This is a live mirror of the Perl 5 development currently hosted at https://github.com/perl/perl5
hv.c calling negative malloc()
[perl5.git] / pod / perlref.pod
CommitLineData
a0d0e21e
LW
1=head1 NAME
2
3perlref - Perl references and nested data structures
4
a1e2a320
GS
5=head1 NOTE
6
7This is complete documentation about all aspects of references.
8For a shorter, tutorial introduction to just the essential features,
9see L<perlreftut>.
10
a0d0e21e
LW
11=head1 DESCRIPTION
12
cb1a09d0 13Before release 5 of Perl it was difficult to represent complex data
5a964f20
TC
14structures, because all references had to be symbolic--and even then
15it was difficult to refer to a variable instead of a symbol table entry.
16Perl now not only makes it easier to use symbolic references to variables,
17but also lets you have "hard" references to any piece of data or code.
18Any scalar may hold a hard reference. Because arrays and hashes contain
19scalars, you can now easily build arrays of arrays, arrays of hashes,
20hashes of arrays, arrays of hashes of functions, and so on.
a0d0e21e
LW
21
22Hard references are smart--they keep track of reference counts for you,
2d24ed35 23automatically freeing the thing referred to when its reference count goes
7b8d334a 24to zero. (Note: the reference counts for values in self-referential or
2d24ed35 25cyclic data structures may not go to zero without a little help; see
7b8d334a 26L<perlobj/"Two-Phased Garbage Collection"> for a detailed explanation.)
2d24ed35
CS
27If that thing happens to be an object, the object is destructed. See
28L<perlobj> for more about objects. (In a sense, everything in Perl is an
29object, but we usually reserve the word for references to objects that
30have been officially "blessed" into a class package.)
31
32Symbolic references are names of variables or other objects, just as a
54310121 33symbolic link in a Unix filesystem contains merely the name of a file.
2d24ed35
CS
34The C<*glob> notation is a kind of symbolic reference. (Symbolic
35references are sometimes called "soft references", but please don't call
36them that; references are confusing enough without useless synonyms.)
37
54310121 38In contrast, hard references are more like hard links in a Unix file
2d24ed35
CS
39system: They are used to access an underlying object without concern for
40what its (other) name is. When the word "reference" is used without an
5a964f20 41adjective, as in the following paragraph, it is usually talking about a
2d24ed35
CS
42hard reference.
43
44References are easy to use in Perl. There is just one overriding
45principle: Perl does no implicit referencing or dereferencing. When a
46scalar is holding a reference, it always behaves as a simple scalar. It
47doesn't magically start being an array or hash or subroutine; you have to
48tell it explicitly to do so, by dereferencing it.
a0d0e21e 49
5a964f20
TC
50=head2 Making References
51
52References can be created in several ways.
a0d0e21e
LW
53
54=over 4
55
56=item 1.
57
58By using the backslash operator on a variable, subroutine, or value.
54310121 59(This works much like the & (address-of) operator in C.) Note
19799a22 60that this typically creates I<another> reference to a variable, because
a0d0e21e
LW
61there's already a reference to the variable in the symbol table. But
62the symbol table reference might go away, and you'll still have the
63reference that the backslash returned. Here are some examples:
64
65 $scalarref = \$foo;
66 $arrayref = \@ARGV;
67 $hashref = \%ENV;
68 $coderef = \&handler;
55497cff 69 $globref = \*foo;
cb1a09d0 70
5a964f20
TC
71It isn't possible to create a true reference to an IO handle (filehandle
72or dirhandle) using the backslash operator. The most you can get is a
73reference to a typeglob, which is actually a complete symbol table entry.
74But see the explanation of the C<*foo{THING}> syntax below. However,
75you can still use type globs and globrefs as though they were IO handles.
a0d0e21e
LW
76
77=item 2.
78
5a964f20 79A reference to an anonymous array can be created using square
a0d0e21e
LW
80brackets:
81
82 $arrayref = [1, 2, ['a', 'b', 'c']];
83
5a964f20 84Here we've created a reference to an anonymous array of three elements
54310121 85whose final element is itself a reference to another anonymous array of three
a0d0e21e 86elements. (The multidimensional syntax described later can be used to
184e9718 87access this. For example, after the above, C<$arrayref-E<gt>[2][1]> would have
a0d0e21e
LW
88the value "b".)
89
cb1a09d0
AD
90Note that taking a reference to an enumerated list is not the same
91as using square brackets--instead it's the same as creating
92a list of references!
93
54310121 94 @list = (\$a, \@b, \%c);
58e0a6ae
GS
95 @list = \($a, @b, %c); # same thing!
96
54310121 97As a special case, C<\(@foo)> returns a list of references to the contents
b6429b1b
GS
98of C<@foo>, not a reference to C<@foo> itself. Likewise for C<%foo>,
99except that the key references are to copies (since the keys are just
100strings rather than full-fledged scalars).
cb1a09d0 101
a0d0e21e
LW
102=item 3.
103
5a964f20 104A reference to an anonymous hash can be created using curly
a0d0e21e
LW
105brackets:
106
107 $hashref = {
108 'Adam' => 'Eve',
109 'Clyde' => 'Bonnie',
110 };
111
5a964f20 112Anonymous hash and array composers like these can be intermixed freely to
a0d0e21e
LW
113produce as complicated a structure as you want. The multidimensional
114syntax described below works for these too. The values above are
115literals, but variables and expressions would work just as well, because
116assignment operators in Perl (even within local() or my()) are executable
117statements, not compile-time declarations.
118
119Because curly brackets (braces) are used for several other things
120including BLOCKs, you may occasionally have to disambiguate braces at the
121beginning of a statement by putting a C<+> or a C<return> in front so
122that Perl realizes the opening brace isn't starting a BLOCK. The economy and
123mnemonic value of using curlies is deemed worth this occasional extra
124hassle.
125
126For example, if you wanted a function to make a new hash and return a
127reference to it, you have these options:
128
129 sub hashem { { @_ } } # silently wrong
130 sub hashem { +{ @_ } } # ok
131 sub hashem { return { @_ } } # ok
132
ebc58f1a
GS
133On the other hand, if you want the other meaning, you can do this:
134
135 sub showem { { @_ } } # ambiguous (currently ok, but may change)
136 sub showem { {; @_ } } # ok
137 sub showem { { return @_ } } # ok
138
139Note how the leading C<+{> and C<{;> always serve to disambiguate
140the expression to mean either the HASH reference, or the BLOCK.
141
a0d0e21e
LW
142=item 4.
143
5a964f20 144A reference to an anonymous subroutine can be created by using
a0d0e21e
LW
145C<sub> without a subname:
146
147 $coderef = sub { print "Boink!\n" };
148
149Note the presence of the semicolon. Except for the fact that the code
150inside isn't executed immediately, a C<sub {}> is not so much a
151declaration as it is an operator, like C<do{}> or C<eval{}>. (However, no
5a964f20 152matter how many times you execute that particular line (unless you're in an
19799a22 153C<eval("...")>), $coderef will still have a reference to the I<same>
a0d0e21e
LW
154anonymous subroutine.)
155
748a9306
LW
156Anonymous subroutines act as closures with respect to my() variables,
157that is, variables visible lexically within the current scope. Closure
158is a notion out of the Lisp world that says if you define an anonymous
159function in a particular lexical context, it pretends to run in that
160context even when it's called outside of the context.
161
162In human terms, it's a funny way of passing arguments to a subroutine when
163you define it as well as when you call it. It's useful for setting up
164little bits of code to run later, such as callbacks. You can even
54310121
PP
165do object-oriented stuff with it, though Perl already provides a different
166mechanism to do that--see L<perlobj>.
748a9306
LW
167
168You can also think of closure as a way to write a subroutine template without
169using eval. (In fact, in version 5.000, eval was the I<only> way to get
170closures. You may wish to use "require 5.001" if you use closures.)
171
172Here's a small example of how closures works:
173
174 sub newprint {
175 my $x = shift;
176 return sub { my $y = shift; print "$x, $y!\n"; };
a0d0e21e 177 }
748a9306
LW
178 $h = newprint("Howdy");
179 $g = newprint("Greetings");
180
181 # Time passes...
182
183 &$h("world");
184 &$g("earthlings");
a0d0e21e 185
748a9306
LW
186This prints
187
188 Howdy, world!
189 Greetings, earthlings!
190
191Note particularly that $x continues to refer to the value passed into
cb1a09d0 192newprint() I<despite> the fact that the "my $x" has seemingly gone out of
748a9306
LW
193scope by the time the anonymous subroutine runs. That's what closure
194is all about.
195
5a964f20 196This applies only to lexical variables, by the way. Dynamic variables
748a9306
LW
197continue to work as they have always worked. Closure is not something
198that most Perl programmers need trouble themselves about to begin with.
a0d0e21e
LW
199
200=item 5.
201
202References are often returned by special subroutines called constructors.
748a9306 203Perl objects are just references to a special kind of object that happens to know
a0d0e21e
LW
204which package it's associated with. Constructors are just special
205subroutines that know how to create that association. They do so by
206starting with an ordinary reference, and it remains an ordinary reference
5a964f20
TC
207even while it's also being an object. Constructors are often
208named new() and called indirectly:
a0d0e21e
LW
209
210 $objref = new Doggie (Tail => 'short', Ears => 'long');
211
5a964f20
TC
212But don't have to be:
213
214 $objref = Doggie->new(Tail => 'short', Ears => 'long');
215
216 use Term::Cap;
217 $terminal = Term::Cap->Tgetent( { OSPEED => 9600 });
218
219 use Tk;
220 $main = MainWindow->new();
221 $menubar = $main->Frame(-relief => "raised",
222 -borderwidth => 2)
223
a0d0e21e
LW
224=item 6.
225
226References of the appropriate type can spring into existence if you
5f05dabc 227dereference them in a context that assumes they exist. Because we haven't
a0d0e21e
LW
228talked about dereferencing yet, we can't show you any examples yet.
229
cb1a09d0
AD
230=item 7.
231
55497cff
PP
232A reference can be created by using a special syntax, lovingly known as
233the *foo{THING} syntax. *foo{THING} returns a reference to the THING
234slot in *foo (which is the symbol table entry which holds everything
235known as foo).
cb1a09d0 236
55497cff
PP
237 $scalarref = *foo{SCALAR};
238 $arrayref = *ARGV{ARRAY};
239 $hashref = *ENV{HASH};
240 $coderef = *handler{CODE};
36477c24 241 $ioref = *STDIN{IO};
55497cff
PP
242 $globref = *foo{GLOB};
243
36477c24
PP
244All of these are self-explanatory except for *foo{IO}. It returns the
245IO 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
248Perl, *foo{FILEHANDLE} is a synonym for *foo{IO}.
55497cff 249
5f05dabc
PP
250*foo{THING} returns undef if that particular THING hasn't been used yet,
251except in the case of scalars. *foo{SCALAR} returns a reference to an
252anonymous scalar if $foo hasn't been used yet. This might change in a
253future release.
254
5a964f20
TC
255*foo{IO} is an alternative to the \*HANDLE mechanism given in
256L<perldata/"Typeglobs and Filehandles"> for passing filehandles
257into or out of subroutines, or storing into larger data structures.
258Its disadvantage is that it won't create a new filehandle for you.
259Its advantage is that you have no risk of clobbering more than you want
260to with a typeglob assignment, although if you assign to a scalar instead
261of a typeglob, you're ok.
36477c24 262
5a964f20 263 splutter(*STDOUT);
36477c24 264 splutter(*STDOUT{IO});
5a964f20 265
cb1a09d0
AD
266 sub splutter {
267 my $fh = shift;
268 print $fh "her um well a hmmm\n";
269 }
270
5a964f20 271 $rec = get_rec(*STDIN);
36477c24 272 $rec = get_rec(*STDIN{IO});
5a964f20 273
cb1a09d0
AD
274 sub get_rec {
275 my $fh = shift;
276 return scalar <$fh>;
277 }
278
a0d0e21e
LW
279=back
280
5a964f20
TC
281=head2 Using References
282
a0d0e21e
LW
283That's it for creating references. By now you're probably dying to
284know how to use references to get back to your long-lost data. There
285are several basic methods.
286
287=over 4
288
289=item 1.
290
6309d9d9
PP
291Anywhere you'd put an identifier (or chain of identifiers) as part
292of a variable or subroutine name, you can replace the identifier with
293a simple scalar variable containing a reference of the correct type:
a0d0e21e
LW
294
295 $bar = $$scalarref;
296 push(@$arrayref, $filename);
297 $$arrayref[0] = "January";
298 $$hashref{"KEY"} = "VALUE";
299 &$coderef(1,2,3);
cb1a09d0 300 print $globref "output\n";
a0d0e21e 301
19799a22 302It's important to understand that we are specifically I<not> dereferencing
a0d0e21e 303C<$arrayref[0]> or C<$hashref{"KEY"}> there. The dereference of the
19799a22 304scalar variable happens I<before> it does any key lookups. Anything more
a0d0e21e
LW
305complicated than a simple scalar variable must use methods 2 or 3 below.
306However, a "simple scalar" includes an identifier that itself uses method
3071 recursively. Therefore, the following prints "howdy".
308
309 $refrefref = \\\"howdy";
310 print $$$$refrefref;
311
312=item 2.
313
6309d9d9
PP
314Anywhere you'd put an identifier (or chain of identifiers) as part of a
315variable or subroutine name, you can replace the identifier with a
316BLOCK returning a reference of the correct type. In other words, the
317previous examples could be written like this:
a0d0e21e
LW
318
319 $bar = ${$scalarref};
320 push(@{$arrayref}, $filename);
321 ${$arrayref}[0] = "January";
322 ${$hashref}{"KEY"} = "VALUE";
323 &{$coderef}(1,2,3);
36477c24 324 $globref->print("output\n"); # iff IO::Handle is loaded
a0d0e21e
LW
325
326Admittedly, it's a little silly to use the curlies in this case, but
327the BLOCK can contain any arbitrary expression, in particular,
328subscripted expressions:
329
54310121 330 &{ $dispatch{$index} }(1,2,3); # call correct routine
a0d0e21e
LW
331
332Because of being able to omit the curlies for the simple case of C<$$x>,
333people often make the mistake of viewing the dereferencing symbols as
334proper operators, and wonder about their precedence. If they were,
5f05dabc 335though, you could use parentheses instead of braces. That's not the case.
a0d0e21e 336Consider the difference below; case 0 is a short-hand version of case 1,
19799a22 337I<not> case 2:
a0d0e21e
LW
338
339 $$hashref{"KEY"} = "VALUE"; # CASE 0
340 ${$hashref}{"KEY"} = "VALUE"; # CASE 1
341 ${$hashref{"KEY"}} = "VALUE"; # CASE 2
342 ${$hashref->{"KEY"}} = "VALUE"; # CASE 3
343
344Case 2 is also deceptive in that you're accessing a variable
345called %hashref, not dereferencing through $hashref to the hash
346it's presumably referencing. That would be case 3.
347
348=item 3.
349
6da72b64
CS
350Subroutine calls and lookups of individual array elements arise often
351enough that it gets cumbersome to use method 2. As a form of
352syntactic sugar, the examples for method 2 may be written:
a0d0e21e 353
6da72b64
CS
354 $arrayref->[0] = "January"; # Array element
355 $hashref->{"KEY"} = "VALUE"; # Hash element
356 $coderef->(1,2,3); # Subroutine call
a0d0e21e 357
6da72b64 358The left side of the arrow can be any expression returning a reference,
19799a22 359including a previous dereference. Note that C<$array[$x]> is I<not> the
a0d0e21e
LW
360same thing as C<$array-E<gt>[$x]> here:
361
362 $array[$x]->{"foo"}->[0] = "January";
363
364This is one of the cases we mentioned earlier in which references could
365spring into existence when in an lvalue context. Before this
366statement, C<$array[$x]> may have been undefined. If so, it's
367automatically defined with a hash reference so that we can look up
368C<{"foo"}> in it. Likewise C<$array[$x]-E<gt>{"foo"}> will automatically get
369defined with an array reference so that we can look up C<[0]> in it.
5a964f20 370This process is called I<autovivification>.
a0d0e21e 371
19799a22 372One more thing here. The arrow is optional I<between> brackets
a0d0e21e
LW
373subscripts, so you can shrink the above down to
374
375 $array[$x]{"foo"}[0] = "January";
376
377Which, in the degenerate case of using only ordinary arrays, gives you
378multidimensional arrays just like C's:
379
380 $score[$x][$y][$z] += 42;
381
382Well, okay, not entirely like C's arrays, actually. C doesn't know how
383to grow its arrays on demand. Perl does.
384
385=item 4.
386
387If a reference happens to be a reference to an object, then there are
388probably methods to access the things referred to, and you should probably
389stick to those methods unless you're in the class package that defines the
390object's methods. In other words, be nice, and don't violate the object's
391encapsulation without a very good reason. Perl does not enforce
392encapsulation. We are not totalitarians here. We do expect some basic
393civility though.
394
395=back
396
397The ref() operator may be used to determine what type of thing the
398reference is pointing to. See L<perlfunc>.
399
5a964f20
TC
400The bless() operator may be used to associate the object a reference
401points to with a package functioning as an object class. See L<perlobj>.
a0d0e21e 402
5f05dabc 403A typeglob may be dereferenced the same way a reference can, because
a0d0e21e
LW
404the dereference syntax always indicates the kind of reference desired.
405So C<${*foo}> and C<${\$foo}> both indicate the same scalar variable.
406
407Here's a trick for interpolating a subroutine call into a string:
408
cb1a09d0
AD
409 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
410
411The way it works is that when the C<@{...}> is seen in the double-quoted
412string, it's evaluated as a block. The block creates a reference to an
413anonymous array containing the results of the call to C<mysub(1,2,3)>. So
414the whole block returns a reference to an array, which is then
415dereferenced by C<@{...}> and stuck into the double-quoted string. This
416chicanery is also useful for arbitrary expressions:
a0d0e21e 417
184e9718 418 print "That yields @{[$n + 5]} widgets\n";
a0d0e21e
LW
419
420=head2 Symbolic references
421
422We said that references spring into existence as necessary if they are
423undefined, but we didn't say what happens if a value used as a
19799a22 424reference is already defined, but I<isn't> a hard reference. If you
a0d0e21e 425use it as a reference in this case, it'll be treated as a symbolic
19799a22 426reference. That is, the value of the scalar is taken to be the I<name>
a0d0e21e
LW
427of a variable, rather than a direct link to a (possibly) anonymous
428value.
429
430People frequently expect it to work like this. So it does.
431
432 $name = "foo";
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)
439 $pack = "THAT";
440 ${"${pack}::$name"} = 5; # Sets $THAT::foo without eval
441
442This is very powerful, and slightly dangerous, in that it's possible
443to intend (with the utmost sincerity) to use a hard reference, and
444accidentally use a symbolic reference instead. To protect against
445that, you can say
446
447 use strict 'refs';
448
449and then only hard references will be allowed for the rest of the enclosing
54310121 450block. An inner block may countermand that with
a0d0e21e
LW
451
452 no strict 'refs';
453
5a964f20
TC
454Only package variables (globals, even if localized) are visible to
455symbolic references. Lexical variables (declared with my()) aren't in
456a symbol table, and thus are invisible to this mechanism. For example:
a0d0e21e 457
5a964f20 458 local $value = 10;
b0c35547 459 $ref = "value";
a0d0e21e
LW
460 {
461 my $value = 20;
462 print $$ref;
54310121 463 }
a0d0e21e
LW
464
465This will still print 10, not 20. Remember that local() affects package
466variables, which are all "global" to the package.
467
748a9306
LW
468=head2 Not-so-symbolic references
469
a6006777
PP
470A new feature contributing to readability in perl version 5.001 is that the
471brackets around a symbolic reference behave more like quotes, just as they
748a9306
LW
472always have within a string. That is,
473
474 $push = "pop on ";
475 print "${push}over";
476
477has always meant to print "pop on over", despite the fact that push is
478a reserved word. This has been generalized to work the same outside
479of quotes, so that
480
481 print ${push} . "over";
482
483and even
484
485 print ${ push } . "over";
486
487will have the same effect. (This would have been a syntax error in
a6006777 488Perl 5.000, though Perl 4 allowed it in the spaceless form.) Note that this
748a9306
LW
489construct is I<not> considered to be a symbolic reference when you're
490using strict refs:
491
492 use strict 'refs';
493 ${ bareword }; # Okay, means $bareword.
494 ${ "bareword" }; # Error, symbolic reference.
495
496Similarly, because of all the subscripting that is done using single
497words, we've applied the same rule to any bareword that is used for
498subscripting a hash. So now, instead of writing
499
500 $array{ "aaa" }{ "bbb" }{ "ccc" }
501
5f05dabc 502you can write just
748a9306
LW
503
504 $array{ aaa }{ bbb }{ ccc }
505
506and not worry about whether the subscripts are reserved words. In the
507rare event that you do wish to do something like
508
509 $array{ shift }
510
511you can force interpretation as a reserved word by adding anything that
512makes it more than a bareword:
513
514 $array{ shift() }
515 $array{ +shift }
516 $array{ shift @_ }
517
518The B<-w> switch will warn you if it interprets a reserved word as a string.
5f05dabc 519But it will no longer warn you about using lowercase words, because the
748a9306
LW
520string is effectively quoted.
521
49399b3f
GA
522=head2 Pseudo-hashes: Using an array as a hash
523
524WARNING: This section describes an experimental feature. Details may
525change without notice in future versions.
526
527Beginning with release 5.005 of Perl you can use an array reference
528in some contexts that would normally require a hash reference. This
529allows you to access array elements using symbolic names, as if they
530were fields in a structure.
531
532For this to work, the array must contain extra information. The first
533element of the array has to be a hash reference that maps field names
534to array indices. Here is an example:
535
536 $struct = [{foo => 1, bar => 2}, "FOO", "BAR"];
537
538 $struct->{foo}; # same as $struct->[1], i.e. "FOO"
539 $struct->{bar}; # same as $struct->[2], i.e. "BAR"
540
541 keys %$struct; # will return ("foo", "bar") in some order
542 values %$struct; # will return ("FOO", "BAR") in same some order
543
544 while (my($k,$v) = each %$struct) {
545 print "$k => $v\n";
546 }
547
548Perl will raise an exception if you try to delete keys from a pseudo-hash
549or try to access nonexistent fields. For better performance, Perl can also
550do the translation from field names to array indices at compile time for
551typed object references. See L<fields>.
552
553
5a964f20
TC
554=head2 Function Templates
555
556As explained above, a closure is an anonymous function with access to the
557lexical variables visible when that function was compiled. It retains
558access to those variables even though it doesn't get run until later,
559such as in a signal handler or a Tk callback.
560
561Using a closure as a function template allows us to generate many functions
c2611fb3 562that act similarly. Suppose you wanted functions named after the colors
5a964f20
TC
563that generated HTML font changes for the various colors:
564
565 print "Be ", red("careful"), "with that ", green("light");
566
567The red() and green() functions would be very similar. To create these,
568we'll assign a closure to a typeglob of the name of the function we're
569trying to build.
570
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>" };
575 }
576
577Now all those different functions appear to exist independently. You can
578call red(), RED(), blue(), BLUE(), green(), etc. This technique saves on
579both compile time and memory use, and is less error-prone as well, since
580syntax checks happen at compile time. It's critical that any variables in
581the anonymous subroutine be lexicals in order to create a proper closure.
582That's the reasons for the C<my> on the loop iteration variable.
583
584This is one of the only places where giving a prototype to a closure makes
585much sense. If you wanted to impose scalar context on the arguments of
586these functions (probably not a wise idea for this particular example),
587you could have written it this way instead:
588
589 *$name = sub ($) { "<FONT COLOR='$name'>$_[0]</FONT>" };
590
591However, since prototype checking happens at compile time, the assignment
592above happens too late to be of much use. You could address this by
593putting the whole loop of assignments within a BEGIN block, forcing it
594to occur during compilation.
595
596Access to lexicals that change over type--like those in the C<for> loop
597above--only works with closures, not general subroutines. In the general
598case, then, named subroutines do not nest properly, although anonymous
599ones do. If you are accustomed to using nested subroutines in other
600programming languages with their own private variables, you'll have to
601work at it a bit in Perl. The intuitive coding of this kind of thing
602incurs mysterious warnings about ``will not stay shared''. For example,
603this won't work:
604
605 sub outer {
606 my $x = $_[0] + 35;
607 sub inner { return $x * 19 } # WRONG
608 return $x + inner();
609 }
610
611A work-around is the following:
612
613 sub outer {
614 my $x = $_[0] + 35;
615 local *inner = sub { return $x * 19 };
616 return $x + inner();
617 }
618
619Now inner() can only be called from within outer(), because of the
620temporary assignments of the closure (anonymous subroutine). But when
621it does, it has normal access to the lexical variable $x from the scope
622of outer().
623
624This has the interesting effect of creating a function local to another
625function, something not normally supported in Perl.
626
cb1a09d0 627=head1 WARNING
748a9306
LW
628
629You may not (usefully) use a reference as the key to a hash. It will be
630converted into a string:
631
632 $x{ \$a } = $a;
633
54310121 634If you try to dereference the key, it won't do a hard dereference, and
184e9718 635you won't accomplish what you're attempting. You might want to do something
cb1a09d0 636more like
748a9306 637
cb1a09d0
AD
638 $r = \@a;
639 $x{ $r } = $r;
640
641And then at least you can use the values(), which will be
642real refs, instead of the keys(), which won't.
643
5a964f20
TC
644The standard Tie::RefHash module provides a convenient workaround to this.
645
cb1a09d0 646=head1 SEE ALSO
a0d0e21e
LW
647
648Besides the obvious documents, source code can be instructive.
649Some rather pathological examples of the use of references can be found
650in the F<t/op/ref.t> regression test in the Perl source directory.
cb1a09d0
AD
651
652See also L<perldsc> and L<perllol> for how to use references to create
5a964f20
TC
653complex data structures, and L<perltoot>, L<perlobj>, and L<perlbot>
654for how to use them to create objects.