}{"KEY"} = "VALUE";
               &{$coderef}(1,2,3);
               $globref->print("output\n");  # iff IO::Handle is loaded

           Admittedly, it's a little silly to use the curlies in this case,
           but the BLOCK can contain any arbitrary expression, in particular,
           subscripted expressions:

               &{ $dispatch{$index} }(1,2,3);      # call correct routine

           Because of being able to omit the curlies for the simple case of
           $$x, people often make the mistake of viewing the dereferencing
           symbols as proper operators, and wonder about their precedence.  If

       3.  Subroutine calls and lookups of individual array elements arise
           often enough that it gets cumbersome to use method 2.  As a form of
           syntactic sugar, the examples for method 2 may be written:

               $arrayref->[0] = "January";   # Array element
               $hashref->{"KEY"} = "VALUE";  # Hash element
               $coderef->(1,2,3);            # Subroutine call

           The left side of the arrow can be any expression returning a
           reference, including a previous dereference.  Note that $array[$x]
           is not the same thing as "$array->[$x]" here:

               $array[$x]->{"foo"}->[0] = "January";

           This is one of the cases we mentioned earlier in which references
           could spring into existence when in an lvalue context.  Before this
           statement, $array[$x] may have been undefined.  If so, it's
           automatically defined with a hash reference so that we can look up
           "{"foo"}" in it.  Likewise "$array[$x]->{"foo"}" will automatically
           get defined with an array reference so that we can look up "[0]" in
           it.  This process is called autovivification.

           One more thing here.  The arrow is optional between brackets
           subscripts, so you can shrink the above down to

               $array[$x]{"foo"}[0] = "January";

           Which, in the degenerate case of using only ordinary arrays, gives
           you multidimensional arrays just like C's:

               $score[$x][$y][$z] += 42;

           Well, okay, not entirely like C's arrays, actually.  C doesn't know
           how to grow its arrays on demand.  Perl does.

       4.  If a reference happens to be a reference to an object, then there
           are probably methods to access the things referred to, and you
           should probably stick to those methods unless you're in the class
           package that defines the object's methods.  In other words, be
           nice, and don't violate the object's encapsulation without a very
           good reason.  Perl does not enforce encapsulation.  We are not
           totalitarians here.  We do expect some basic civility though.

       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";
           }


       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";

   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 with

           no strict 'refs';

       Only package variables (globals, even if localized) are visible to
       symbolic references.  Lexical variables (declared with my()) aren't in

   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 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

           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 {

       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.

WARNING
       You may not (usefully) use a reference as the key to a hash.  It will
       be converted into a string:

           $x{ \$a } = $a;

       If you try to dereference the key, it won't do a hard dereference, and
       you won't accomplish what you're attempting.  You might want to do
       something more like

           $r = \@a;
           $x{ $r } = $r;

       And then at least you can use the values(), which will be real refs,
       instead of the keys(), which won't.

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

SEE ALSO
       Besides the obvious documents, source code can be instructive.  Some
       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 v5.10.1                      2009-02-12                        PERLREF(1)
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