Before release 5 of Perl it was difficult to represent complex datastructures, because all references had to be symbolic--and even thenit 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 containscalars, 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 goesto zero. (Reference counts for values in self-referential orcyclic data structures may not go to zero without a little help; seeCircular References for a detailed explanation.)If that thing happens to be an object, the object is destructed. Seeperlobj for more about objects. (In a sense, everything in Perl is anobject, but we usually reserve the word for references to objects thathave been officially "blessed" into a class package.)

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

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

References are easy to use in Perl. There is just one overridingprinciple: Perl does no implicit referencing or dereferencing. When ascalar is holding a reference, it always behaves as a simple scalar. Itdoesn't magically start being an array or hash or subroutine; you have totell it explicitly to do so, by dereferencing it.

References are easy to use in Perl. There is just one overridingprinciple: 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 totell it explicitly to do so, by dereferencing it.

That said, be aware that Perl version 5.14 introduces an exceptionto the rule, for syntactic convenience. Experimental array and hash containerfunction behavior allows array and hash references to be handled by Perl asif they had been explicitly syntactically dereferenced. SeeSyntactical Enhancements in perl5140deltaand perlfunc for details.

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, becausethere's already a reference to the variable in the symbol table. Butthe symbol table reference might go away, and you'll still have thereference 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 (filehandleor dirhandle) using the backslash operator. The most you can get is areference 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 squarebrackets:

 $arrayref = [1, 2, ['a', 'b', 'c']];

Here we've created a reference to an anonymous array of three elementswhose final element is itself a reference to another anonymous array of threeelements. (The multidimensional syntax described later can be used toaccess this. For example, after the above, $arrayref->[2][1] would havethe value "b".)

Taking a reference to an enumerated list is not the sameas using square brackets--instead it's the same as creatinga 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 contentsof @foo, not a reference to @foo itself. Likewise for %foo,except that the key references are to copies (since the keys are juststrings rather than full-fledged scalars).

3.

A reference to an anonymous hash can be created using curlybrackets:

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

Anonymous hash and array composers like these can be intermixed freely toproduce as complicated a structure as you want. The multidimensionalsyntax described below works for these too. The values above areliterals, but variables and expressions would work just as well, becauseassignment operators in Perl (even within local() or my()) are executablestatements, not compile-time declarations.

Because curly brackets (braces) are used for several other thingsincluding BLOCKs, you may occasionally have to disambiguate braces at thebeginning of a statement by putting a + or a return in front sothat Perl realizes the opening brace isn't starting a BLOCK. The economy andmnemonic value of using curlies is deemed worth this occasional extrahassle.

For example, if you wanted a function to make a new hash and return areference 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 disambiguatethe expression to mean either the HASH reference, or the BLOCK.

4.

A reference to an anonymous subroutine can be created by usingsub without a subname:

 $coderef = sub { print "Boink!\n" };

Note the semicolon. Except for the codeinside not being immediately executed, a sub {} is not so much adeclaration as it is an operator, like do{} or eval{}. (However, nomatter how many times you execute that particular line (unless you're in aneval("...")), $coderef will still have a reference to the sameanonymous subroutine.)

Anonymous subroutines act as closures with respect to my() variables,that is, variables lexically visible within the current scope. Closureis a notion out of the Lisp world that says if you define an anonymousfunction in a particular lexical context, it pretends to run in thatcontext even when it's called outside the context.

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

You might also think of closure as a way to write a subroutinetemplate without using eval(). Here's a small example of howclosures 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 passedinto newprint() despite "my $x" having gone out of scope by thetime the anonymous subroutine runs. That's what a closure is allabout.

This applies only to lexical variables, by the way. Dynamic variablescontinue to work as they have always worked. Closure is not somethingthat most Perl programmers need trouble themselves about to begin with.

5.

References are often returned by special subroutines called constructors. Perlobjects are just references to a special type of object that happens to knowwhich package it's associated with. Constructors are just special subroutinesthat know how to create that association. They do so by starting with anordinary reference, and it remains an ordinary reference even while it's alsobeing an object. Constructors are often named new(). You can call themindirectly:

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

But that can produce ambiguous syntax in certain cases, so it's oftenbetter 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 youdereference them in a context that assumes they exist. Because we haven'ttalked about dereferencing yet, we can't show you any examples yet.

7.

A reference can be created by using a special syntax, lovingly known asthe *foo{THING} syntax. *foo{THING} returns a reference to the THINGslot in *foo (which is the symbol table entry which holds everythingknown 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 returnsthe IO handle, used for file handles (open), sockets(socket and socketpair), and directoryhandles (opendir). For compatibility with previousversions of Perl, *foo{FILEHANDLE} is a synonym for *foo{IO}, though itis deprecated as of 5.8.0. If deprecation warnings are in effect, it will warnof 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 ananonymous scalar if $foo hasn't been used yet. This might change in afuture release.

*foo{IO} is an alternative to the *HANDLE mechanism given inTypeglobs and Filehandles in perldata for passing filehandlesinto 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 thanyou want to with a typeglob assignment. (It still conflates fileand directory handles, though.) However, if you assign the incomingvalue to a scalar instead of a typeglob as we do in the examplesbelow, 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 toknow how to use references to get back to your long-lost data. Thereare several basic methods.

1.

Anywhere you'd put an identifier (or chain of identifiers) as partof a variable or subroutine name, you can replace the identifier witha 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 thescalar variable happens before it does any key lookups. Anything morecomplicated than a simple scalar variable must use methods 2 or 3 below.However, a "simple scalar" includes an identifier that itself uses method1 recursively. Therefore, the following prints "howdy".

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

2.

Anywhere you'd put an identifier (or chain of identifiers) as part of avariable or subroutine name, you can replace the identifier with aBLOCK returning a reference of the correct type. In other words, theprevious 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, butthe 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 asproper 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 variablecalled %hashref, not dereferencing through $hashref to the hashit's presumably referencing. That would be case 3.

3.

Subroutine calls and lookups of individual array elements arise oftenenough that it gets cumbersome to use method 2. As a form ofsyntactic 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 thesame thing as $array->[$x] here:

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

This is one of the cases we mentioned earlier in which references couldspring into existence when in an lvalue context. Before thisstatement, $array[$x] may have been undefined. If so, it'sautomatically defined with a hash reference so that we can look up{"foo"} in it. Likewise $array[$x]->{"foo"} will automatically getdefined 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 bracketssubscripts, so you can shrink the above down to

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

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

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

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

4.

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

Using a string or number as a reference produces a symbolic reference,as explained above. Using a reference as a number produces aninteger representing its storage location in memory. The onlyuseful thing to be done with this is to compare two referencesnumerically 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 wellas the numeric address expressed in hex. The ref() operator returnsjust the type of thing the reference is pointing to, without theaddress. See ref for details and examples of its use.

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

A typeglob may be dereferenced the same way a reference can, becausethe 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-quotedstring, it's evaluated as a block. The block creates a reference to ananonymous array containing the results of the call to mysub(1,2,3). Sothe whole block returns a reference to an array, which is thendereferenced by @{...} and stuck into the double-quoted string. Thischicanery is also useful for arbitrary expressions:

 print "That yields @{[$n + 5]} widgets\n";

Similarly, an expression that returns a reference to a scalar can bedereferenced via ${...}. Thus, the above expression may be writtenas:

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

Circular References

It is possible to create a "circular reference" in Perl, which can leadto memory leaks. A circular reference occurs when two referencescontain 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 tomemory leaks.

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

You can break circular references by creating a "weak reference". Aweak reference does not increment the reference count for a variable,which means that the object can go out of scope and be destroyed. Youcan weaken a reference with the weaken function exported by theScalar::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. Thenext time you look at the value of the $foo->{bar} key, it willbe undef.

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

Symbolic references

We said that references spring into existence as necessary if they areundefined, but we didn't say what happens if a value used as areference is already defined, but isn't a hard reference. If youuse it as a reference, it'll be treated as a symbolicreference. That is, the value of the scalar is taken to be the nameof a variable, rather than a direct link to a (possibly) anonymousvalue.

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 possibleto intend (with the utmost sincerity) to use a hard reference, andaccidentally use a symbolic reference instead. To protect againstthat, you can say

 use strict 'refs';

and then only hard references will be allowed for the rest of the enclosingblock. An inner block may countermand that with

 no strict 'refs';

Only package variables (globals, even if localized) are visible tosymbolic references. Lexical variables (declared with my()) aren't ina 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 packagevariables, which are all "global" to the package.

Not-so-symbolic references

Since Perl verion 5.001, brackets around a symbolic reference can simplyserve to isolate an identifier or variable name from the rest of anexpression, 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 isa reserved word. In 5.001, this was generalized to work the samewithout the enclosing double quotes, so that

 print ${push} . "over";

and even

 print ${ push } . "over";

will have the same effect. (This would have been a syntax error inPerl 5.000, though Perl 4 allowed it in the spaceless form.) Thisconstruct is not considered to be a symbolic reference when you'reusing 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

 $array{ "aaa" }{ "bbb" }{ "ccc" }

you can write just

 $array{ aaa }{ bbb }{ ccc }

and not worry about whether the subscripts are reserved words. In therare event that you do wish to do something like

 $array{ shift }

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

 $array{ shift() }
  $array{ +shift }
  $array{ shift @_ }

The use warnings pragma or the -w switch will warn you if itinterprets a reserved word as a string.But it will no longer warn you about using lowercase words, because thestring is effectively quoted.

Pseudo-hashes: Using an array as a hash

Pseudo-hashes have been removed from Perl. The 'fields' pragmaremains available.

Function Templates

As explained above, an anonymous function with access to the lexicalvariables visible when that function was compiled, creates a closure. Itretains access to those variables even though it doesn't get run untillater, such as in a signal handler or a Tk callback.

Using a closure as a function template allows us to generate many functionsthat act similarly. Suppose you wanted functions named after the colorsthat 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'retrying 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 cancall red(), RED(), blue(), BLUE(), green(), etc. This technique saves onboth compile time and memory use, and is less error-prone as well, sincesyntax checks happen at compile time. It's critical that any variables inthe 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 makesmuch sense. If you wanted to impose scalar context on the arguments ofthese 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 assignmentabove happens too late to be of much use. You could address this byputting the whole loop of assignments within a BEGIN block, forcing itto occur during compilation.

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

This is because named subroutines are created at compile time so theirlexical variables get assigned to the parent lexicals from the firstexecution of the parent block. If a parent scope is entered a secondtime, its lexicals are created again, while the nested subs stillreference the old ones.

Anonymous subroutines get to capture each time you execute the suboperator, as they are created on the fly. If you are accustomed to usingnested subroutines in other programming languages with their own privatevariables, you'll have to work at it a bit in Perl. The intuitive codingof this type of thing incurs mysterious warnings about "will not stayshared" 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 thetemporary assignments of the anonymous subroutine. But when it does,it has normal access to the lexical variable $x from the scope ofouter() at the time outer is invoked.

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

WARNING

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

 $x{ \$a } = $a;

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

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

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

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

Reference pointer data structure structure struct

Daftar Isi

NAME

NOTE

DESCRIPTION