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a0d0e21e 1=head1 NAME
d74e8afc 2X<reference> X<pointer> X<data structure> X<structure> X<struct>
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3
4perlref - Perl references and nested data structures
5
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6=head1 NOTE
7
8This is complete documentation about all aspects of references.
9For a shorter, tutorial introduction to just the essential features,
10see L<perlreftut>.
11
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12=head1 DESCRIPTION
13
cb1a09d0 14Before release 5 of Perl it was difficult to represent complex data
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15structures, because all references had to be symbolic--and even then
16it was difficult to refer to a variable instead of a symbol table entry.
17Perl now not only makes it easier to use symbolic references to variables,
18but also lets you have "hard" references to any piece of data or code.
19Any scalar may hold a hard reference. Because arrays and hashes contain
20scalars, you can now easily build arrays of arrays, arrays of hashes,
21hashes of arrays, arrays of hashes of functions, and so on.
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22
23Hard references are smart--they keep track of reference counts for you,
2d24ed35 24automatically freeing the thing referred to when its reference count goes
7c2ea1c7 25to zero. (Reference counts for values in self-referential or
2d24ed35 26cyclic data structures may not go to zero without a little help; see
7b8d334a 27L<perlobj/"Two-Phased Garbage Collection"> for a detailed explanation.)
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28If that thing happens to be an object, the object is destructed. See
29L<perlobj> for more about objects. (In a sense, everything in Perl is an
30object, but we usually reserve the word for references to objects that
31have been officially "blessed" into a class package.)
32
33Symbolic references are names of variables or other objects, just as a
54310121 34symbolic link in a Unix filesystem contains merely the name of a file.
d1be9408 35The C<*glob> notation is something of a symbolic reference. (Symbolic
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36references are sometimes called "soft references", but please don't call
37them that; references are confusing enough without useless synonyms.)
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38X<reference, symbolic> X<reference, soft>
39X<symbolic reference> X<soft reference>
2d24ed35 40
54310121 41In contrast, hard references are more like hard links in a Unix file
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42system: They are used to access an underlying object without concern for
43what its (other) name is. When the word "reference" is used without an
5a964f20 44adjective, as in the following paragraph, it is usually talking about a
2d24ed35 45hard reference.
d74e8afc 46X<reference, hard> X<hard reference>
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47
48References are easy to use in Perl. There is just one overriding
49principle: Perl does no implicit referencing or dereferencing. When a
50scalar is holding a reference, it always behaves as a simple scalar. It
51doesn't magically start being an array or hash or subroutine; you have to
52tell it explicitly to do so, by dereferencing it.
a0d0e21e 53
5a964f20 54=head2 Making References
d74e8afc 55X<reference, creation> X<referencing>
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56
57References can be created in several ways.
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58
59=over 4
60
61=item 1.
d74e8afc 62X<\> X<backslash>
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63
64By using the backslash operator on a variable, subroutine, or value.
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65(This works much like the & (address-of) operator in C.)
66This typically creates I<another> reference to a variable, because
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67there's already a reference to the variable in the symbol table. But
68the symbol table reference might go away, and you'll still have the
69reference that the backslash returned. Here are some examples:
70
71 $scalarref = \$foo;
72 $arrayref = \@ARGV;
73 $hashref = \%ENV;
74 $coderef = \&handler;
55497cff 75 $globref = \*foo;
cb1a09d0 76
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77It isn't possible to create a true reference to an IO handle (filehandle
78or dirhandle) using the backslash operator. The most you can get is a
79reference to a typeglob, which is actually a complete symbol table entry.
80But see the explanation of the C<*foo{THING}> syntax below. However,
81you can still use type globs and globrefs as though they were IO handles.
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82
83=item 2.
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84X<array, anonymous> X<[> X<[]> X<square bracket>
85X<bracket, square> X<arrayref> X<array reference> X<reference, array>
a0d0e21e 86
5a964f20 87A reference to an anonymous array can be created using square
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88brackets:
89
90 $arrayref = [1, 2, ['a', 'b', 'c']];
91
5a964f20 92Here we've created a reference to an anonymous array of three elements
54310121 93whose final element is itself a reference to another anonymous array of three
a0d0e21e 94elements. (The multidimensional syntax described later can be used to
c47ff5f1 95access this. For example, after the above, C<< $arrayref->[2][1] >> would have
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96the value "b".)
97
7c2ea1c7 98Taking a reference to an enumerated list is not the same
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99as using square brackets--instead it's the same as creating
100a list of references!
101
54310121 102 @list = (\$a, \@b, \%c);
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103 @list = \($a, @b, %c); # same thing!
104
54310121 105As a special case, C<\(@foo)> returns a list of references to the contents
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106of C<@foo>, not a reference to C<@foo> itself. Likewise for C<%foo>,
107except that the key references are to copies (since the keys are just
108strings rather than full-fledged scalars).
cb1a09d0 109
a0d0e21e 110=item 3.
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111X<hash, anonymous> X<{> X<{}> X<curly bracket>
112X<bracket, curly> X<brace> X<hashref> X<hash reference> X<reference, hash>
a0d0e21e 113
5a964f20 114A reference to an anonymous hash can be created using curly
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115brackets:
116
117 $hashref = {
118 'Adam' => 'Eve',
119 'Clyde' => 'Bonnie',
120 };
121
5a964f20 122Anonymous hash and array composers like these can be intermixed freely to
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123produce as complicated a structure as you want. The multidimensional
124syntax described below works for these too. The values above are
125literals, but variables and expressions would work just as well, because
126assignment operators in Perl (even within local() or my()) are executable
127statements, not compile-time declarations.
128
129Because curly brackets (braces) are used for several other things
130including BLOCKs, you may occasionally have to disambiguate braces at the
131beginning of a statement by putting a C<+> or a C<return> in front so
132that Perl realizes the opening brace isn't starting a BLOCK. The economy and
133mnemonic value of using curlies is deemed worth this occasional extra
134hassle.
135
136For example, if you wanted a function to make a new hash and return a
137reference to it, you have these options:
138
139 sub hashem { { @_ } } # silently wrong
140 sub hashem { +{ @_ } } # ok
141 sub hashem { return { @_ } } # ok
142
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143On the other hand, if you want the other meaning, you can do this:
144
145 sub showem { { @_ } } # ambiguous (currently ok, but may change)
146 sub showem { {; @_ } } # ok
147 sub showem { { return @_ } } # ok
148
7c2ea1c7 149The leading C<+{> and C<{;> always serve to disambiguate
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150the expression to mean either the HASH reference, or the BLOCK.
151
a0d0e21e 152=item 4.
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153X<subroutine, anonymous> X<subroutine, reference> X<reference, subroutine>
154X<scope, lexical> X<closure> X<lexical> X<lexical scope>
a0d0e21e 155
5a964f20 156A reference to an anonymous subroutine can be created by using
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157C<sub> without a subname:
158
159 $coderef = sub { print "Boink!\n" };
160
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161Note the semicolon. Except for the code
162inside not being immediately executed, a C<sub {}> is not so much a
a0d0e21e 163declaration as it is an operator, like C<do{}> or C<eval{}>. (However, no
5a964f20 164matter how many times you execute that particular line (unless you're in an
19799a22 165C<eval("...")>), $coderef will still have a reference to the I<same>
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166anonymous subroutine.)
167
748a9306 168Anonymous subroutines act as closures with respect to my() variables,
7c2ea1c7 169that is, variables lexically visible within the current scope. Closure
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170is a notion out of the Lisp world that says if you define an anonymous
171function in a particular lexical context, it pretends to run in that
7c2ea1c7 172context even when it's called outside the context.
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173
174In human terms, it's a funny way of passing arguments to a subroutine when
175you define it as well as when you call it. It's useful for setting up
176little bits of code to run later, such as callbacks. You can even
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177do object-oriented stuff with it, though Perl already provides a different
178mechanism to do that--see L<perlobj>.
748a9306 179
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180You might also think of closure as a way to write a subroutine
181template without using eval(). Here's a small example of how
182closures work:
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183
184 sub newprint {
185 my $x = shift;
186 return sub { my $y = shift; print "$x, $y!\n"; };
a0d0e21e 187 }
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188 $h = newprint("Howdy");
189 $g = newprint("Greetings");
190
191 # Time passes...
192
193 &$h("world");
194 &$g("earthlings");
a0d0e21e 195
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196This prints
197
198 Howdy, world!
199 Greetings, earthlings!
200
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201Note particularly that $x continues to refer to the value passed
202into newprint() I<despite> "my $x" having gone out of scope by the
203time the anonymous subroutine runs. That's what a closure is all
204about.
748a9306 205
5a964f20 206This applies only to lexical variables, by the way. Dynamic variables
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207continue to work as they have always worked. Closure is not something
208that most Perl programmers need trouble themselves about to begin with.
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209
210=item 5.
d74e8afc 211X<constructor> X<new>
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212
213References are often returned by special subroutines called constructors.
7c2ea1c7 214Perl objects are just references to a special type of object that happens to know
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215which package it's associated with. Constructors are just special
216subroutines that know how to create that association. They do so by
217starting with an ordinary reference, and it remains an ordinary reference
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218even while it's also being an object. Constructors are often
219named new() and called indirectly:
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220
221 $objref = new Doggie (Tail => 'short', Ears => 'long');
222
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223But don't have to be:
224
225 $objref = Doggie->new(Tail => 'short', Ears => 'long');
226
227 use Term::Cap;
228 $terminal = Term::Cap->Tgetent( { OSPEED => 9600 });
229
230 use Tk;
231 $main = MainWindow->new();
232 $menubar = $main->Frame(-relief => "raised",
233 -borderwidth => 2)
234
a0d0e21e 235=item 6.
d74e8afc 236X<autovivification>
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237
238References of the appropriate type can spring into existence if you
5f05dabc 239dereference them in a context that assumes they exist. Because we haven't
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240talked about dereferencing yet, we can't show you any examples yet.
241
cb1a09d0 242=item 7.
d74e8afc 243X<*foo{THING}> X<*>
cb1a09d0 244
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245A reference can be created by using a special syntax, lovingly known as
246the *foo{THING} syntax. *foo{THING} returns a reference to the THING
247slot in *foo (which is the symbol table entry which holds everything
248known as foo).
cb1a09d0 249
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250 $scalarref = *foo{SCALAR};
251 $arrayref = *ARGV{ARRAY};
252 $hashref = *ENV{HASH};
253 $coderef = *handler{CODE};
36477c24 254 $ioref = *STDIN{IO};
55497cff 255 $globref = *foo{GLOB};
c0bd1adc 256 $formatref = *foo{FORMAT};
55497cff 257
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258All of these are self-explanatory except for C<*foo{IO}>. It returns
259the IO handle, used for file handles (L<perlfunc/open>), sockets
260(L<perlfunc/socket> and L<perlfunc/socketpair>), and directory
261handles (L<perlfunc/opendir>). For compatibility with previous
39b99f21 262versions of Perl, C<*foo{FILEHANDLE}> is a synonym for C<*foo{IO}>, though it
263is deprecated as of 5.8.0. If deprecation warnings are in effect, it will warn
264of its use.
55497cff 265
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266C<*foo{THING}> returns undef if that particular THING hasn't been used yet,
267except in the case of scalars. C<*foo{SCALAR}> returns a reference to an
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268anonymous scalar if $foo hasn't been used yet. This might change in a
269future release.
270
7c2ea1c7 271C<*foo{IO}> is an alternative to the C<*HANDLE> mechanism given in
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272L<perldata/"Typeglobs and Filehandles"> for passing filehandles
273into or out of subroutines, or storing into larger data structures.
274Its disadvantage is that it won't create a new filehandle for you.
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275Its advantage is that you have less risk of clobbering more than
276you want to with a typeglob assignment. (It still conflates file
277and directory handles, though.) However, if you assign the incoming
278value to a scalar instead of a typeglob as we do in the examples
279below, there's no risk of that happening.
36477c24 280
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281 splutter(*STDOUT); # pass the whole glob
282 splutter(*STDOUT{IO}); # pass both file and dir handles
5a964f20 283
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284 sub splutter {
285 my $fh = shift;
286 print $fh "her um well a hmmm\n";
287 }
288
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289 $rec = get_rec(*STDIN); # pass the whole glob
290 $rec = get_rec(*STDIN{IO}); # pass both file and dir handles
5a964f20 291
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292 sub get_rec {
293 my $fh = shift;
294 return scalar <$fh>;
295 }
296
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297=back
298
5a964f20 299=head2 Using References
d74e8afc 300X<reference, use> X<dereferencing> X<dereference>
5a964f20 301
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302That's it for creating references. By now you're probably dying to
303know how to use references to get back to your long-lost data. There
304are several basic methods.
305
306=over 4
307
308=item 1.
309
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310Anywhere you'd put an identifier (or chain of identifiers) as part
311of a variable or subroutine name, you can replace the identifier with
312a simple scalar variable containing a reference of the correct type:
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313
314 $bar = $$scalarref;
315 push(@$arrayref, $filename);
316 $$arrayref[0] = "January";
317 $$hashref{"KEY"} = "VALUE";
318 &$coderef(1,2,3);
cb1a09d0 319 print $globref "output\n";
a0d0e21e 320
19799a22 321It's important to understand that we are specifically I<not> dereferencing
a0d0e21e 322C<$arrayref[0]> or C<$hashref{"KEY"}> there. The dereference of the
19799a22 323scalar variable happens I<before> it does any key lookups. Anything more
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324complicated than a simple scalar variable must use methods 2 or 3 below.
325However, a "simple scalar" includes an identifier that itself uses method
3261 recursively. Therefore, the following prints "howdy".
327
328 $refrefref = \\\"howdy";
329 print $$$$refrefref;
330
331=item 2.
d74e8afc 332X<${}> X<@{}> X<%{}>
a0d0e21e 333
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334Anywhere you'd put an identifier (or chain of identifiers) as part of a
335variable or subroutine name, you can replace the identifier with a
336BLOCK returning a reference of the correct type. In other words, the
337previous examples could be written like this:
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338
339 $bar = ${$scalarref};
340 push(@{$arrayref}, $filename);
341 ${$arrayref}[0] = "January";
342 ${$hashref}{"KEY"} = "VALUE";
343 &{$coderef}(1,2,3);
36477c24 344 $globref->print("output\n"); # iff IO::Handle is loaded
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345
346Admittedly, it's a little silly to use the curlies in this case, but
347the BLOCK can contain any arbitrary expression, in particular,
348subscripted expressions:
349
54310121 350 &{ $dispatch{$index} }(1,2,3); # call correct routine
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351
352Because of being able to omit the curlies for the simple case of C<$$x>,
353people often make the mistake of viewing the dereferencing symbols as
354proper operators, and wonder about their precedence. If they were,
5f05dabc 355though, you could use parentheses instead of braces. That's not the case.
a0d0e21e 356Consider the difference below; case 0 is a short-hand version of case 1,
19799a22 357I<not> case 2:
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358
359 $$hashref{"KEY"} = "VALUE"; # CASE 0
360 ${$hashref}{"KEY"} = "VALUE"; # CASE 1
361 ${$hashref{"KEY"}} = "VALUE"; # CASE 2
362 ${$hashref->{"KEY"}} = "VALUE"; # CASE 3
363
364Case 2 is also deceptive in that you're accessing a variable
365called %hashref, not dereferencing through $hashref to the hash
366it's presumably referencing. That would be case 3.
367
368=item 3.
d74e8afc 369X<autovivification> X<< -> >> X<arrow>
a0d0e21e 370
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371Subroutine calls and lookups of individual array elements arise often
372enough that it gets cumbersome to use method 2. As a form of
373syntactic sugar, the examples for method 2 may be written:
a0d0e21e 374
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375 $arrayref->[0] = "January"; # Array element
376 $hashref->{"KEY"} = "VALUE"; # Hash element
377 $coderef->(1,2,3); # Subroutine call
a0d0e21e 378
6da72b64 379The left side of the arrow can be any expression returning a reference,
19799a22 380including a previous dereference. Note that C<$array[$x]> is I<not> the
c47ff5f1 381same thing as C<< $array->[$x] >> here:
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382
383 $array[$x]->{"foo"}->[0] = "January";
384
385This is one of the cases we mentioned earlier in which references could
386spring into existence when in an lvalue context. Before this
387statement, C<$array[$x]> may have been undefined. If so, it's
388automatically defined with a hash reference so that we can look up
c47ff5f1 389C<{"foo"}> in it. Likewise C<< $array[$x]->{"foo"} >> will automatically get
a0d0e21e 390defined with an array reference so that we can look up C<[0]> in it.
5a964f20 391This process is called I<autovivification>.
a0d0e21e 392
19799a22 393One more thing here. The arrow is optional I<between> brackets
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394subscripts, so you can shrink the above down to
395
396 $array[$x]{"foo"}[0] = "January";
397
398Which, in the degenerate case of using only ordinary arrays, gives you
399multidimensional arrays just like C's:
400
401 $score[$x][$y][$z] += 42;
402
403Well, okay, not entirely like C's arrays, actually. C doesn't know how
404to grow its arrays on demand. Perl does.
405
406=item 4.
d74e8afc 407X<encapsulation>
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408
409If a reference happens to be a reference to an object, then there are
410probably methods to access the things referred to, and you should probably
411stick to those methods unless you're in the class package that defines the
412object's methods. In other words, be nice, and don't violate the object's
413encapsulation without a very good reason. Perl does not enforce
414encapsulation. We are not totalitarians here. We do expect some basic
415civility though.
416
417=back
418
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419Using a string or number as a reference produces a symbolic reference,
420as explained above. Using a reference as a number produces an
421integer representing its storage location in memory. The only
422useful thing to be done with this is to compare two references
423numerically to see whether they refer to the same location.
d74e8afc 424X<reference, numeric context>
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425
426 if ($ref1 == $ref2) { # cheap numeric compare of references
427 print "refs 1 and 2 refer to the same thing\n";
428 }
429
430Using a reference as a string produces both its referent's type,
431including any package blessing as described in L<perlobj>, as well
432as the numeric address expressed in hex. The ref() operator returns
433just the type of thing the reference is pointing to, without the
434address. See L<perlfunc/ref> for details and examples of its use.
d74e8afc 435X<reference, string context>
a0d0e21e 436
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437The bless() operator may be used to associate the object a reference
438points to with a package functioning as an object class. See L<perlobj>.
a0d0e21e 439
5f05dabc 440A typeglob may be dereferenced the same way a reference can, because
7c2ea1c7 441the dereference syntax always indicates the type of reference desired.
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442So C<${*foo}> and C<${\$foo}> both indicate the same scalar variable.
443
444Here's a trick for interpolating a subroutine call into a string:
445
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446 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
447
448The way it works is that when the C<@{...}> is seen in the double-quoted
449string, it's evaluated as a block. The block creates a reference to an
450anonymous array containing the results of the call to C<mysub(1,2,3)>. So
451the whole block returns a reference to an array, which is then
452dereferenced by C<@{...}> and stuck into the double-quoted string. This
453chicanery is also useful for arbitrary expressions:
a0d0e21e 454
184e9718 455 print "That yields @{[$n + 5]} widgets\n";
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456
457=head2 Symbolic references
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458X<reference, symbolic> X<reference, soft>
459X<symbolic reference> X<soft reference>
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460
461We said that references spring into existence as necessary if they are
462undefined, but we didn't say what happens if a value used as a
19799a22 463reference is already defined, but I<isn't> a hard reference. If you
7c2ea1c7 464use it as a reference, it'll be treated as a symbolic
19799a22 465reference. That is, the value of the scalar is taken to be the I<name>
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466of a variable, rather than a direct link to a (possibly) anonymous
467value.
468
469People frequently expect it to work like this. So it does.
470
471 $name = "foo";
472 $$name = 1; # Sets $foo
473 ${$name} = 2; # Sets $foo
474 ${$name x 2} = 3; # Sets $foofoo
475 $name->[0] = 4; # Sets $foo[0]
476 @$name = (); # Clears @foo
477 &$name(); # Calls &foo() (as in Perl 4)
478 $pack = "THAT";
479 ${"${pack}::$name"} = 5; # Sets $THAT::foo without eval
480
7c2ea1c7 481This is powerful, and slightly dangerous, in that it's possible
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482to intend (with the utmost sincerity) to use a hard reference, and
483accidentally use a symbolic reference instead. To protect against
484that, you can say
485
486 use strict 'refs';
487
488and then only hard references will be allowed for the rest of the enclosing
54310121 489block. An inner block may countermand that with
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490
491 no strict 'refs';
492
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493Only package variables (globals, even if localized) are visible to
494symbolic references. Lexical variables (declared with my()) aren't in
495a symbol table, and thus are invisible to this mechanism. For example:
a0d0e21e 496
5a964f20 497 local $value = 10;
b0c35547 498 $ref = "value";
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499 {
500 my $value = 20;
501 print $$ref;
54310121 502 }
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503
504This will still print 10, not 20. Remember that local() affects package
505variables, which are all "global" to the package.
506
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507=head2 Not-so-symbolic references
508
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509A new feature contributing to readability in perl version 5.001 is that the
510brackets around a symbolic reference behave more like quotes, just as they
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511always have within a string. That is,
512
513 $push = "pop on ";
514 print "${push}over";
515
7c2ea1c7 516has always meant to print "pop on over", even though push is
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517a reserved word. This has been generalized to work the same outside
518of quotes, so that
519
520 print ${push} . "over";
521
522and even
523
524 print ${ push } . "over";
525
526will have the same effect. (This would have been a syntax error in
7c2ea1c7 527Perl 5.000, though Perl 4 allowed it in the spaceless form.) This
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528construct is I<not> considered to be a symbolic reference when you're
529using strict refs:
530
531 use strict 'refs';
532 ${ bareword }; # Okay, means $bareword.
533 ${ "bareword" }; # Error, symbolic reference.
534
535Similarly, because of all the subscripting that is done using single
536words, we've applied the same rule to any bareword that is used for
537subscripting a hash. So now, instead of writing
538
539 $array{ "aaa" }{ "bbb" }{ "ccc" }
540
5f05dabc 541you can write just
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542
543 $array{ aaa }{ bbb }{ ccc }
544
545and not worry about whether the subscripts are reserved words. In the
546rare event that you do wish to do something like
547
548 $array{ shift }
549
550you can force interpretation as a reserved word by adding anything that
551makes it more than a bareword:
552
553 $array{ shift() }
554 $array{ +shift }
555 $array{ shift @_ }
556
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557The C<use warnings> pragma or the B<-w> switch will warn you if it
558interprets a reserved word as a string.
5f05dabc 559But it will no longer warn you about using lowercase words, because the
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560string is effectively quoted.
561
49399b3f 562=head2 Pseudo-hashes: Using an array as a hash
d74e8afc 563X<pseudo-hash> X<pseudo hash> X<pseudohash>
49399b3f 564
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565Pseudo-hashes have been removed from Perl. The 'fields' pragma
566remains available.
e0478e5a 567
5a964f20 568=head2 Function Templates
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569X<scope, lexical> X<closure> X<lexical> X<lexical scope>
570X<subroutine, nested> X<sub, nested> X<subroutine, local> X<sub, local>
5a964f20 571
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572As explained above, an anonymous function with access to the lexical
573variables visible when that function was compiled, creates a closure. It
574retains access to those variables even though it doesn't get run until
575later, such as in a signal handler or a Tk callback.
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576
577Using a closure as a function template allows us to generate many functions
c2611fb3 578that act similarly. Suppose you wanted functions named after the colors
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579that generated HTML font changes for the various colors:
580
581 print "Be ", red("careful"), "with that ", green("light");
582
7c2ea1c7 583The red() and green() functions would be similar. To create these,
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584we'll assign a closure to a typeglob of the name of the function we're
585trying to build.
586
587 @colors = qw(red blue green yellow orange purple violet);
588 for my $name (@colors) {
589 no strict 'refs'; # allow symbol table manipulation
590 *$name = *{uc $name} = sub { "<FONT COLOR='$name'>@_</FONT>" };
591 }
592
593Now all those different functions appear to exist independently. You can
594call red(), RED(), blue(), BLUE(), green(), etc. This technique saves on
595both compile time and memory use, and is less error-prone as well, since
596syntax checks happen at compile time. It's critical that any variables in
597the anonymous subroutine be lexicals in order to create a proper closure.
598That's the reasons for the C<my> on the loop iteration variable.
599
600This is one of the only places where giving a prototype to a closure makes
601much sense. If you wanted to impose scalar context on the arguments of
602these functions (probably not a wise idea for this particular example),
603you could have written it this way instead:
604
605 *$name = sub ($) { "<FONT COLOR='$name'>$_[0]</FONT>" };
606
607However, since prototype checking happens at compile time, the assignment
608above happens too late to be of much use. You could address this by
609putting the whole loop of assignments within a BEGIN block, forcing it
610to occur during compilation.
611
612Access to lexicals that change over type--like those in the C<for> loop
613above--only works with closures, not general subroutines. In the general
614case, then, named subroutines do not nest properly, although anonymous
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615ones do. Thus is because named subroutines are created (and capture any
616outer lexicals) only once at compile time, whereas anonymous subroutines
617get to capture each time you execute the 'sub' operator. If you are
618accustomed to using nested subroutines in other programming languages with
619their own private variables, you'll have to work at it a bit in Perl. The
620intuitive coding of this type of thing incurs mysterious warnings about
b432a672 621"will not stay shared". For example, this won't work:
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622
623 sub outer {
624 my $x = $_[0] + 35;
625 sub inner { return $x * 19 } # WRONG
626 return $x + inner();
b432a672 627 }
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628
629A work-around is the following:
630
631 sub outer {
632 my $x = $_[0] + 35;
633 local *inner = sub { return $x * 19 };
634 return $x + inner();
b432a672 635 }
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636
637Now inner() can only be called from within outer(), because of the
638temporary assignments of the closure (anonymous subroutine). But when
639it does, it has normal access to the lexical variable $x from the scope
640of outer().
641
642This has the interesting effect of creating a function local to another
643function, something not normally supported in Perl.
644
cb1a09d0 645=head1 WARNING
d74e8afc 646X<reference, string context> X<reference, use as hash key>
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647
648You may not (usefully) use a reference as the key to a hash. It will be
649converted into a string:
650
651 $x{ \$a } = $a;
652
54310121 653If you try to dereference the key, it won't do a hard dereference, and
184e9718 654you won't accomplish what you're attempting. You might want to do something
cb1a09d0 655more like
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657 $r = \@a;
658 $x{ $r } = $r;
659
660And then at least you can use the values(), which will be
661real refs, instead of the keys(), which won't.
662
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663The standard Tie::RefHash module provides a convenient workaround to this.
664
cb1a09d0 665=head1 SEE ALSO
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666
667Besides the obvious documents, source code can be instructive.
7c2ea1c7 668Some pathological examples of the use of references can be found
a0d0e21e 669in the F<t/op/ref.t> regression test in the Perl source directory.
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670
671See also L<perldsc> and L<perllol> for how to use references to create
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672complex data structures, and L<perltoot>, L<perlobj>, and L<perlbot>
673for how to use them to create objects.