This is a live mirror of the Perl 5 development currently hosted at https://github.com/perl/perl5
Accidental interpolation of $@ in Pod::Html [PATCH]
[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
7c2ea1c7 24to zero. (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.
d1be9408 34The C<*glob> notation is something of a symbolic reference. (Symbolic
2d24ed35
CS
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.
7c2ea1c7
GS
59(This works much like the & (address-of) operator in C.)
60This 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
c47ff5f1 87access this. For example, after the above, C<< $arrayref->[2][1] >> would have
a0d0e21e
LW
88the value "b".)
89
7c2ea1c7 90Taking a reference to an enumerated list is not the same
cb1a09d0
AD
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
7c2ea1c7 139The leading C<+{> and C<{;> always serve to disambiguate
ebc58f1a
GS
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
7c2ea1c7
GS
149Note the semicolon. Except for the code
150inside not being immediately executed, a C<sub {}> is not so much a
a0d0e21e 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 156Anonymous subroutines act as closures with respect to my() variables,
7c2ea1c7 157that is, variables lexically visible within the current scope. Closure
748a9306
LW
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
7c2ea1c7 160context even when it's called outside the context.
748a9306
LW
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 167
7c2ea1c7
GS
168You might also think of closure as a way to write a subroutine
169template without using eval(). Here's a small example of how
170closures work:
748a9306
LW
171
172 sub newprint {
173 my $x = shift;
174 return sub { my $y = shift; print "$x, $y!\n"; };
a0d0e21e 175 }
748a9306
LW
176 $h = newprint("Howdy");
177 $g = newprint("Greetings");
178
179 # Time passes...
180
181 &$h("world");
182 &$g("earthlings");
a0d0e21e 183
748a9306
LW
184This prints
185
186 Howdy, world!
187 Greetings, earthlings!
188
7c2ea1c7
GS
189Note particularly that $x continues to refer to the value passed
190into newprint() I<despite> "my $x" having gone out of scope by the
191time the anonymous subroutine runs. That's what a closure is all
192about.
748a9306 193
5a964f20 194This applies only to lexical variables, by the way. Dynamic variables
748a9306
LW
195continue to work as they have always worked. Closure is not something
196that most Perl programmers need trouble themselves about to begin with.
a0d0e21e
LW
197
198=item 5.
199
200References are often returned by special subroutines called constructors.
7c2ea1c7 201Perl objects are just references to a special type of object that happens to know
a0d0e21e
LW
202which package it's associated with. Constructors are just special
203subroutines that know how to create that association. They do so by
204starting with an ordinary reference, and it remains an ordinary reference
5a964f20
TC
205even while it's also being an object. Constructors are often
206named new() and called indirectly:
a0d0e21e
LW
207
208 $objref = new Doggie (Tail => 'short', Ears => 'long');
209
5a964f20
TC
210But don't have to be:
211
212 $objref = Doggie->new(Tail => 'short', Ears => 'long');
213
214 use Term::Cap;
215 $terminal = Term::Cap->Tgetent( { OSPEED => 9600 });
216
217 use Tk;
218 $main = MainWindow->new();
219 $menubar = $main->Frame(-relief => "raised",
220 -borderwidth => 2)
221
a0d0e21e
LW
222=item 6.
223
224References of the appropriate type can spring into existence if you
5f05dabc 225dereference them in a context that assumes they exist. Because we haven't
a0d0e21e
LW
226talked about dereferencing yet, we can't show you any examples yet.
227
cb1a09d0
AD
228=item 7.
229
55497cff
PP
230A reference can be created by using a special syntax, lovingly known as
231the *foo{THING} syntax. *foo{THING} returns a reference to the THING
232slot in *foo (which is the symbol table entry which holds everything
233known as foo).
cb1a09d0 234
55497cff
PP
235 $scalarref = *foo{SCALAR};
236 $arrayref = *ARGV{ARRAY};
237 $hashref = *ENV{HASH};
238 $coderef = *handler{CODE};
36477c24 239 $ioref = *STDIN{IO};
55497cff
PP
240 $globref = *foo{GLOB};
241
7c2ea1c7
GS
242All of these are self-explanatory except for C<*foo{IO}>. It returns
243the IO handle, used for file handles (L<perlfunc/open>), sockets
244(L<perlfunc/socket> and L<perlfunc/socketpair>), and directory
245handles (L<perlfunc/opendir>). For compatibility with previous
39b99f21 246versions of Perl, C<*foo{FILEHANDLE}> is a synonym for C<*foo{IO}>, though it
247is deprecated as of 5.8.0. If deprecation warnings are in effect, it will warn
248of its use.
55497cff 249
7c2ea1c7
GS
250C<*foo{THING}> returns undef if that particular THING hasn't been used yet,
251except in the case of scalars. C<*foo{SCALAR}> returns a reference to an
5f05dabc
PP
252anonymous scalar if $foo hasn't been used yet. This might change in a
253future release.
254
7c2ea1c7 255C<*foo{IO}> is an alternative to the C<*HANDLE> mechanism given in
5a964f20
TC
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.
7c2ea1c7
GS
259Its advantage is that you have less risk of clobbering more than
260you want to with a typeglob assignment. (It still conflates file
261and directory handles, though.) However, if you assign the incoming
262value to a scalar instead of a typeglob as we do in the examples
263below, there's no risk of that happening.
36477c24 264
7c2ea1c7
GS
265 splutter(*STDOUT); # pass the whole glob
266 splutter(*STDOUT{IO}); # pass both file and dir handles
5a964f20 267
cb1a09d0
AD
268 sub splutter {
269 my $fh = shift;
270 print $fh "her um well a hmmm\n";
271 }
272
7c2ea1c7
GS
273 $rec = get_rec(*STDIN); # pass the whole glob
274 $rec = get_rec(*STDIN{IO}); # pass both file and dir handles
5a964f20 275
cb1a09d0
AD
276 sub get_rec {
277 my $fh = shift;
278 return scalar <$fh>;
279 }
280
a0d0e21e
LW
281=back
282
5a964f20
TC
283=head2 Using References
284
a0d0e21e
LW
285That's it for creating references. By now you're probably dying to
286know how to use references to get back to your long-lost data. There
287are several basic methods.
288
289=over 4
290
291=item 1.
292
6309d9d9
PP
293Anywhere you'd put an identifier (or chain of identifiers) as part
294of a variable or subroutine name, you can replace the identifier with
295a simple scalar variable containing a reference of the correct type:
a0d0e21e
LW
296
297 $bar = $$scalarref;
298 push(@$arrayref, $filename);
299 $$arrayref[0] = "January";
300 $$hashref{"KEY"} = "VALUE";
301 &$coderef(1,2,3);
cb1a09d0 302 print $globref "output\n";
a0d0e21e 303
19799a22 304It's important to understand that we are specifically I<not> dereferencing
a0d0e21e 305C<$arrayref[0]> or C<$hashref{"KEY"}> there. The dereference of the
19799a22 306scalar variable happens I<before> it does any key lookups. Anything more
a0d0e21e
LW
307complicated than a simple scalar variable must use methods 2 or 3 below.
308However, a "simple scalar" includes an identifier that itself uses method
3091 recursively. Therefore, the following prints "howdy".
310
311 $refrefref = \\\"howdy";
312 print $$$$refrefref;
313
314=item 2.
315
6309d9d9
PP
316Anywhere you'd put an identifier (or chain of identifiers) as part of a
317variable or subroutine name, you can replace the identifier with a
318BLOCK returning a reference of the correct type. In other words, the
319previous examples could be written like this:
a0d0e21e
LW
320
321 $bar = ${$scalarref};
322 push(@{$arrayref}, $filename);
323 ${$arrayref}[0] = "January";
324 ${$hashref}{"KEY"} = "VALUE";
325 &{$coderef}(1,2,3);
36477c24 326 $globref->print("output\n"); # iff IO::Handle is loaded
a0d0e21e
LW
327
328Admittedly, it's a little silly to use the curlies in this case, but
329the BLOCK can contain any arbitrary expression, in particular,
330subscripted expressions:
331
54310121 332 &{ $dispatch{$index} }(1,2,3); # call correct routine
a0d0e21e
LW
333
334Because of being able to omit the curlies for the simple case of C<$$x>,
335people often make the mistake of viewing the dereferencing symbols as
336proper operators, and wonder about their precedence. If they were,
5f05dabc 337though, you could use parentheses instead of braces. That's not the case.
a0d0e21e 338Consider the difference below; case 0 is a short-hand version of case 1,
19799a22 339I<not> case 2:
a0d0e21e
LW
340
341 $$hashref{"KEY"} = "VALUE"; # CASE 0
342 ${$hashref}{"KEY"} = "VALUE"; # CASE 1
343 ${$hashref{"KEY"}} = "VALUE"; # CASE 2
344 ${$hashref->{"KEY"}} = "VALUE"; # CASE 3
345
346Case 2 is also deceptive in that you're accessing a variable
347called %hashref, not dereferencing through $hashref to the hash
348it's presumably referencing. That would be case 3.
349
350=item 3.
351
6da72b64
CS
352Subroutine calls and lookups of individual array elements arise often
353enough that it gets cumbersome to use method 2. As a form of
354syntactic sugar, the examples for method 2 may be written:
a0d0e21e 355
6da72b64
CS
356 $arrayref->[0] = "January"; # Array element
357 $hashref->{"KEY"} = "VALUE"; # Hash element
358 $coderef->(1,2,3); # Subroutine call
a0d0e21e 359
6da72b64 360The left side of the arrow can be any expression returning a reference,
19799a22 361including a previous dereference. Note that C<$array[$x]> is I<not> the
c47ff5f1 362same thing as C<< $array->[$x] >> here:
a0d0e21e
LW
363
364 $array[$x]->{"foo"}->[0] = "January";
365
366This is one of the cases we mentioned earlier in which references could
367spring into existence when in an lvalue context. Before this
368statement, C<$array[$x]> may have been undefined. If so, it's
369automatically defined with a hash reference so that we can look up
c47ff5f1 370C<{"foo"}> in it. Likewise C<< $array[$x]->{"foo"} >> will automatically get
a0d0e21e 371defined with an array reference so that we can look up C<[0]> in it.
5a964f20 372This process is called I<autovivification>.
a0d0e21e 373
19799a22 374One more thing here. The arrow is optional I<between> brackets
a0d0e21e
LW
375subscripts, so you can shrink the above down to
376
377 $array[$x]{"foo"}[0] = "January";
378
379Which, in the degenerate case of using only ordinary arrays, gives you
380multidimensional arrays just like C's:
381
382 $score[$x][$y][$z] += 42;
383
384Well, okay, not entirely like C's arrays, actually. C doesn't know how
385to grow its arrays on demand. Perl does.
386
387=item 4.
388
389If a reference happens to be a reference to an object, then there are
390probably methods to access the things referred to, and you should probably
391stick to those methods unless you're in the class package that defines the
392object's methods. In other words, be nice, and don't violate the object's
393encapsulation without a very good reason. Perl does not enforce
394encapsulation. We are not totalitarians here. We do expect some basic
395civility though.
396
397=back
398
7c2ea1c7
GS
399Using a string or number as a reference produces a symbolic reference,
400as explained above. Using a reference as a number produces an
401integer representing its storage location in memory. The only
402useful thing to be done with this is to compare two references
403numerically to see whether they refer to the same location.
404
405 if ($ref1 == $ref2) { # cheap numeric compare of references
406 print "refs 1 and 2 refer to the same thing\n";
407 }
408
409Using a reference as a string produces both its referent's type,
410including any package blessing as described in L<perlobj>, as well
411as the numeric address expressed in hex. The ref() operator returns
412just the type of thing the reference is pointing to, without the
413address. See L<perlfunc/ref> for details and examples of its use.
a0d0e21e 414
5a964f20
TC
415The bless() operator may be used to associate the object a reference
416points to with a package functioning as an object class. See L<perlobj>.
a0d0e21e 417
5f05dabc 418A typeglob may be dereferenced the same way a reference can, because
7c2ea1c7 419the dereference syntax always indicates the type of reference desired.
a0d0e21e
LW
420So C<${*foo}> and C<${\$foo}> both indicate the same scalar variable.
421
422Here's a trick for interpolating a subroutine call into a string:
423
cb1a09d0
AD
424 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
425
426The way it works is that when the C<@{...}> is seen in the double-quoted
427string, it's evaluated as a block. The block creates a reference to an
428anonymous array containing the results of the call to C<mysub(1,2,3)>. So
429the whole block returns a reference to an array, which is then
430dereferenced by C<@{...}> and stuck into the double-quoted string. This
431chicanery is also useful for arbitrary expressions:
a0d0e21e 432
184e9718 433 print "That yields @{[$n + 5]} widgets\n";
a0d0e21e
LW
434
435=head2 Symbolic references
436
437We said that references spring into existence as necessary if they are
438undefined, but we didn't say what happens if a value used as a
19799a22 439reference is already defined, but I<isn't> a hard reference. If you
7c2ea1c7 440use it as a reference, it'll be treated as a symbolic
19799a22 441reference. That is, the value of the scalar is taken to be the I<name>
a0d0e21e
LW
442of a variable, rather than a direct link to a (possibly) anonymous
443value.
444
445People frequently expect it to work like this. So it does.
446
447 $name = "foo";
448 $$name = 1; # Sets $foo
449 ${$name} = 2; # Sets $foo
450 ${$name x 2} = 3; # Sets $foofoo
451 $name->[0] = 4; # Sets $foo[0]
452 @$name = (); # Clears @foo
453 &$name(); # Calls &foo() (as in Perl 4)
454 $pack = "THAT";
455 ${"${pack}::$name"} = 5; # Sets $THAT::foo without eval
456
7c2ea1c7 457This is powerful, and slightly dangerous, in that it's possible
a0d0e21e
LW
458to intend (with the utmost sincerity) to use a hard reference, and
459accidentally use a symbolic reference instead. To protect against
460that, you can say
461
462 use strict 'refs';
463
464and then only hard references will be allowed for the rest of the enclosing
54310121 465block. An inner block may countermand that with
a0d0e21e
LW
466
467 no strict 'refs';
468
5a964f20
TC
469Only package variables (globals, even if localized) are visible to
470symbolic references. Lexical variables (declared with my()) aren't in
471a symbol table, and thus are invisible to this mechanism. For example:
a0d0e21e 472
5a964f20 473 local $value = 10;
b0c35547 474 $ref = "value";
a0d0e21e
LW
475 {
476 my $value = 20;
477 print $$ref;
54310121 478 }
a0d0e21e
LW
479
480This will still print 10, not 20. Remember that local() affects package
481variables, which are all "global" to the package.
482
748a9306
LW
483=head2 Not-so-symbolic references
484
a6006777
PP
485A new feature contributing to readability in perl version 5.001 is that the
486brackets around a symbolic reference behave more like quotes, just as they
748a9306
LW
487always have within a string. That is,
488
489 $push = "pop on ";
490 print "${push}over";
491
7c2ea1c7 492has always meant to print "pop on over", even though push is
748a9306
LW
493a reserved word. This has been generalized to work the same outside
494of quotes, so that
495
496 print ${push} . "over";
497
498and even
499
500 print ${ push } . "over";
501
502will have the same effect. (This would have been a syntax error in
7c2ea1c7 503Perl 5.000, though Perl 4 allowed it in the spaceless form.) This
748a9306
LW
504construct is I<not> considered to be a symbolic reference when you're
505using strict refs:
506
507 use strict 'refs';
508 ${ bareword }; # Okay, means $bareword.
509 ${ "bareword" }; # Error, symbolic reference.
510
511Similarly, because of all the subscripting that is done using single
512words, we've applied the same rule to any bareword that is used for
513subscripting a hash. So now, instead of writing
514
515 $array{ "aaa" }{ "bbb" }{ "ccc" }
516
5f05dabc 517you can write just
748a9306
LW
518
519 $array{ aaa }{ bbb }{ ccc }
520
521and not worry about whether the subscripts are reserved words. In the
522rare event that you do wish to do something like
523
524 $array{ shift }
525
526you can force interpretation as a reserved word by adding anything that
527makes it more than a bareword:
528
529 $array{ shift() }
530 $array{ +shift }
531 $array{ shift @_ }
532
9f1b1f2d
GS
533The C<use warnings> pragma or the B<-w> switch will warn you if it
534interprets a reserved word as a string.
5f05dabc 535But it will no longer warn you about using lowercase words, because the
748a9306
LW
536string is effectively quoted.
537
49399b3f
GA
538=head2 Pseudo-hashes: Using an array as a hash
539
6d822dc4
MS
540Pseudo-hashes have been removed from Perl. The 'fields' pragma
541remains available.
e0478e5a 542
5a964f20
TC
543=head2 Function Templates
544
b5c19bd7
DM
545As explained above, an anonymous function with access to the lexical
546variables visible when that function was compiled, creates a closure. It
547retains access to those variables even though it doesn't get run until
548later, such as in a signal handler or a Tk callback.
5a964f20
TC
549
550Using a closure as a function template allows us to generate many functions
c2611fb3 551that act similarly. Suppose you wanted functions named after the colors
5a964f20
TC
552that generated HTML font changes for the various colors:
553
554 print "Be ", red("careful"), "with that ", green("light");
555
7c2ea1c7 556The red() and green() functions would be similar. To create these,
5a964f20
TC
557we'll assign a closure to a typeglob of the name of the function we're
558trying to build.
559
560 @colors = qw(red blue green yellow orange purple violet);
561 for my $name (@colors) {
562 no strict 'refs'; # allow symbol table manipulation
563 *$name = *{uc $name} = sub { "<FONT COLOR='$name'>@_</FONT>" };
564 }
565
566Now all those different functions appear to exist independently. You can
567call red(), RED(), blue(), BLUE(), green(), etc. This technique saves on
568both compile time and memory use, and is less error-prone as well, since
569syntax checks happen at compile time. It's critical that any variables in
570the anonymous subroutine be lexicals in order to create a proper closure.
571That's the reasons for the C<my> on the loop iteration variable.
572
573This is one of the only places where giving a prototype to a closure makes
574much sense. If you wanted to impose scalar context on the arguments of
575these functions (probably not a wise idea for this particular example),
576you could have written it this way instead:
577
578 *$name = sub ($) { "<FONT COLOR='$name'>$_[0]</FONT>" };
579
580However, since prototype checking happens at compile time, the assignment
581above happens too late to be of much use. You could address this by
582putting the whole loop of assignments within a BEGIN block, forcing it
583to occur during compilation.
584
585Access to lexicals that change over type--like those in the C<for> loop
586above--only works with closures, not general subroutines. In the general
587case, then, named subroutines do not nest properly, although anonymous
b5c19bd7
DM
588ones do. Thus is because named subroutines are created (and capture any
589outer lexicals) only once at compile time, whereas anonymous subroutines
590get to capture each time you execute the 'sub' operator. If you are
591accustomed to using nested subroutines in other programming languages with
592their own private variables, you'll have to work at it a bit in Perl. The
593intuitive coding of this type of thing incurs mysterious warnings about
b432a672 594"will not stay shared". For example, this won't work:
5a964f20
TC
595
596 sub outer {
597 my $x = $_[0] + 35;
598 sub inner { return $x * 19 } # WRONG
599 return $x + inner();
b432a672 600 }
5a964f20
TC
601
602A work-around is the following:
603
604 sub outer {
605 my $x = $_[0] + 35;
606 local *inner = sub { return $x * 19 };
607 return $x + inner();
b432a672 608 }
5a964f20
TC
609
610Now inner() can only be called from within outer(), because of the
611temporary assignments of the closure (anonymous subroutine). But when
612it does, it has normal access to the lexical variable $x from the scope
613of outer().
614
615This has the interesting effect of creating a function local to another
616function, something not normally supported in Perl.
617
cb1a09d0 618=head1 WARNING
748a9306
LW
619
620You may not (usefully) use a reference as the key to a hash. It will be
621converted into a string:
622
623 $x{ \$a } = $a;
624
54310121 625If you try to dereference the key, it won't do a hard dereference, and
184e9718 626you won't accomplish what you're attempting. You might want to do something
cb1a09d0 627more like
748a9306 628
cb1a09d0
AD
629 $r = \@a;
630 $x{ $r } = $r;
631
632And then at least you can use the values(), which will be
633real refs, instead of the keys(), which won't.
634
5a964f20
TC
635The standard Tie::RefHash module provides a convenient workaround to this.
636
cb1a09d0 637=head1 SEE ALSO
a0d0e21e
LW
638
639Besides the obvious documents, source code can be instructive.
7c2ea1c7 640Some pathological examples of the use of references can be found
a0d0e21e 641in the F<t/op/ref.t> regression test in the Perl source directory.
cb1a09d0
AD
642
643See also L<perldsc> and L<perllol> for how to use references to create
5a964f20
TC
644complex data structures, and L<perltoot>, L<perlobj>, and L<perlbot>
645for how to use them to create objects.