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};
257 All of these are self-explanatory except for C<*foo{IO}>. It returns
258 the IO handle, used for file handles (L<perlfunc/open>), sockets
259 (L<perlfunc/socket> and L<perlfunc/socketpair>), and directory
260 handles (L<perlfunc/opendir>). For compatibility with previous
261 versions of Perl, C<*foo{FILEHANDLE}> is a synonym for C<*foo{IO}>, though it
262 is deprecated as of 5.8.0. If deprecation warnings are in effect, it will warn
265 C<*foo{THING}> returns undef if that particular THING hasn't been used yet,
266 except in the case of scalars. C<*foo{SCALAR}> returns a reference to an
267 anonymous scalar if $foo hasn't been used yet. This might change in a
270 C<*foo{IO}> is an alternative to the C<*HANDLE> mechanism given in
271 L<perldata/"Typeglobs and Filehandles"> for passing filehandles
272 into or out of subroutines, or storing into larger data structures.
273 Its disadvantage is that it won't create a new filehandle for you.
274 Its advantage is that you have less risk of clobbering more than
275 you want to with a typeglob assignment. (It still conflates file
276 and directory handles, though.) However, if you assign the incoming
277 value to a scalar instead of a typeglob as we do in the examples
278 below, there's no risk of that happening.
280 splutter(*STDOUT); # pass the whole glob
281 splutter(*STDOUT{IO}); # pass both file and dir handles
285 print $fh "her um well a hmmm\n";
288 $rec = get_rec(*STDIN); # pass the whole glob
289 $rec = get_rec(*STDIN{IO}); # pass both file and dir handles
298 =head2 Using References
299 X<reference, use> X<dereferencing> X<dereference>
301 That's it for creating references. By now you're probably dying to
302 know how to use references to get back to your long-lost data. There
303 are several basic methods.
309 Anywhere you'd put an identifier (or chain of identifiers) as part
310 of a variable or subroutine name, you can replace the identifier with
311 a simple scalar variable containing a reference of the correct type:
314 push(@$arrayref, $filename);
315 $$arrayref[0] = "January";
316 $$hashref{"KEY"} = "VALUE";
318 print $globref "output\n";
320 It's important to understand that we are specifically I<not> dereferencing
321 C<$arrayref[0]> or C<$hashref{"KEY"}> there. The dereference of the
322 scalar variable happens I<before> it does any key lookups. Anything more
323 complicated than a simple scalar variable must use methods 2 or 3 below.
324 However, a "simple scalar" includes an identifier that itself uses method
325 1 recursively. Therefore, the following prints "howdy".
327 $refrefref = \\\"howdy";
333 Anywhere you'd put an identifier (or chain of identifiers) as part of a
334 variable or subroutine name, you can replace the identifier with a
335 BLOCK returning a reference of the correct type. In other words, the
336 previous examples could be written like this:
338 $bar = ${$scalarref};
339 push(@{$arrayref}, $filename);
340 ${$arrayref}[0] = "January";
341 ${$hashref}{"KEY"} = "VALUE";
343 $globref->print("output\n"); # iff IO::Handle is loaded
345 Admittedly, it's a little silly to use the curlies in this case, but
346 the BLOCK can contain any arbitrary expression, in particular,
347 subscripted expressions:
349 &{ $dispatch{$index} }(1,2,3); # call correct routine
351 Because of being able to omit the curlies for the simple case of C<$$x>,
352 people often make the mistake of viewing the dereferencing symbols as
353 proper operators, and wonder about their precedence. If they were,
354 though, you could use parentheses instead of braces. That's not the case.
355 Consider the difference below; case 0 is a short-hand version of case 1,
358 $$hashref{"KEY"} = "VALUE"; # CASE 0
359 ${$hashref}{"KEY"} = "VALUE"; # CASE 1
360 ${$hashref{"KEY"}} = "VALUE"; # CASE 2
361 ${$hashref->{"KEY"}} = "VALUE"; # CASE 3
363 Case 2 is also deceptive in that you're accessing a variable
364 called %hashref, not dereferencing through $hashref to the hash
365 it's presumably referencing. That would be case 3.
368 X<autovivification> X<< -> >> X<arrow>
370 Subroutine calls and lookups of individual array elements arise often
371 enough that it gets cumbersome to use method 2. As a form of
372 syntactic sugar, the examples for method 2 may be written:
374 $arrayref->[0] = "January"; # Array element
375 $hashref->{"KEY"} = "VALUE"; # Hash element
376 $coderef->(1,2,3); # Subroutine call
378 The left side of the arrow can be any expression returning a reference,
379 including a previous dereference. Note that C<$array[$x]> is I<not> the
380 same thing as C<< $array->[$x] >> here:
382 $array[$x]->{"foo"}->[0] = "January";
384 This is one of the cases we mentioned earlier in which references could
385 spring into existence when in an lvalue context. Before this
386 statement, C<$array[$x]> may have been undefined. If so, it's
387 automatically defined with a hash reference so that we can look up
388 C<{"foo"}> in it. Likewise C<< $array[$x]->{"foo"} >> will automatically get
389 defined with an array reference so that we can look up C<[0]> in it.
390 This process is called I<autovivification>.
392 One more thing here. The arrow is optional I<between> brackets
393 subscripts, so you can shrink the above down to
395 $array[$x]{"foo"}[0] = "January";
397 Which, in the degenerate case of using only ordinary arrays, gives you
398 multidimensional arrays just like C's:
400 $score[$x][$y][$z] += 42;
402 Well, okay, not entirely like C's arrays, actually. C doesn't know how
403 to grow its arrays on demand. Perl does.
408 If a reference happens to be a reference to an object, then there are
409 probably methods to access the things referred to, and you should probably
410 stick to those methods unless you're in the class package that defines the
411 object's methods. In other words, be nice, and don't violate the object's
412 encapsulation without a very good reason. Perl does not enforce
413 encapsulation. We are not totalitarians here. We do expect some basic
418 Using a string or number as a reference produces a symbolic reference,
419 as explained above. Using a reference as a number produces an
420 integer representing its storage location in memory. The only
421 useful thing to be done with this is to compare two references
422 numerically to see whether they refer to the same location.
423 X<reference, numeric context>
425 if ($ref1 == $ref2) { # cheap numeric compare of references
426 print "refs 1 and 2 refer to the same thing\n";
429 Using a reference as a string produces both its referent's type,
430 including any package blessing as described in L<perlobj>, as well
431 as the numeric address expressed in hex. The ref() operator returns
432 just the type of thing the reference is pointing to, without the
433 address. See L<perlfunc/ref> for details and examples of its use.
434 X<reference, string context>
436 The bless() operator may be used to associate the object a reference
437 points to with a package functioning as an object class. See L<perlobj>.
439 A typeglob may be dereferenced the same way a reference can, because
440 the dereference syntax always indicates the type of reference desired.
441 So C<${*foo}> and C<${\$foo}> both indicate the same scalar variable.
443 Here's a trick for interpolating a subroutine call into a string:
445 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
447 The way it works is that when the C<@{...}> is seen in the double-quoted
448 string, it's evaluated as a block. The block creates a reference to an
449 anonymous array containing the results of the call to C<mysub(1,2,3)>. So
450 the whole block returns a reference to an array, which is then
451 dereferenced by C<@{...}> and stuck into the double-quoted string. This
452 chicanery is also useful for arbitrary expressions:
454 print "That yields @{[$n + 5]} widgets\n";
456 =head2 Symbolic references
457 X<reference, symbolic> X<reference, soft>
458 X<symbolic reference> X<soft reference>
460 We said that references spring into existence as necessary if they are
461 undefined, but we didn't say what happens if a value used as a
462 reference is already defined, but I<isn't> a hard reference. If you
463 use it as a reference, it'll be treated as a symbolic
464 reference. That is, the value of the scalar is taken to be the I<name>
465 of a variable, rather than a direct link to a (possibly) anonymous
468 People frequently expect it to work like this. So it does.
471 $$name = 1; # Sets $foo
472 ${$name} = 2; # Sets $foo
473 ${$name x 2} = 3; # Sets $foofoo
474 $name->[0] = 4; # Sets $foo[0]
475 @$name = (); # Clears @foo
476 &$name(); # Calls &foo() (as in Perl 4)
478 ${"${pack}::$name"} = 5; # Sets $THAT::foo without eval
480 This is powerful, and slightly dangerous, in that it's possible
481 to intend (with the utmost sincerity) to use a hard reference, and
482 accidentally use a symbolic reference instead. To protect against
487 and then only hard references will be allowed for the rest of the enclosing
488 block. An inner block may countermand that with
492 Only package variables (globals, even if localized) are visible to
493 symbolic references. Lexical variables (declared with my()) aren't in
494 a symbol table, and thus are invisible to this mechanism. For example:
503 This will still print 10, not 20. Remember that local() affects package
504 variables, which are all "global" to the package.
506 =head2 Not-so-symbolic references
508 A new feature contributing to readability in perl version 5.001 is that the
509 brackets around a symbolic reference behave more like quotes, just as they
510 always have within a string. That is,
515 has always meant to print "pop on over", even though push is
516 a reserved word. This has been generalized to work the same outside
519 print ${push} . "over";
523 print ${ push } . "over";
525 will have the same effect. (This would have been a syntax error in
526 Perl 5.000, though Perl 4 allowed it in the spaceless form.) This
527 construct is I<not> considered to be a symbolic reference when you're
531 ${ bareword }; # Okay, means $bareword.
532 ${ "bareword" }; # Error, symbolic reference.
534 Similarly, because of all the subscripting that is done using single
535 words, we've applied the same rule to any bareword that is used for
536 subscripting a hash. So now, instead of writing
538 $array{ "aaa" }{ "bbb" }{ "ccc" }
542 $array{ aaa }{ bbb }{ ccc }
544 and not worry about whether the subscripts are reserved words. In the
545 rare event that you do wish to do something like
549 you can force interpretation as a reserved word by adding anything that
550 makes it more than a bareword:
556 The C<use warnings> pragma or the B<-w> switch will warn you if it
557 interprets a reserved word as a string.
558 But it will no longer warn you about using lowercase words, because the
559 string is effectively quoted.
561 =head2 Pseudo-hashes: Using an array as a hash
562 X<pseudo-hash> X<pseudo hash> X<pseudohash>
564 Pseudo-hashes have been removed from Perl. The 'fields' pragma
567 =head2 Function Templates
568 X<scope, lexical> X<closure> X<lexical> X<lexical scope>
569 X<subroutine, nested> X<sub, nested> X<subroutine, local> X<sub, local>
571 As explained above, an anonymous function with access to the lexical
572 variables visible when that function was compiled, creates a closure. It
573 retains access to those variables even though it doesn't get run until
574 later, such as in a signal handler or a Tk callback.
576 Using a closure as a function template allows us to generate many functions
577 that act similarly. Suppose you wanted functions named after the colors
578 that generated HTML font changes for the various colors:
580 print "Be ", red("careful"), "with that ", green("light");
582 The red() and green() functions would be similar. To create these,
583 we'll assign a closure to a typeglob of the name of the function we're
586 @colors = qw(red blue green yellow orange purple violet);
587 for my $name (@colors) {
588 no strict 'refs'; # allow symbol table manipulation
589 *$name = *{uc $name} = sub { "<FONT COLOR='$name'>@_</FONT>" };
592 Now all those different functions appear to exist independently. You can
593 call red(), RED(), blue(), BLUE(), green(), etc. This technique saves on
594 both compile time and memory use, and is less error-prone as well, since
595 syntax checks happen at compile time. It's critical that any variables in
596 the anonymous subroutine be lexicals in order to create a proper closure.
597 That's the reasons for the C<my> on the loop iteration variable.
599 This is one of the only places where giving a prototype to a closure makes
600 much sense. If you wanted to impose scalar context on the arguments of
601 these functions (probably not a wise idea for this particular example),
602 you could have written it this way instead:
604 *$name = sub ($) { "<FONT COLOR='$name'>$_[0]</FONT>" };
606 However, since prototype checking happens at compile time, the assignment
607 above happens too late to be of much use. You could address this by
608 putting the whole loop of assignments within a BEGIN block, forcing it
609 to occur during compilation.
611 Access to lexicals that change over type--like those in the C<for> loop
612 above--only works with closures, not general subroutines. In the general
613 case, then, named subroutines do not nest properly, although anonymous
614 ones do. Thus is because named subroutines are created (and capture any
615 outer lexicals) only once at compile time, whereas anonymous subroutines
616 get to capture each time you execute the 'sub' operator. If you are
617 accustomed to using nested subroutines in other programming languages with
618 their own private variables, you'll have to work at it a bit in Perl. The
619 intuitive coding of this type of thing incurs mysterious warnings about
620 "will not stay shared". For example, this won't work:
624 sub inner { return $x * 19 } # WRONG
628 A work-around is the following:
632 local *inner = sub { return $x * 19 };
636 Now inner() can only be called from within outer(), because of the
637 temporary assignments of the closure (anonymous subroutine). But when
638 it does, it has normal access to the lexical variable $x from the scope
641 This has the interesting effect of creating a function local to another
642 function, something not normally supported in Perl.
645 X<reference, string context> X<reference, use as hash key>
647 You may not (usefully) use a reference as the key to a hash. It will be
648 converted into a string:
652 If you try to dereference the key, it won't do a hard dereference, and
653 you won't accomplish what you're attempting. You might want to do something
659 And then at least you can use the values(), which will be
660 real refs, instead of the keys(), which won't.
662 The standard Tie::RefHash module provides a convenient workaround to this.
666 Besides the obvious documents, source code can be instructive.
667 Some pathological examples of the use of references can be found
668 in the F<t/op/ref.t> regression test in the Perl source directory.
670 See also L<perldsc> and L<perllol> for how to use references to create
671 complex data structures, and L<perltoot>, L<perlobj>, and L<perlbot>
672 for how to use them to create objects.