3 perlref - Perl references and nested data structures
7 This is complete documentation about all aspects of references.
8 For a shorter, tutorial introduction to just the essential features,
13 Before release 5 of Perl it was difficult to represent complex data
14 structures, because all references had to be symbolic--and even then
15 it was difficult to refer to a variable instead of a symbol table entry.
16 Perl now not only makes it easier to use symbolic references to variables,
17 but also lets you have "hard" references to any piece of data or code.
18 Any scalar may hold a hard reference. Because arrays and hashes contain
19 scalars, you can now easily build arrays of arrays, arrays of hashes,
20 hashes of arrays, arrays of hashes of functions, and so on.
22 Hard references are smart--they keep track of reference counts for you,
23 automatically freeing the thing referred to when its reference count goes
24 to zero. (Note: the reference counts for values in self-referential or
25 cyclic data structures may not go to zero without a little help; see
26 L<perlobj/"Two-Phased Garbage Collection"> for a detailed explanation.)
27 If that thing happens to be an object, the object is destructed. See
28 L<perlobj> for more about objects. (In a sense, everything in Perl is an
29 object, but we usually reserve the word for references to objects that
30 have been officially "blessed" into a class package.)
32 Symbolic references are names of variables or other objects, just as a
33 symbolic link in a Unix filesystem contains merely the name of a file.
34 The C<*glob> notation is a kind of symbolic reference. (Symbolic
35 references are sometimes called "soft references", but please don't call
36 them that; references are confusing enough without useless synonyms.)
38 In contrast, hard references are more like hard links in a Unix file
39 system: They are used to access an underlying object without concern for
40 what its (other) name is. When the word "reference" is used without an
41 adjective, as in the following paragraph, it is usually talking about a
44 References are easy to use in Perl. There is just one overriding
45 principle: Perl does no implicit referencing or dereferencing. When a
46 scalar is holding a reference, it always behaves as a simple scalar. It
47 doesn't magically start being an array or hash or subroutine; you have to
48 tell it explicitly to do so, by dereferencing it.
50 =head2 Making References
52 References can be created in several ways.
58 By using the backslash operator on a variable, subroutine, or value.
59 (This works much like the & (address-of) operator in C.) Note
60 that this typically creates I<ANOTHER> reference to a variable, because
61 there's already a reference to the variable in the symbol table. But
62 the symbol table reference might go away, and you'll still have the
63 reference that the backslash returned. Here are some examples:
71 It isn't possible to create a true reference to an IO handle (filehandle
72 or dirhandle) using the backslash operator. The most you can get is a
73 reference to a typeglob, which is actually a complete symbol table entry.
74 But see the explanation of the C<*foo{THING}> syntax below. However,
75 you can still use type globs and globrefs as though they were IO handles.
79 A reference to an anonymous array can be created using square
82 $arrayref = [1, 2, ['a', 'b', 'c']];
84 Here we've created a reference to an anonymous array of three elements
85 whose final element is itself a reference to another anonymous array of three
86 elements. (The multidimensional syntax described later can be used to
87 access this. For example, after the above, C<$arrayref-E<gt>[2][1]> would have
90 Note that taking a reference to an enumerated list is not the same
91 as using square brackets--instead it's the same as creating
94 @list = (\$a, \@b, \%c);
95 @list = \($a, @b, %c); # same thing!
97 As a special case, C<\(@foo)> returns a list of references to the contents
98 of C<@foo>, not a reference to C<@foo> itself. Likewise for C<%foo>,
99 except that the key references are to copies (since the keys are just
100 strings rather than full-fledged scalars).
104 A reference to an anonymous hash can be created using curly
112 Anonymous hash and array composers like these can be intermixed freely to
113 produce as complicated a structure as you want. The multidimensional
114 syntax described below works for these too. The values above are
115 literals, but variables and expressions would work just as well, because
116 assignment operators in Perl (even within local() or my()) are executable
117 statements, not compile-time declarations.
119 Because curly brackets (braces) are used for several other things
120 including BLOCKs, you may occasionally have to disambiguate braces at the
121 beginning of a statement by putting a C<+> or a C<return> in front so
122 that Perl realizes the opening brace isn't starting a BLOCK. The economy and
123 mnemonic value of using curlies is deemed worth this occasional extra
126 For example, if you wanted a function to make a new hash and return a
127 reference to it, you have these options:
129 sub hashem { { @_ } } # silently wrong
130 sub hashem { +{ @_ } } # ok
131 sub hashem { return { @_ } } # ok
133 On the other hand, if you want the other meaning, you can do this:
135 sub showem { { @_ } } # ambiguous (currently ok, but may change)
136 sub showem { {; @_ } } # ok
137 sub showem { { return @_ } } # ok
139 Note how the leading C<+{> and C<{;> always serve to disambiguate
140 the expression to mean either the HASH reference, or the BLOCK.
144 A reference to an anonymous subroutine can be created by using
145 C<sub> without a subname:
147 $coderef = sub { print "Boink!\n" };
149 Note the presence of the semicolon. Except for the fact that the code
150 inside isn't executed immediately, a C<sub {}> is not so much a
151 declaration as it is an operator, like C<do{}> or C<eval{}>. (However, no
152 matter how many times you execute that particular line (unless you're in an
153 C<eval("...")>), C<$coderef> will still have a reference to the I<SAME>
154 anonymous subroutine.)
156 Anonymous subroutines act as closures with respect to my() variables,
157 that is, variables visible lexically within the current scope. Closure
158 is a notion out of the Lisp world that says if you define an anonymous
159 function in a particular lexical context, it pretends to run in that
160 context even when it's called outside of the context.
162 In human terms, it's a funny way of passing arguments to a subroutine when
163 you define it as well as when you call it. It's useful for setting up
164 little bits of code to run later, such as callbacks. You can even
165 do object-oriented stuff with it, though Perl already provides a different
166 mechanism to do that--see L<perlobj>.
168 You can also think of closure as a way to write a subroutine template without
169 using eval. (In fact, in version 5.000, eval was the I<only> way to get
170 closures. You may wish to use "require 5.001" if you use closures.)
172 Here's a small example of how closures works:
176 return sub { my $y = shift; print "$x, $y!\n"; };
178 $h = newprint("Howdy");
179 $g = newprint("Greetings");
189 Greetings, earthlings!
191 Note particularly that $x continues to refer to the value passed into
192 newprint() I<despite> the fact that the "my $x" has seemingly gone out of
193 scope by the time the anonymous subroutine runs. That's what closure
196 This applies only to lexical variables, by the way. Dynamic variables
197 continue to work as they have always worked. Closure is not something
198 that most Perl programmers need trouble themselves about to begin with.
202 References are often returned by special subroutines called constructors.
203 Perl objects are just references to a special kind of object that happens to know
204 which package it's associated with. Constructors are just special
205 subroutines that know how to create that association. They do so by
206 starting with an ordinary reference, and it remains an ordinary reference
207 even while it's also being an object. Constructors are often
208 named new() and called indirectly:
210 $objref = new Doggie (Tail => 'short', Ears => 'long');
212 But don't have to be:
214 $objref = Doggie->new(Tail => 'short', Ears => 'long');
217 $terminal = Term::Cap->Tgetent( { OSPEED => 9600 });
220 $main = MainWindow->new();
221 $menubar = $main->Frame(-relief => "raised",
226 References of the appropriate type can spring into existence if you
227 dereference them in a context that assumes they exist. Because we haven't
228 talked about dereferencing yet, we can't show you any examples yet.
232 A reference can be created by using a special syntax, lovingly known as
233 the *foo{THING} syntax. *foo{THING} returns a reference to the THING
234 slot in *foo (which is the symbol table entry which holds everything
237 $scalarref = *foo{SCALAR};
238 $arrayref = *ARGV{ARRAY};
239 $hashref = *ENV{HASH};
240 $coderef = *handler{CODE};
242 $globref = *foo{GLOB};
244 All of these are self-explanatory except for *foo{IO}. It returns the
245 IO handle, used for file handles (L<perlfunc/open>), sockets
246 (L<perlfunc/socket> and L<perlfunc/socketpair>), and directory handles
247 (L<perlfunc/opendir>). For compatibility with previous versions of
248 Perl, *foo{FILEHANDLE} is a synonym for *foo{IO}.
250 *foo{THING} returns undef if that particular THING hasn't been used yet,
251 except in the case of scalars. *foo{SCALAR} returns a reference to an
252 anonymous scalar if $foo hasn't been used yet. This might change in a
255 *foo{IO} is an alternative to the \*HANDLE mechanism given in
256 L<perldata/"Typeglobs and Filehandles"> for passing filehandles
257 into or out of subroutines, or storing into larger data structures.
258 Its disadvantage is that it won't create a new filehandle for you.
259 Its advantage is that you have no risk of clobbering more than you want
260 to with a typeglob assignment, although if you assign to a scalar instead
261 of a typeglob, you're ok.
264 splutter(*STDOUT{IO});
268 print $fh "her um well a hmmm\n";
271 $rec = get_rec(*STDIN);
272 $rec = get_rec(*STDIN{IO});
281 =head2 Using References
283 That's it for creating references. By now you're probably dying to
284 know how to use references to get back to your long-lost data. There
285 are several basic methods.
291 Anywhere you'd put an identifier (or chain of identifiers) as part
292 of a variable or subroutine name, you can replace the identifier with
293 a simple scalar variable containing a reference of the correct type:
296 push(@$arrayref, $filename);
297 $$arrayref[0] = "January";
298 $$hashref{"KEY"} = "VALUE";
300 print $globref "output\n";
302 It's important to understand that we are specifically I<NOT> dereferencing
303 C<$arrayref[0]> or C<$hashref{"KEY"}> there. The dereference of the
304 scalar variable happens I<BEFORE> it does any key lookups. Anything more
305 complicated than a simple scalar variable must use methods 2 or 3 below.
306 However, a "simple scalar" includes an identifier that itself uses method
307 1 recursively. Therefore, the following prints "howdy".
309 $refrefref = \\\"howdy";
314 Anywhere you'd put an identifier (or chain of identifiers) as part of a
315 variable or subroutine name, you can replace the identifier with a
316 BLOCK returning a reference of the correct type. In other words, the
317 previous examples could be written like this:
319 $bar = ${$scalarref};
320 push(@{$arrayref}, $filename);
321 ${$arrayref}[0] = "January";
322 ${$hashref}{"KEY"} = "VALUE";
324 $globref->print("output\n"); # iff IO::Handle is loaded
326 Admittedly, it's a little silly to use the curlies in this case, but
327 the BLOCK can contain any arbitrary expression, in particular,
328 subscripted expressions:
330 &{ $dispatch{$index} }(1,2,3); # call correct routine
332 Because of being able to omit the curlies for the simple case of C<$$x>,
333 people often make the mistake of viewing the dereferencing symbols as
334 proper operators, and wonder about their precedence. If they were,
335 though, you could use parentheses instead of braces. That's not the case.
336 Consider the difference below; case 0 is a short-hand version of case 1,
339 $$hashref{"KEY"} = "VALUE"; # CASE 0
340 ${$hashref}{"KEY"} = "VALUE"; # CASE 1
341 ${$hashref{"KEY"}} = "VALUE"; # CASE 2
342 ${$hashref->{"KEY"}} = "VALUE"; # CASE 3
344 Case 2 is also deceptive in that you're accessing a variable
345 called %hashref, not dereferencing through $hashref to the hash
346 it's presumably referencing. That would be case 3.
350 Subroutine calls and lookups of individual array elements arise often
351 enough that it gets cumbersome to use method 2. As a form of
352 syntactic sugar, the examples for method 2 may be written:
354 $arrayref->[0] = "January"; # Array element
355 $hashref->{"KEY"} = "VALUE"; # Hash element
356 $coderef->(1,2,3); # Subroutine call
358 The left side of the arrow can be any expression returning a reference,
359 including a previous dereference. Note that C<$array[$x]> is I<NOT> the
360 same thing as C<$array-E<gt>[$x]> here:
362 $array[$x]->{"foo"}->[0] = "January";
364 This is one of the cases we mentioned earlier in which references could
365 spring into existence when in an lvalue context. Before this
366 statement, C<$array[$x]> may have been undefined. If so, it's
367 automatically defined with a hash reference so that we can look up
368 C<{"foo"}> in it. Likewise C<$array[$x]-E<gt>{"foo"}> will automatically get
369 defined with an array reference so that we can look up C<[0]> in it.
370 This process is called I<autovivification>.
372 One more thing here. The arrow is optional I<BETWEEN> brackets
373 subscripts, so you can shrink the above down to
375 $array[$x]{"foo"}[0] = "January";
377 Which, in the degenerate case of using only ordinary arrays, gives you
378 multidimensional arrays just like C's:
380 $score[$x][$y][$z] += 42;
382 Well, okay, not entirely like C's arrays, actually. C doesn't know how
383 to grow its arrays on demand. Perl does.
387 If a reference happens to be a reference to an object, then there are
388 probably methods to access the things referred to, and you should probably
389 stick to those methods unless you're in the class package that defines the
390 object's methods. In other words, be nice, and don't violate the object's
391 encapsulation without a very good reason. Perl does not enforce
392 encapsulation. We are not totalitarians here. We do expect some basic
397 The ref() operator may be used to determine what type of thing the
398 reference is pointing to. See L<perlfunc>.
400 The bless() operator may be used to associate the object a reference
401 points to with a package functioning as an object class. See L<perlobj>.
403 A typeglob may be dereferenced the same way a reference can, because
404 the dereference syntax always indicates the kind of reference desired.
405 So C<${*foo}> and C<${\$foo}> both indicate the same scalar variable.
407 Here's a trick for interpolating a subroutine call into a string:
409 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
411 The way it works is that when the C<@{...}> is seen in the double-quoted
412 string, it's evaluated as a block. The block creates a reference to an
413 anonymous array containing the results of the call to C<mysub(1,2,3)>. So
414 the whole block returns a reference to an array, which is then
415 dereferenced by C<@{...}> and stuck into the double-quoted string. This
416 chicanery is also useful for arbitrary expressions:
418 print "That yields @{[$n + 5]} widgets\n";
420 =head2 Symbolic references
422 We said that references spring into existence as necessary if they are
423 undefined, but we didn't say what happens if a value used as a
424 reference is already defined, but I<ISN'T> a hard reference. If you
425 use it as a reference in this case, it'll be treated as a symbolic
426 reference. That is, the value of the scalar is taken to be the I<NAME>
427 of a variable, rather than a direct link to a (possibly) anonymous
430 People frequently expect it to work like this. So it does.
433 $$name = 1; # Sets $foo
434 ${$name} = 2; # Sets $foo
435 ${$name x 2} = 3; # Sets $foofoo
436 $name->[0] = 4; # Sets $foo[0]
437 @$name = (); # Clears @foo
438 &$name(); # Calls &foo() (as in Perl 4)
440 ${"${pack}::$name"} = 5; # Sets $THAT::foo without eval
442 This is very powerful, and slightly dangerous, in that it's possible
443 to intend (with the utmost sincerity) to use a hard reference, and
444 accidentally use a symbolic reference instead. To protect against
449 and then only hard references will be allowed for the rest of the enclosing
450 block. An inner block may countermand that with
454 Only package variables (globals, even if localized) are visible to
455 symbolic references. Lexical variables (declared with my()) aren't in
456 a symbol table, and thus are invisible to this mechanism. For example:
465 This will still print 10, not 20. Remember that local() affects package
466 variables, which are all "global" to the package.
468 =head2 Not-so-symbolic references
470 A new feature contributing to readability in perl version 5.001 is that the
471 brackets around a symbolic reference behave more like quotes, just as they
472 always have within a string. That is,
477 has always meant to print "pop on over", despite the fact that push is
478 a reserved word. This has been generalized to work the same outside
481 print ${push} . "over";
485 print ${ push } . "over";
487 will have the same effect. (This would have been a syntax error in
488 Perl 5.000, though Perl 4 allowed it in the spaceless form.) Note that this
489 construct is I<not> considered to be a symbolic reference when you're
493 ${ bareword }; # Okay, means $bareword.
494 ${ "bareword" }; # Error, symbolic reference.
496 Similarly, because of all the subscripting that is done using single
497 words, we've applied the same rule to any bareword that is used for
498 subscripting a hash. So now, instead of writing
500 $array{ "aaa" }{ "bbb" }{ "ccc" }
504 $array{ aaa }{ bbb }{ ccc }
506 and not worry about whether the subscripts are reserved words. In the
507 rare event that you do wish to do something like
511 you can force interpretation as a reserved word by adding anything that
512 makes it more than a bareword:
518 The B<-w> switch will warn you if it interprets a reserved word as a string.
519 But it will no longer warn you about using lowercase words, because the
520 string is effectively quoted.
522 =head2 Pseudo-hashes: Using an array as a hash
524 WARNING: This section describes an experimental feature. Details may
525 change without notice in future versions.
527 Beginning with release 5.005 of Perl you can use an array reference
528 in some contexts that would normally require a hash reference. This
529 allows you to access array elements using symbolic names, as if they
530 were fields in a structure.
532 For this to work, the array must contain extra information. The first
533 element of the array has to be a hash reference that maps field names
534 to array indices. Here is an example:
536 $struct = [{foo => 1, bar => 2}, "FOO", "BAR"];
538 $struct->{foo}; # same as $struct->[1], i.e. "FOO"
539 $struct->{bar}; # same as $struct->[2], i.e. "BAR"
541 keys %$struct; # will return ("foo", "bar") in some order
542 values %$struct; # will return ("FOO", "BAR") in same some order
544 while (my($k,$v) = each %$struct) {
548 Perl will raise an exception if you try to delete keys from a pseudo-hash
549 or try to access nonexistent fields. For better performance, Perl can also
550 do the translation from field names to array indices at compile time for
551 typed object references. See L<fields>.
554 =head2 Function Templates
556 As explained above, a closure is an anonymous function with access to the
557 lexical variables visible when that function was compiled. It retains
558 access to those variables even though it doesn't get run until later,
559 such as in a signal handler or a Tk callback.
561 Using a closure as a function template allows us to generate many functions
562 that act similarly. Suppopose you wanted functions named after the colors
563 that generated HTML font changes for the various colors:
565 print "Be ", red("careful"), "with that ", green("light");
567 The red() and green() functions would be very similar. To create these,
568 we'll assign a closure to a typeglob of the name of the function we're
571 @colors = qw(red blue green yellow orange purple violet);
572 for my $name (@colors) {
573 no strict 'refs'; # allow symbol table manipulation
574 *$name = *{uc $name} = sub { "<FONT COLOR='$name'>@_</FONT>" };
577 Now all those different functions appear to exist independently. You can
578 call red(), RED(), blue(), BLUE(), green(), etc. This technique saves on
579 both compile time and memory use, and is less error-prone as well, since
580 syntax checks happen at compile time. It's critical that any variables in
581 the anonymous subroutine be lexicals in order to create a proper closure.
582 That's the reasons for the C<my> on the loop iteration variable.
584 This is one of the only places where giving a prototype to a closure makes
585 much sense. If you wanted to impose scalar context on the arguments of
586 these functions (probably not a wise idea for this particular example),
587 you could have written it this way instead:
589 *$name = sub ($) { "<FONT COLOR='$name'>$_[0]</FONT>" };
591 However, since prototype checking happens at compile time, the assignment
592 above happens too late to be of much use. You could address this by
593 putting the whole loop of assignments within a BEGIN block, forcing it
594 to occur during compilation.
596 Access to lexicals that change over type--like those in the C<for> loop
597 above--only works with closures, not general subroutines. In the general
598 case, then, named subroutines do not nest properly, although anonymous
599 ones do. If you are accustomed to using nested subroutines in other
600 programming languages with their own private variables, you'll have to
601 work at it a bit in Perl. The intuitive coding of this kind of thing
602 incurs mysterious warnings about ``will not stay shared''. For example,
607 sub inner { return $x * 19 } # WRONG
611 A work-around is the following:
615 local *inner = sub { return $x * 19 };
619 Now inner() can only be called from within outer(), because of the
620 temporary assignments of the closure (anonymous subroutine). But when
621 it does, it has normal access to the lexical variable $x from the scope
624 This has the interesting effect of creating a function local to another
625 function, something not normally supported in Perl.
629 You may not (usefully) use a reference as the key to a hash. It will be
630 converted into a string:
634 If you try to dereference the key, it won't do a hard dereference, and
635 you won't accomplish what you're attempting. You might want to do something
641 And then at least you can use the values(), which will be
642 real refs, instead of the keys(), which won't.
644 The standard Tie::RefHash module provides a convenient workaround to this.
648 Besides the obvious documents, source code can be instructive.
649 Some rather pathological examples of the use of references can be found
650 in the F<t/op/ref.t> regression test in the Perl source directory.
652 See also L<perldsc> and L<perllol> for how to use references to create
653 complex data structures, and L<perltoot>, L<perlobj>, and L<perlbot>
654 for how to use them to create objects.