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1=head1 NAME
2
3perlref - Perl references and nested data structures
4
5=head1 DESCRIPTION
6
cb1a09d0 7Before release 5 of Perl it was difficult to represent complex data
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8structures, because all references had to be symbolic--and even then
9it was difficult to refer to a variable instead of a symbol table entry.
10Perl now not only makes it easier to use symbolic references to variables,
11but also lets you have "hard" references to any piece of data or code.
12Any scalar may hold a hard reference. Because arrays and hashes contain
13scalars, you can now easily build arrays of arrays, arrays of hashes,
14hashes of arrays, arrays of hashes of functions, and so on.
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15
16Hard references are smart--they keep track of reference counts for you,
2d24ed35 17automatically freeing the thing referred to when its reference count goes
7b8d334a 18to zero. (Note: the reference counts for values in self-referential or
2d24ed35 19cyclic data structures may not go to zero without a little help; see
7b8d334a 20L<perlobj/"Two-Phased Garbage Collection"> for a detailed explanation.)
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21If that thing happens to be an object, the object is destructed. See
22L<perlobj> for more about objects. (In a sense, everything in Perl is an
23object, but we usually reserve the word for references to objects that
24have been officially "blessed" into a class package.)
25
26Symbolic references are names of variables or other objects, just as a
54310121 27symbolic link in a Unix filesystem contains merely the name of a file.
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28The C<*glob> notation is a kind of symbolic reference. (Symbolic
29references are sometimes called "soft references", but please don't call
30them that; references are confusing enough without useless synonyms.)
31
54310121 32In contrast, hard references are more like hard links in a Unix file
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33system: They are used to access an underlying object without concern for
34what its (other) name is. When the word "reference" is used without an
5a964f20 35adjective, as in the following paragraph, it is usually talking about a
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36hard reference.
37
38References are easy to use in Perl. There is just one overriding
39principle: Perl does no implicit referencing or dereferencing. When a
40scalar is holding a reference, it always behaves as a simple scalar. It
41doesn't magically start being an array or hash or subroutine; you have to
42tell it explicitly to do so, by dereferencing it.
a0d0e21e 43
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44=head2 Making References
45
46References can be created in several ways.
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47
48=over 4
49
50=item 1.
51
52By using the backslash operator on a variable, subroutine, or value.
54310121 53(This works much like the & (address-of) operator in C.) Note
5f05dabc 54that this typically creates I<ANOTHER> reference to a variable, because
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55there's already a reference to the variable in the symbol table. But
56the symbol table reference might go away, and you'll still have the
57reference that the backslash returned. Here are some examples:
58
59 $scalarref = \$foo;
60 $arrayref = \@ARGV;
61 $hashref = \%ENV;
62 $coderef = \&handler;
55497cff 63 $globref = \*foo;
cb1a09d0 64
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65It isn't possible to create a true reference to an IO handle (filehandle
66or dirhandle) using the backslash operator. The most you can get is a
67reference to a typeglob, which is actually a complete symbol table entry.
68But see the explanation of the C<*foo{THING}> syntax below. However,
69you can still use type globs and globrefs as though they were IO handles.
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70
71=item 2.
72
5a964f20 73A reference to an anonymous array can be created using square
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74brackets:
75
76 $arrayref = [1, 2, ['a', 'b', 'c']];
77
5a964f20 78Here we've created a reference to an anonymous array of three elements
54310121 79whose final element is itself a reference to another anonymous array of three
a0d0e21e 80elements. (The multidimensional syntax described later can be used to
184e9718 81access this. For example, after the above, C<$arrayref-E<gt>[2][1]> would have
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82the value "b".)
83
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84Note that taking a reference to an enumerated list is not the same
85as using square brackets--instead it's the same as creating
86a list of references!
87
54310121 88 @list = (\$a, \@b, \%c);
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89 @list = \($a, @b, %c); # same thing!
90
54310121 91As a special case, C<\(@foo)> returns a list of references to the contents
58e0a6ae 92of C<@foo>, not a reference to C<@foo> itself. Likewise for C<%foo>.
cb1a09d0 93
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94=item 3.
95
5a964f20 96A reference to an anonymous hash can be created using curly
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97brackets:
98
99 $hashref = {
100 'Adam' => 'Eve',
101 'Clyde' => 'Bonnie',
102 };
103
5a964f20 104Anonymous hash and array composers like these can be intermixed freely to
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105produce as complicated a structure as you want. The multidimensional
106syntax described below works for these too. The values above are
107literals, but variables and expressions would work just as well, because
108assignment operators in Perl (even within local() or my()) are executable
109statements, not compile-time declarations.
110
111Because curly brackets (braces) are used for several other things
112including BLOCKs, you may occasionally have to disambiguate braces at the
113beginning of a statement by putting a C<+> or a C<return> in front so
114that Perl realizes the opening brace isn't starting a BLOCK. The economy and
115mnemonic value of using curlies is deemed worth this occasional extra
116hassle.
117
118For example, if you wanted a function to make a new hash and return a
119reference to it, you have these options:
120
121 sub hashem { { @_ } } # silently wrong
122 sub hashem { +{ @_ } } # ok
123 sub hashem { return { @_ } } # ok
124
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125On the other hand, if you want the other meaning, you can do this:
126
127 sub showem { { @_ } } # ambiguous (currently ok, but may change)
128 sub showem { {; @_ } } # ok
129 sub showem { { return @_ } } # ok
130
131Note how the leading C<+{> and C<{;> always serve to disambiguate
132the expression to mean either the HASH reference, or the BLOCK.
133
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134=item 4.
135
5a964f20 136A reference to an anonymous subroutine can be created by using
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137C<sub> without a subname:
138
139 $coderef = sub { print "Boink!\n" };
140
141Note the presence of the semicolon. Except for the fact that the code
142inside isn't executed immediately, a C<sub {}> is not so much a
143declaration as it is an operator, like C<do{}> or C<eval{}>. (However, no
5a964f20 144matter how many times you execute that particular line (unless you're in an
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145C<eval("...")>), C<$coderef> will still have a reference to the I<SAME>
146anonymous subroutine.)
147
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148Anonymous subroutines act as closures with respect to my() variables,
149that is, variables visible lexically within the current scope. Closure
150is a notion out of the Lisp world that says if you define an anonymous
151function in a particular lexical context, it pretends to run in that
152context even when it's called outside of the context.
153
154In human terms, it's a funny way of passing arguments to a subroutine when
155you define it as well as when you call it. It's useful for setting up
156little bits of code to run later, such as callbacks. You can even
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157do object-oriented stuff with it, though Perl already provides a different
158mechanism to do that--see L<perlobj>.
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159
160You can also think of closure as a way to write a subroutine template without
161using eval. (In fact, in version 5.000, eval was the I<only> way to get
162closures. You may wish to use "require 5.001" if you use closures.)
163
164Here's a small example of how closures works:
165
166 sub newprint {
167 my $x = shift;
168 return sub { my $y = shift; print "$x, $y!\n"; };
a0d0e21e 169 }
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170 $h = newprint("Howdy");
171 $g = newprint("Greetings");
172
173 # Time passes...
174
175 &$h("world");
176 &$g("earthlings");
a0d0e21e 177
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178This prints
179
180 Howdy, world!
181 Greetings, earthlings!
182
183Note particularly that $x continues to refer to the value passed into
cb1a09d0 184newprint() I<despite> the fact that the "my $x" has seemingly gone out of
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185scope by the time the anonymous subroutine runs. That's what closure
186is all about.
187
5a964f20 188This applies only to lexical variables, by the way. Dynamic variables
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189continue to work as they have always worked. Closure is not something
190that most Perl programmers need trouble themselves about to begin with.
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191
192=item 5.
193
194References are often returned by special subroutines called constructors.
748a9306 195Perl objects are just references to a special kind of object that happens to know
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196which package it's associated with. Constructors are just special
197subroutines that know how to create that association. They do so by
198starting with an ordinary reference, and it remains an ordinary reference
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199even while it's also being an object. Constructors are often
200named new() and called indirectly:
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201
202 $objref = new Doggie (Tail => 'short', Ears => 'long');
203
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204But don't have to be:
205
206 $objref = Doggie->new(Tail => 'short', Ears => 'long');
207
208 use Term::Cap;
209 $terminal = Term::Cap->Tgetent( { OSPEED => 9600 });
210
211 use Tk;
212 $main = MainWindow->new();
213 $menubar = $main->Frame(-relief => "raised",
214 -borderwidth => 2)
215
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216=item 6.
217
218References of the appropriate type can spring into existence if you
5f05dabc 219dereference them in a context that assumes they exist. Because we haven't
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220talked about dereferencing yet, we can't show you any examples yet.
221
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222=item 7.
223
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224A reference can be created by using a special syntax, lovingly known as
225the *foo{THING} syntax. *foo{THING} returns a reference to the THING
226slot in *foo (which is the symbol table entry which holds everything
227known as foo).
cb1a09d0 228
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229 $scalarref = *foo{SCALAR};
230 $arrayref = *ARGV{ARRAY};
231 $hashref = *ENV{HASH};
232 $coderef = *handler{CODE};
36477c24 233 $ioref = *STDIN{IO};
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234 $globref = *foo{GLOB};
235
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236All of these are self-explanatory except for *foo{IO}. It returns the
237IO handle, used for file handles (L<perlfunc/open>), sockets
238(L<perlfunc/socket> and L<perlfunc/socketpair>), and directory handles
239(L<perlfunc/opendir>). For compatibility with previous versions of
240Perl, *foo{FILEHANDLE} is a synonym for *foo{IO}.
55497cff 241
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242*foo{THING} returns undef if that particular THING hasn't been used yet,
243except in the case of scalars. *foo{SCALAR} returns a reference to an
244anonymous scalar if $foo hasn't been used yet. This might change in a
245future release.
246
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247*foo{IO} is an alternative to the \*HANDLE mechanism given in
248L<perldata/"Typeglobs and Filehandles"> for passing filehandles
249into or out of subroutines, or storing into larger data structures.
250Its disadvantage is that it won't create a new filehandle for you.
251Its advantage is that you have no risk of clobbering more than you want
252to with a typeglob assignment, although if you assign to a scalar instead
253of a typeglob, you're ok.
36477c24 254
5a964f20 255 splutter(*STDOUT);
36477c24 256 splutter(*STDOUT{IO});
5a964f20 257
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258 sub splutter {
259 my $fh = shift;
260 print $fh "her um well a hmmm\n";
261 }
262
5a964f20 263 $rec = get_rec(*STDIN);
36477c24 264 $rec = get_rec(*STDIN{IO});
5a964f20 265
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266 sub get_rec {
267 my $fh = shift;
268 return scalar <$fh>;
269 }
270
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271=back
272
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273=head2 Using References
274
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275That's it for creating references. By now you're probably dying to
276know how to use references to get back to your long-lost data. There
277are several basic methods.
278
279=over 4
280
281=item 1.
282
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283Anywhere you'd put an identifier (or chain of identifiers) as part
284of a variable or subroutine name, you can replace the identifier with
285a simple scalar variable containing a reference of the correct type:
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286
287 $bar = $$scalarref;
288 push(@$arrayref, $filename);
289 $$arrayref[0] = "January";
290 $$hashref{"KEY"} = "VALUE";
291 &$coderef(1,2,3);
cb1a09d0 292 print $globref "output\n";
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293
294It's important to understand that we are specifically I<NOT> dereferencing
295C<$arrayref[0]> or C<$hashref{"KEY"}> there. The dereference of the
296scalar variable happens I<BEFORE> it does any key lookups. Anything more
297complicated than a simple scalar variable must use methods 2 or 3 below.
298However, a "simple scalar" includes an identifier that itself uses method
2991 recursively. Therefore, the following prints "howdy".
300
301 $refrefref = \\\"howdy";
302 print $$$$refrefref;
303
304=item 2.
305
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306Anywhere you'd put an identifier (or chain of identifiers) as part of a
307variable or subroutine name, you can replace the identifier with a
308BLOCK returning a reference of the correct type. In other words, the
309previous examples could be written like this:
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310
311 $bar = ${$scalarref};
312 push(@{$arrayref}, $filename);
313 ${$arrayref}[0] = "January";
314 ${$hashref}{"KEY"} = "VALUE";
315 &{$coderef}(1,2,3);
36477c24 316 $globref->print("output\n"); # iff IO::Handle is loaded
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317
318Admittedly, it's a little silly to use the curlies in this case, but
319the BLOCK can contain any arbitrary expression, in particular,
320subscripted expressions:
321
54310121 322 &{ $dispatch{$index} }(1,2,3); # call correct routine
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323
324Because of being able to omit the curlies for the simple case of C<$$x>,
325people often make the mistake of viewing the dereferencing symbols as
326proper operators, and wonder about their precedence. If they were,
5f05dabc 327though, you could use parentheses instead of braces. That's not the case.
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328Consider the difference below; case 0 is a short-hand version of case 1,
329I<NOT> case 2:
330
331 $$hashref{"KEY"} = "VALUE"; # CASE 0
332 ${$hashref}{"KEY"} = "VALUE"; # CASE 1
333 ${$hashref{"KEY"}} = "VALUE"; # CASE 2
334 ${$hashref->{"KEY"}} = "VALUE"; # CASE 3
335
336Case 2 is also deceptive in that you're accessing a variable
337called %hashref, not dereferencing through $hashref to the hash
338it's presumably referencing. That would be case 3.
339
340=item 3.
341
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342Subroutine calls and lookups of individual array elements arise often
343enough that it gets cumbersome to use method 2. As a form of
344syntactic sugar, the examples for method 2 may be written:
a0d0e21e 345
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346 $arrayref->[0] = "January"; # Array element
347 $hashref->{"KEY"} = "VALUE"; # Hash element
348 $coderef->(1,2,3); # Subroutine call
a0d0e21e 349
6da72b64 350The left side of the arrow can be any expression returning a reference,
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351including a previous dereference. Note that C<$array[$x]> is I<NOT> the
352same thing as C<$array-E<gt>[$x]> here:
353
354 $array[$x]->{"foo"}->[0] = "January";
355
356This is one of the cases we mentioned earlier in which references could
357spring into existence when in an lvalue context. Before this
358statement, C<$array[$x]> may have been undefined. If so, it's
359automatically defined with a hash reference so that we can look up
360C<{"foo"}> in it. Likewise C<$array[$x]-E<gt>{"foo"}> will automatically get
361defined with an array reference so that we can look up C<[0]> in it.
5a964f20 362This process is called I<autovivification>.
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363
364One more thing here. The arrow is optional I<BETWEEN> brackets
365subscripts, so you can shrink the above down to
366
367 $array[$x]{"foo"}[0] = "January";
368
369Which, in the degenerate case of using only ordinary arrays, gives you
370multidimensional arrays just like C's:
371
372 $score[$x][$y][$z] += 42;
373
374Well, okay, not entirely like C's arrays, actually. C doesn't know how
375to grow its arrays on demand. Perl does.
376
377=item 4.
378
379If a reference happens to be a reference to an object, then there are
380probably methods to access the things referred to, and you should probably
381stick to those methods unless you're in the class package that defines the
382object's methods. In other words, be nice, and don't violate the object's
383encapsulation without a very good reason. Perl does not enforce
384encapsulation. We are not totalitarians here. We do expect some basic
385civility though.
386
387=back
388
389The ref() operator may be used to determine what type of thing the
390reference is pointing to. See L<perlfunc>.
391
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392The bless() operator may be used to associate the object a reference
393points to with a package functioning as an object class. See L<perlobj>.
a0d0e21e 394
5f05dabc 395A typeglob may be dereferenced the same way a reference can, because
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396the dereference syntax always indicates the kind of reference desired.
397So C<${*foo}> and C<${\$foo}> both indicate the same scalar variable.
398
399Here's a trick for interpolating a subroutine call into a string:
400
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401 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
402
403The way it works is that when the C<@{...}> is seen in the double-quoted
404string, it's evaluated as a block. The block creates a reference to an
405anonymous array containing the results of the call to C<mysub(1,2,3)>. So
406the whole block returns a reference to an array, which is then
407dereferenced by C<@{...}> and stuck into the double-quoted string. This
408chicanery is also useful for arbitrary expressions:
a0d0e21e 409
184e9718 410 print "That yields @{[$n + 5]} widgets\n";
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411
412=head2 Symbolic references
413
414We said that references spring into existence as necessary if they are
415undefined, but we didn't say what happens if a value used as a
416reference is already defined, but I<ISN'T> a hard reference. If you
417use it as a reference in this case, it'll be treated as a symbolic
418reference. That is, the value of the scalar is taken to be the I<NAME>
419of a variable, rather than a direct link to a (possibly) anonymous
420value.
421
422People frequently expect it to work like this. So it does.
423
424 $name = "foo";
425 $$name = 1; # Sets $foo
426 ${$name} = 2; # Sets $foo
427 ${$name x 2} = 3; # Sets $foofoo
428 $name->[0] = 4; # Sets $foo[0]
429 @$name = (); # Clears @foo
430 &$name(); # Calls &foo() (as in Perl 4)
431 $pack = "THAT";
432 ${"${pack}::$name"} = 5; # Sets $THAT::foo without eval
433
434This is very powerful, and slightly dangerous, in that it's possible
435to intend (with the utmost sincerity) to use a hard reference, and
436accidentally use a symbolic reference instead. To protect against
437that, you can say
438
439 use strict 'refs';
440
441and then only hard references will be allowed for the rest of the enclosing
54310121 442block. An inner block may countermand that with
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443
444 no strict 'refs';
445
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446Only package variables (globals, even if localized) are visible to
447symbolic references. Lexical variables (declared with my()) aren't in
448a symbol table, and thus are invisible to this mechanism. For example:
a0d0e21e 449
5a964f20 450 local $value = 10;
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451 $ref = \$value;
452 {
453 my $value = 20;
454 print $$ref;
54310121 455 }
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456
457This will still print 10, not 20. Remember that local() affects package
458variables, which are all "global" to the package.
459
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460=head2 Not-so-symbolic references
461
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462A new feature contributing to readability in perl version 5.001 is that the
463brackets around a symbolic reference behave more like quotes, just as they
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464always have within a string. That is,
465
466 $push = "pop on ";
467 print "${push}over";
468
469has always meant to print "pop on over", despite the fact that push is
470a reserved word. This has been generalized to work the same outside
471of quotes, so that
472
473 print ${push} . "over";
474
475and even
476
477 print ${ push } . "over";
478
479will have the same effect. (This would have been a syntax error in
a6006777 480Perl 5.000, though Perl 4 allowed it in the spaceless form.) Note that this
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481construct is I<not> considered to be a symbolic reference when you're
482using strict refs:
483
484 use strict 'refs';
485 ${ bareword }; # Okay, means $bareword.
486 ${ "bareword" }; # Error, symbolic reference.
487
488Similarly, because of all the subscripting that is done using single
489words, we've applied the same rule to any bareword that is used for
490subscripting a hash. So now, instead of writing
491
492 $array{ "aaa" }{ "bbb" }{ "ccc" }
493
5f05dabc 494you can write just
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495
496 $array{ aaa }{ bbb }{ ccc }
497
498and not worry about whether the subscripts are reserved words. In the
499rare event that you do wish to do something like
500
501 $array{ shift }
502
503you can force interpretation as a reserved word by adding anything that
504makes it more than a bareword:
505
506 $array{ shift() }
507 $array{ +shift }
508 $array{ shift @_ }
509
510The B<-w> switch will warn you if it interprets a reserved word as a string.
5f05dabc 511But it will no longer warn you about using lowercase words, because the
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512string is effectively quoted.
513
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514=head2 Function Templates
515
516As explained above, a closure is an anonymous function with access to the
517lexical variables visible when that function was compiled. It retains
518access to those variables even though it doesn't get run until later,
519such as in a signal handler or a Tk callback.
520
521Using a closure as a function template allows us to generate many functions
522that act similarly. Suppopose you wanted functions named after the colors
523that generated HTML font changes for the various colors:
524
525 print "Be ", red("careful"), "with that ", green("light");
526
527The red() and green() functions would be very similar. To create these,
528we'll assign a closure to a typeglob of the name of the function we're
529trying to build.
530
531 @colors = qw(red blue green yellow orange purple violet);
532 for my $name (@colors) {
533 no strict 'refs'; # allow symbol table manipulation
534 *$name = *{uc $name} = sub { "<FONT COLOR='$name'>@_</FONT>" };
535 }
536
537Now all those different functions appear to exist independently. You can
538call red(), RED(), blue(), BLUE(), green(), etc. This technique saves on
539both compile time and memory use, and is less error-prone as well, since
540syntax checks happen at compile time. It's critical that any variables in
541the anonymous subroutine be lexicals in order to create a proper closure.
542That's the reasons for the C<my> on the loop iteration variable.
543
544This is one of the only places where giving a prototype to a closure makes
545much sense. If you wanted to impose scalar context on the arguments of
546these functions (probably not a wise idea for this particular example),
547you could have written it this way instead:
548
549 *$name = sub ($) { "<FONT COLOR='$name'>$_[0]</FONT>" };
550
551However, since prototype checking happens at compile time, the assignment
552above happens too late to be of much use. You could address this by
553putting the whole loop of assignments within a BEGIN block, forcing it
554to occur during compilation.
555
556Access to lexicals that change over type--like those in the C<for> loop
557above--only works with closures, not general subroutines. In the general
558case, then, named subroutines do not nest properly, although anonymous
559ones do. If you are accustomed to using nested subroutines in other
560programming languages with their own private variables, you'll have to
561work at it a bit in Perl. The intuitive coding of this kind of thing
562incurs mysterious warnings about ``will not stay shared''. For example,
563this won't work:
564
565 sub outer {
566 my $x = $_[0] + 35;
567 sub inner { return $x * 19 } # WRONG
568 return $x + inner();
569 }
570
571A work-around is the following:
572
573 sub outer {
574 my $x = $_[0] + 35;
575 local *inner = sub { return $x * 19 };
576 return $x + inner();
577 }
578
579Now inner() can only be called from within outer(), because of the
580temporary assignments of the closure (anonymous subroutine). But when
581it does, it has normal access to the lexical variable $x from the scope
582of outer().
583
584This has the interesting effect of creating a function local to another
585function, something not normally supported in Perl.
586
cb1a09d0 587=head1 WARNING
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588
589You may not (usefully) use a reference as the key to a hash. It will be
590converted into a string:
591
592 $x{ \$a } = $a;
593
54310121 594If you try to dereference the key, it won't do a hard dereference, and
184e9718 595you won't accomplish what you're attempting. You might want to do something
cb1a09d0 596more like
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598 $r = \@a;
599 $x{ $r } = $r;
600
601And then at least you can use the values(), which will be
602real refs, instead of the keys(), which won't.
603
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604The standard Tie::RefHash module provides a convenient workaround to this.
605
cb1a09d0 606=head1 SEE ALSO
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607
608Besides the obvious documents, source code can be instructive.
609Some rather pathological examples of the use of references can be found
610in the F<t/op/ref.t> regression test in the Perl source directory.
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611
612See also L<perldsc> and L<perllol> for how to use references to create
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613complex data structures, and L<perltoot>, L<perlobj>, and L<perlbot>
614for how to use them to create objects.