2 X<reference> X<pointer> X<data structure> X<structure> X<struct>
4 perlref - Perl references and nested data structures
8 This is complete documentation about all aspects of references.
9 For a shorter, tutorial introduction to just the essential features,
14 Before release 5 of Perl it was difficult to represent complex data
15 structures, because all references had to be symbolic--and even then
16 it was difficult to refer to a variable instead of a symbol table entry.
17 Perl now not only makes it easier to use symbolic references to variables,
18 but also lets you have "hard" references to any piece of data or code.
19 Any scalar may hold a hard reference. Because arrays and hashes contain
20 scalars, you can now easily build arrays of arrays, arrays of hashes,
21 hashes of arrays, arrays of hashes of functions, and so on.
23 Hard references are smart--they keep track of reference counts for you,
24 automatically freeing the thing referred to when its reference count goes
25 to zero. (Reference counts for values in self-referential or
26 cyclic data structures may not go to zero without a little help; see
27 L<perlobj/"Two-Phased Garbage Collection"> for a detailed explanation.)
28 If that thing happens to be an object, the object is destructed. See
29 L<perlobj> for more about objects. (In a sense, everything in Perl is an
30 object, but we usually reserve the word for references to objects that
31 have been officially "blessed" into a class package.)
33 Symbolic references are names of variables or other objects, just as a
34 symbolic link in a Unix filesystem contains merely the name of a file.
35 The C<*glob> notation is something of a symbolic reference. (Symbolic
36 references are sometimes called "soft references", but please don't call
37 them that; references are confusing enough without useless synonyms.)
38 X<reference, symbolic> X<reference, soft>
39 X<symbolic reference> X<soft reference>
41 In contrast, hard references are more like hard links in a Unix file
42 system: They are used to access an underlying object without concern for
43 what its (other) name is. When the word "reference" is used without an
44 adjective, as in the following paragraph, it is usually talking about a
46 X<reference, hard> X<hard reference>
48 References are easy to use in Perl. There is just one overriding
49 principle: Perl does no implicit referencing or dereferencing. When a
50 scalar is holding a reference, it always behaves as a simple scalar. It
51 doesn't magically start being an array or hash or subroutine; you have to
52 tell it explicitly to do so, by dereferencing it.
54 =head2 Making References
55 X<reference, creation> X<referencing>
57 References can be created in several ways.
64 By using the backslash operator on a variable, subroutine, or value.
65 (This works much like the & (address-of) operator in C.)
66 This typically creates I<another> reference to a variable, because
67 there's already a reference to the variable in the symbol table. But
68 the symbol table reference might go away, and you'll still have the
69 reference that the backslash returned. Here are some examples:
77 It isn't possible to create a true reference to an IO handle (filehandle
78 or dirhandle) using the backslash operator. The most you can get is a
79 reference to a typeglob, which is actually a complete symbol table entry.
80 But see the explanation of the C<*foo{THING}> syntax below. However,
81 you can still use type globs and globrefs as though they were IO handles.
84 X<array, anonymous> X<[> X<[]> X<square bracket>
85 X<bracket, square> X<arrayref> X<array reference> X<reference, array>
87 A reference to an anonymous array can be created using square
90 $arrayref = [1, 2, ['a', 'b', 'c']];
92 Here we've created a reference to an anonymous array of three elements
93 whose final element is itself a reference to another anonymous array of three
94 elements. (The multidimensional syntax described later can be used to
95 access this. For example, after the above, C<< $arrayref->[2][1] >> would have
98 Taking a reference to an enumerated list is not the same
99 as using square brackets--instead it's the same as creating
100 a list of references!
102 @list = (\$a, \@b, \%c);
103 @list = \($a, @b, %c); # same thing!
105 As a special case, C<\(@foo)> returns a list of references to the contents
106 of C<@foo>, not a reference to C<@foo> itself. Likewise for C<%foo>,
107 except that the key references are to copies (since the keys are just
108 strings rather than full-fledged scalars).
111 X<hash, anonymous> X<{> X<{}> X<curly bracket>
112 X<bracket, curly> X<brace> X<hashref> X<hash reference> X<reference, hash>
114 A reference to an anonymous hash can be created using curly
122 Anonymous hash and array composers like these can be intermixed freely to
123 produce as complicated a structure as you want. The multidimensional
124 syntax described below works for these too. The values above are
125 literals, but variables and expressions would work just as well, because
126 assignment operators in Perl (even within local() or my()) are executable
127 statements, not compile-time declarations.
129 Because curly brackets (braces) are used for several other things
130 including BLOCKs, you may occasionally have to disambiguate braces at the
131 beginning of a statement by putting a C<+> or a C<return> in front so
132 that Perl realizes the opening brace isn't starting a BLOCK. The economy and
133 mnemonic value of using curlies is deemed worth this occasional extra
136 For example, if you wanted a function to make a new hash and return a
137 reference to it, you have these options:
139 sub hashem { { @_ } } # silently wrong
140 sub hashem { +{ @_ } } # ok
141 sub hashem { return { @_ } } # ok
143 On the other hand, if you want the other meaning, you can do this:
145 sub showem { { @_ } } # ambiguous (currently ok, but may change)
146 sub showem { {; @_ } } # ok
147 sub showem { { return @_ } } # ok
149 The leading C<+{> and C<{;> always serve to disambiguate
150 the expression to mean either the HASH reference, or the BLOCK.
153 X<subroutine, anonymous> X<subroutine, reference> X<reference, subroutine>
154 X<scope, lexical> X<closure> X<lexical> X<lexical scope>
156 A reference to an anonymous subroutine can be created by using
157 C<sub> without a subname:
159 $coderef = sub { print "Boink!\n" };
161 Note the semicolon. Except for the code
162 inside not being immediately executed, a C<sub {}> is not so much a
163 declaration as it is an operator, like C<do{}> or C<eval{}>. (However, no
164 matter how many times you execute that particular line (unless you're in an
165 C<eval("...")>), $coderef will still have a reference to the I<same>
166 anonymous subroutine.)
168 Anonymous subroutines act as closures with respect to my() variables,
169 that is, variables lexically visible within the current scope. Closure
170 is a notion out of the Lisp world that says if you define an anonymous
171 function in a particular lexical context, it pretends to run in that
172 context even when it's called outside the context.
174 In human terms, it's a funny way of passing arguments to a subroutine when
175 you define it as well as when you call it. It's useful for setting up
176 little bits of code to run later, such as callbacks. You can even
177 do object-oriented stuff with it, though Perl already provides a different
178 mechanism to do that--see L<perlobj>.
180 You might also think of closure as a way to write a subroutine
181 template without using eval(). Here's a small example of how
186 return sub { my $y = shift; print "$x, $y!\n"; };
188 $h = newprint("Howdy");
189 $g = newprint("Greetings");
199 Greetings, earthlings!
201 Note particularly that $x continues to refer to the value passed
202 into newprint() I<despite> "my $x" having gone out of scope by the
203 time the anonymous subroutine runs. That's what a closure is all
206 This applies only to lexical variables, by the way. Dynamic variables
207 continue to work as they have always worked. Closure is not something
208 that most Perl programmers need trouble themselves about to begin with.
211 X<constructor> X<new>
213 References are often returned by special subroutines called constructors.
214 Perl objects are just references to a special type of object that happens to know
215 which package it's associated with. Constructors are just special
216 subroutines that know how to create that association. They do so by
217 starting with an ordinary reference, and it remains an ordinary reference
218 even while it's also being an object. Constructors are often
219 named new() and called indirectly:
221 $objref = new Doggie (Tail => 'short', Ears => 'long');
223 But don't have to be:
225 $objref = Doggie->new(Tail => 'short', Ears => 'long');
228 $terminal = Term::Cap->Tgetent( { OSPEED => 9600 });
231 $main = MainWindow->new();
232 $menubar = $main->Frame(-relief => "raised",
238 References of the appropriate type can spring into existence if you
239 dereference them in a context that assumes they exist. Because we haven't
240 talked about dereferencing yet, we can't show you any examples yet.
245 A reference can be created by using a special syntax, lovingly known as
246 the *foo{THING} syntax. *foo{THING} returns a reference to the THING
247 slot in *foo (which is the symbol table entry which holds everything
250 $scalarref = *foo{SCALAR};
251 $arrayref = *ARGV{ARRAY};
252 $hashref = *ENV{HASH};
253 $coderef = *handler{CODE};
255 $globref = *foo{GLOB};
256 $formatref = *foo{FORMAT};
258 All of these are self-explanatory except for C<*foo{IO}>. It returns
259 the IO handle, used for file handles (L<perlfunc/open>), sockets
260 (L<perlfunc/socket> and L<perlfunc/socketpair>), and directory
261 handles (L<perlfunc/opendir>). For compatibility with previous
262 versions of Perl, C<*foo{FILEHANDLE}> is a synonym for C<*foo{IO}>, though it
263 is deprecated as of 5.8.0. If deprecation warnings are in effect, it will warn
266 C<*foo{THING}> returns undef if that particular THING hasn't been used yet,
267 except in the case of scalars. C<*foo{SCALAR}> returns a reference to an
268 anonymous scalar if $foo hasn't been used yet. This might change in a
271 C<*foo{IO}> is an alternative to the C<*HANDLE> mechanism given in
272 L<perldata/"Typeglobs and Filehandles"> for passing filehandles
273 into or out of subroutines, or storing into larger data structures.
274 Its disadvantage is that it won't create a new filehandle for you.
275 Its advantage is that you have less risk of clobbering more than
276 you want to with a typeglob assignment. (It still conflates file
277 and directory handles, though.) However, if you assign the incoming
278 value to a scalar instead of a typeglob as we do in the examples
279 below, there's no risk of that happening.
281 splutter(*STDOUT); # pass the whole glob
282 splutter(*STDOUT{IO}); # pass both file and dir handles
286 print $fh "her um well a hmmm\n";
289 $rec = get_rec(*STDIN); # pass the whole glob
290 $rec = get_rec(*STDIN{IO}); # pass both file and dir handles
299 =head2 Using References
300 X<reference, use> X<dereferencing> X<dereference>
302 That's it for creating references. By now you're probably dying to
303 know how to use references to get back to your long-lost data. There
304 are several basic methods.
310 Anywhere you'd put an identifier (or chain of identifiers) as part
311 of a variable or subroutine name, you can replace the identifier with
312 a simple scalar variable containing a reference of the correct type:
315 push(@$arrayref, $filename);
316 $$arrayref[0] = "January";
317 $$hashref{"KEY"} = "VALUE";
319 print $globref "output\n";
321 It's important to understand that we are specifically I<not> dereferencing
322 C<$arrayref[0]> or C<$hashref{"KEY"}> there. The dereference of the
323 scalar variable happens I<before> it does any key lookups. Anything more
324 complicated than a simple scalar variable must use methods 2 or 3 below.
325 However, a "simple scalar" includes an identifier that itself uses method
326 1 recursively. Therefore, the following prints "howdy".
328 $refrefref = \\\"howdy";
334 Anywhere you'd put an identifier (or chain of identifiers) as part of a
335 variable or subroutine name, you can replace the identifier with a
336 BLOCK returning a reference of the correct type. In other words, the
337 previous examples could be written like this:
339 $bar = ${$scalarref};
340 push(@{$arrayref}, $filename);
341 ${$arrayref}[0] = "January";
342 ${$hashref}{"KEY"} = "VALUE";
344 $globref->print("output\n"); # iff IO::Handle is loaded
346 Admittedly, it's a little silly to use the curlies in this case, but
347 the BLOCK can contain any arbitrary expression, in particular,
348 subscripted expressions:
350 &{ $dispatch{$index} }(1,2,3); # call correct routine
352 Because of being able to omit the curlies for the simple case of C<$$x>,
353 people often make the mistake of viewing the dereferencing symbols as
354 proper operators, and wonder about their precedence. If they were,
355 though, you could use parentheses instead of braces. That's not the case.
356 Consider the difference below; case 0 is a short-hand version of case 1,
359 $$hashref{"KEY"} = "VALUE"; # CASE 0
360 ${$hashref}{"KEY"} = "VALUE"; # CASE 1
361 ${$hashref{"KEY"}} = "VALUE"; # CASE 2
362 ${$hashref->{"KEY"}} = "VALUE"; # CASE 3
364 Case 2 is also deceptive in that you're accessing a variable
365 called %hashref, not dereferencing through $hashref to the hash
366 it's presumably referencing. That would be case 3.
369 X<autovivification> X<< -> >> X<arrow>
371 Subroutine calls and lookups of individual array elements arise often
372 enough that it gets cumbersome to use method 2. As a form of
373 syntactic sugar, the examples for method 2 may be written:
375 $arrayref->[0] = "January"; # Array element
376 $hashref->{"KEY"} = "VALUE"; # Hash element
377 $coderef->(1,2,3); # Subroutine call
379 The left side of the arrow can be any expression returning a reference,
380 including a previous dereference. Note that C<$array[$x]> is I<not> the
381 same thing as C<< $array->[$x] >> here:
383 $array[$x]->{"foo"}->[0] = "January";
385 This is one of the cases we mentioned earlier in which references could
386 spring into existence when in an lvalue context. Before this
387 statement, C<$array[$x]> may have been undefined. If so, it's
388 automatically defined with a hash reference so that we can look up
389 C<{"foo"}> in it. Likewise C<< $array[$x]->{"foo"} >> will automatically get
390 defined with an array reference so that we can look up C<[0]> in it.
391 This process is called I<autovivification>.
393 One more thing here. The arrow is optional I<between> brackets
394 subscripts, so you can shrink the above down to
396 $array[$x]{"foo"}[0] = "January";
398 Which, in the degenerate case of using only ordinary arrays, gives you
399 multidimensional arrays just like C's:
401 $score[$x][$y][$z] += 42;
403 Well, okay, not entirely like C's arrays, actually. C doesn't know how
404 to grow its arrays on demand. Perl does.
409 If a reference happens to be a reference to an object, then there are
410 probably methods to access the things referred to, and you should probably
411 stick to those methods unless you're in the class package that defines the
412 object's methods. In other words, be nice, and don't violate the object's
413 encapsulation without a very good reason. Perl does not enforce
414 encapsulation. We are not totalitarians here. We do expect some basic
419 Using a string or number as a reference produces a symbolic reference,
420 as explained above. Using a reference as a number produces an
421 integer representing its storage location in memory. The only
422 useful thing to be done with this is to compare two references
423 numerically to see whether they refer to the same location.
424 X<reference, numeric context>
426 if ($ref1 == $ref2) { # cheap numeric compare of references
427 print "refs 1 and 2 refer to the same thing\n";
430 Using a reference as a string produces both its referent's type,
431 including any package blessing as described in L<perlobj>, as well
432 as the numeric address expressed in hex. The ref() operator returns
433 just the type of thing the reference is pointing to, without the
434 address. See L<perlfunc/ref> for details and examples of its use.
435 X<reference, string context>
437 The bless() operator may be used to associate the object a reference
438 points to with a package functioning as an object class. See L<perlobj>.
440 A typeglob may be dereferenced the same way a reference can, because
441 the dereference syntax always indicates the type of reference desired.
442 So C<${*foo}> and C<${\$foo}> both indicate the same scalar variable.
444 Here's a trick for interpolating a subroutine call into a string:
446 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
448 The way it works is that when the C<@{...}> is seen in the double-quoted
449 string, it's evaluated as a block. The block creates a reference to an
450 anonymous array containing the results of the call to C<mysub(1,2,3)>. So
451 the whole block returns a reference to an array, which is then
452 dereferenced by C<@{...}> and stuck into the double-quoted string. This
453 chicanery is also useful for arbitrary expressions:
455 print "That yields @{[$n + 5]} widgets\n";
457 =head2 Symbolic references
458 X<reference, symbolic> X<reference, soft>
459 X<symbolic reference> X<soft reference>
461 We said that references spring into existence as necessary if they are
462 undefined, but we didn't say what happens if a value used as a
463 reference is already defined, but I<isn't> a hard reference. If you
464 use it as a reference, it'll be treated as a symbolic
465 reference. That is, the value of the scalar is taken to be the I<name>
466 of a variable, rather than a direct link to a (possibly) anonymous
469 People frequently expect it to work like this. So it does.
472 $$name = 1; # Sets $foo
473 ${$name} = 2; # Sets $foo
474 ${$name x 2} = 3; # Sets $foofoo
475 $name->[0] = 4; # Sets $foo[0]
476 @$name = (); # Clears @foo
477 &$name(); # Calls &foo() (as in Perl 4)
479 ${"${pack}::$name"} = 5; # Sets $THAT::foo without eval
481 This is powerful, and slightly dangerous, in that it's possible
482 to intend (with the utmost sincerity) to use a hard reference, and
483 accidentally use a symbolic reference instead. To protect against
488 and then only hard references will be allowed for the rest of the enclosing
489 block. An inner block may countermand that with
493 Only package variables (globals, even if localized) are visible to
494 symbolic references. Lexical variables (declared with my()) aren't in
495 a symbol table, and thus are invisible to this mechanism. For example:
504 This will still print 10, not 20. Remember that local() affects package
505 variables, which are all "global" to the package.
507 =head2 Not-so-symbolic references
509 A new feature contributing to readability in perl version 5.001 is that the
510 brackets around a symbolic reference behave more like quotes, just as they
511 always have within a string. That is,
516 has always meant to print "pop on over", even though push is
517 a reserved word. This has been generalized to work the same outside
520 print ${push} . "over";
524 print ${ push } . "over";
526 will have the same effect. (This would have been a syntax error in
527 Perl 5.000, though Perl 4 allowed it in the spaceless form.) This
528 construct is I<not> considered to be a symbolic reference when you're
532 ${ bareword }; # Okay, means $bareword.
533 ${ "bareword" }; # Error, symbolic reference.
535 Similarly, because of all the subscripting that is done using single
536 words, we've applied the same rule to any bareword that is used for
537 subscripting a hash. So now, instead of writing
539 $array{ "aaa" }{ "bbb" }{ "ccc" }
543 $array{ aaa }{ bbb }{ ccc }
545 and not worry about whether the subscripts are reserved words. In the
546 rare event that you do wish to do something like
550 you can force interpretation as a reserved word by adding anything that
551 makes it more than a bareword:
557 The C<use warnings> pragma or the B<-w> switch will warn you if it
558 interprets a reserved word as a string.
559 But it will no longer warn you about using lowercase words, because the
560 string is effectively quoted.
562 =head2 Pseudo-hashes: Using an array as a hash
563 X<pseudo-hash> X<pseudo hash> X<pseudohash>
565 Pseudo-hashes have been removed from Perl. The 'fields' pragma
568 =head2 Function Templates
569 X<scope, lexical> X<closure> X<lexical> X<lexical scope>
570 X<subroutine, nested> X<sub, nested> X<subroutine, local> X<sub, local>
572 As explained above, an anonymous function with access to the lexical
573 variables visible when that function was compiled, creates a closure. It
574 retains access to those variables even though it doesn't get run until
575 later, such as in a signal handler or a Tk callback.
577 Using a closure as a function template allows us to generate many functions
578 that act similarly. Suppose you wanted functions named after the colors
579 that generated HTML font changes for the various colors:
581 print "Be ", red("careful"), "with that ", green("light");
583 The red() and green() functions would be similar. To create these,
584 we'll assign a closure to a typeglob of the name of the function we're
587 @colors = qw(red blue green yellow orange purple violet);
588 for my $name (@colors) {
589 no strict 'refs'; # allow symbol table manipulation
590 *$name = *{uc $name} = sub { "<FONT COLOR='$name'>@_</FONT>" };
593 Now all those different functions appear to exist independently. You can
594 call red(), RED(), blue(), BLUE(), green(), etc. This technique saves on
595 both compile time and memory use, and is less error-prone as well, since
596 syntax checks happen at compile time. It's critical that any variables in
597 the anonymous subroutine be lexicals in order to create a proper closure.
598 That's the reasons for the C<my> on the loop iteration variable.
600 This is one of the only places where giving a prototype to a closure makes
601 much sense. If you wanted to impose scalar context on the arguments of
602 these functions (probably not a wise idea for this particular example),
603 you could have written it this way instead:
605 *$name = sub ($) { "<FONT COLOR='$name'>$_[0]</FONT>" };
607 However, since prototype checking happens at compile time, the assignment
608 above happens too late to be of much use. You could address this by
609 putting the whole loop of assignments within a BEGIN block, forcing it
610 to occur during compilation.
612 Access to lexicals that change over type--like those in the C<for> loop
613 above--only works with closures, not general subroutines. In the general
614 case, then, named subroutines do not nest properly, although anonymous
615 ones do. Thus is because named subroutines are created (and capture any
616 outer lexicals) only once at compile time, whereas anonymous subroutines
617 get to capture each time you execute the 'sub' operator. If you are
618 accustomed to using nested subroutines in other programming languages with
619 their own private variables, you'll have to work at it a bit in Perl. The
620 intuitive coding of this type of thing incurs mysterious warnings about
621 "will not stay shared". For example, this won't work:
625 sub inner { return $x * 19 } # WRONG
629 A work-around is the following:
633 local *inner = sub { return $x * 19 };
637 Now inner() can only be called from within outer(), because of the
638 temporary assignments of the closure (anonymous subroutine). But when
639 it does, it has normal access to the lexical variable $x from the scope
642 This has the interesting effect of creating a function local to another
643 function, something not normally supported in Perl.
646 X<reference, string context> X<reference, use as hash key>
648 You may not (usefully) use a reference as the key to a hash. It will be
649 converted into a string:
653 If you try to dereference the key, it won't do a hard dereference, and
654 you won't accomplish what you're attempting. You might want to do something
660 And then at least you can use the values(), which will be
661 real refs, instead of the keys(), which won't.
663 The standard Tie::RefHash module provides a convenient workaround to this.
667 Besides the obvious documents, source code can be instructive.
668 Some pathological examples of the use of references can be found
669 in the F<t/op/ref.t> regression test in the Perl source directory.
671 See also L<perldsc> and L<perllol> for how to use references to create
672 complex data structures, and L<perltoot>, L<perlobj>, and L<perlbot>
673 for how to use them to create objects.