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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
2b4f771d 27L</"Circular References"> 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
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54References are easy to use in Perl. There is just one overriding
55principle: in general, Perl does no implicit referencing or dereferencing.
56When a scalar is holding a reference, it always behaves as a simple scalar.
57It doesn't magically start being an array or hash or subroutine; you have to
58tell it explicitly to do so, by dereferencing it.
59
60That said, be aware that Perl version 5.14 introduces an exception
61to the rule, for syntactic convenience. Experimental array and hash container
62function behavior allows array and hash references to be handled by Perl as
63if they had been explicitly syntactically dereferenced. See
64L<perl5140delta/"Syntactical Enhancements">
65and L<perlfunc> for details.
66
5a964f20 67=head2 Making References
d74e8afc 68X<reference, creation> X<referencing>
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69
70References can be created in several ways.
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71
72=over 4
73
74=item 1.
d74e8afc 75X<\> X<backslash>
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76
77By using the backslash operator on a variable, subroutine, or value.
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78(This works much like the & (address-of) operator in C.)
79This typically creates I<another> reference to a variable, because
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80there's already a reference to the variable in the symbol table. But
81the symbol table reference might go away, and you'll still have the
82reference that the backslash returned. Here are some examples:
83
84 $scalarref = \$foo;
85 $arrayref = \@ARGV;
86 $hashref = \%ENV;
87 $coderef = \&handler;
55497cff 88 $globref = \*foo;
cb1a09d0 89
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90It isn't possible to create a true reference to an IO handle (filehandle
91or dirhandle) using the backslash operator. The most you can get is a
92reference to a typeglob, which is actually a complete symbol table entry.
93But see the explanation of the C<*foo{THING}> syntax below. However,
94you can still use type globs and globrefs as though they were IO handles.
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95
96=item 2.
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97X<array, anonymous> X<[> X<[]> X<square bracket>
98X<bracket, square> X<arrayref> X<array reference> X<reference, array>
a0d0e21e 99
5a964f20 100A reference to an anonymous array can be created using square
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101brackets:
102
103 $arrayref = [1, 2, ['a', 'b', 'c']];
104
5a964f20 105Here we've created a reference to an anonymous array of three elements
54310121 106whose final element is itself a reference to another anonymous array of three
a0d0e21e 107elements. (The multidimensional syntax described later can be used to
c47ff5f1 108access this. For example, after the above, C<< $arrayref->[2][1] >> would have
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109the value "b".)
110
7c2ea1c7 111Taking a reference to an enumerated list is not the same
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112as using square brackets--instead it's the same as creating
113a list of references!
114
54310121 115 @list = (\$a, \@b, \%c);
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116 @list = \($a, @b, %c); # same thing!
117
54310121 118As a special case, C<\(@foo)> returns a list of references to the contents
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119of C<@foo>, not a reference to C<@foo> itself. Likewise for C<%foo>,
120except that the key references are to copies (since the keys are just
121strings rather than full-fledged scalars).
cb1a09d0 122
a0d0e21e 123=item 3.
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124X<hash, anonymous> X<{> X<{}> X<curly bracket>
125X<bracket, curly> X<brace> X<hashref> X<hash reference> X<reference, hash>
a0d0e21e 126
5a964f20 127A reference to an anonymous hash can be created using curly
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128brackets:
129
130 $hashref = {
131 'Adam' => 'Eve',
132 'Clyde' => 'Bonnie',
133 };
134
5a964f20 135Anonymous hash and array composers like these can be intermixed freely to
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136produce as complicated a structure as you want. The multidimensional
137syntax described below works for these too. The values above are
138literals, but variables and expressions would work just as well, because
139assignment operators in Perl (even within local() or my()) are executable
140statements, not compile-time declarations.
141
142Because curly brackets (braces) are used for several other things
143including BLOCKs, you may occasionally have to disambiguate braces at the
144beginning of a statement by putting a C<+> or a C<return> in front so
145that Perl realizes the opening brace isn't starting a BLOCK. The economy and
146mnemonic value of using curlies is deemed worth this occasional extra
147hassle.
148
149For example, if you wanted a function to make a new hash and return a
150reference to it, you have these options:
151
152 sub hashem { { @_ } } # silently wrong
153 sub hashem { +{ @_ } } # ok
154 sub hashem { return { @_ } } # ok
155
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156On the other hand, if you want the other meaning, you can do this:
157
158 sub showem { { @_ } } # ambiguous (currently ok, but may change)
159 sub showem { {; @_ } } # ok
160 sub showem { { return @_ } } # ok
161
7c2ea1c7 162The leading C<+{> and C<{;> always serve to disambiguate
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163the expression to mean either the HASH reference, or the BLOCK.
164
a0d0e21e 165=item 4.
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166X<subroutine, anonymous> X<subroutine, reference> X<reference, subroutine>
167X<scope, lexical> X<closure> X<lexical> X<lexical scope>
a0d0e21e 168
5a964f20 169A reference to an anonymous subroutine can be created by using
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170C<sub> without a subname:
171
172 $coderef = sub { print "Boink!\n" };
173
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174Note the semicolon. Except for the code
175inside not being immediately executed, a C<sub {}> is not so much a
a0d0e21e 176declaration as it is an operator, like C<do{}> or C<eval{}>. (However, no
5a964f20 177matter how many times you execute that particular line (unless you're in an
19799a22 178C<eval("...")>), $coderef will still have a reference to the I<same>
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179anonymous subroutine.)
180
748a9306 181Anonymous subroutines act as closures with respect to my() variables,
7c2ea1c7 182that is, variables lexically visible within the current scope. Closure
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183is a notion out of the Lisp world that says if you define an anonymous
184function in a particular lexical context, it pretends to run in that
7c2ea1c7 185context even when it's called outside the context.
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186
187In human terms, it's a funny way of passing arguments to a subroutine when
188you define it as well as when you call it. It's useful for setting up
189little bits of code to run later, such as callbacks. You can even
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190do object-oriented stuff with it, though Perl already provides a different
191mechanism to do that--see L<perlobj>.
748a9306 192
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193You might also think of closure as a way to write a subroutine
194template without using eval(). Here's a small example of how
195closures work:
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196
197 sub newprint {
198 my $x = shift;
199 return sub { my $y = shift; print "$x, $y!\n"; };
a0d0e21e 200 }
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201 $h = newprint("Howdy");
202 $g = newprint("Greetings");
203
204 # Time passes...
205
206 &$h("world");
207 &$g("earthlings");
a0d0e21e 208
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209This prints
210
211 Howdy, world!
212 Greetings, earthlings!
213
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214Note particularly that $x continues to refer to the value passed
215into newprint() I<despite> "my $x" having gone out of scope by the
216time the anonymous subroutine runs. That's what a closure is all
217about.
748a9306 218
5a964f20 219This applies only to lexical variables, by the way. Dynamic variables
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220continue to work as they have always worked. Closure is not something
221that most Perl programmers need trouble themselves about to begin with.
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222
223=item 5.
d74e8afc 224X<constructor> X<new>
a0d0e21e 225
63acfd00 226References are often returned by special subroutines called constructors. Perl
227objects are just references to a special type of object that happens to know
228which package it's associated with. Constructors are just special subroutines
229that know how to create that association. They do so by starting with an
230ordinary reference, and it remains an ordinary reference even while it's also
231being an object. Constructors are often named C<new()>. You I<can> call them
232indirectly:
233
234 $objref = new Doggie( Tail => 'short', Ears => 'long' );
235
236But that can produce ambiguous syntax in certain cases, so it's often
237better to use the direct method invocation approach:
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238
239 $objref = Doggie->new(Tail => 'short', Ears => 'long');
240
241 use Term::Cap;
242 $terminal = Term::Cap->Tgetent( { OSPEED => 9600 });
243
244 use Tk;
245 $main = MainWindow->new();
246 $menubar = $main->Frame(-relief => "raised",
247 -borderwidth => 2)
248
a0d0e21e 249=item 6.
d74e8afc 250X<autovivification>
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251
252References of the appropriate type can spring into existence if you
5f05dabc 253dereference them in a context that assumes they exist. Because we haven't
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254talked about dereferencing yet, we can't show you any examples yet.
255
cb1a09d0 256=item 7.
d74e8afc 257X<*foo{THING}> X<*>
cb1a09d0 258
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259A reference can be created by using a special syntax, lovingly known as
260the *foo{THING} syntax. *foo{THING} returns a reference to the THING
261slot in *foo (which is the symbol table entry which holds everything
262known as foo).
cb1a09d0 263
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264 $scalarref = *foo{SCALAR};
265 $arrayref = *ARGV{ARRAY};
266 $hashref = *ENV{HASH};
267 $coderef = *handler{CODE};
36477c24 268 $ioref = *STDIN{IO};
55497cff 269 $globref = *foo{GLOB};
c0bd1adc 270 $formatref = *foo{FORMAT};
55497cff 271
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272All of these are self-explanatory except for C<*foo{IO}>. It returns
273the IO handle, used for file handles (L<perlfunc/open>), sockets
274(L<perlfunc/socket> and L<perlfunc/socketpair>), and directory
275handles (L<perlfunc/opendir>). For compatibility with previous
39b99f21 276versions of Perl, C<*foo{FILEHANDLE}> is a synonym for C<*foo{IO}>, though it
277is deprecated as of 5.8.0. If deprecation warnings are in effect, it will warn
278of its use.
55497cff 279
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280C<*foo{THING}> returns undef if that particular THING hasn't been used yet,
281except in the case of scalars. C<*foo{SCALAR}> returns a reference to an
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282anonymous scalar if $foo hasn't been used yet. This might change in a
283future release.
284
7c2ea1c7 285C<*foo{IO}> is an alternative to the C<*HANDLE> mechanism given in
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286L<perldata/"Typeglobs and Filehandles"> for passing filehandles
287into or out of subroutines, or storing into larger data structures.
288Its disadvantage is that it won't create a new filehandle for you.
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289Its advantage is that you have less risk of clobbering more than
290you want to with a typeglob assignment. (It still conflates file
291and directory handles, though.) However, if you assign the incoming
292value to a scalar instead of a typeglob as we do in the examples
293below, there's no risk of that happening.
36477c24 294
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295 splutter(*STDOUT); # pass the whole glob
296 splutter(*STDOUT{IO}); # pass both file and dir handles
5a964f20 297
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298 sub splutter {
299 my $fh = shift;
300 print $fh "her um well a hmmm\n";
301 }
302
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303 $rec = get_rec(*STDIN); # pass the whole glob
304 $rec = get_rec(*STDIN{IO}); # pass both file and dir handles
5a964f20 305
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306 sub get_rec {
307 my $fh = shift;
308 return scalar <$fh>;
309 }
310
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311=back
312
5a964f20 313=head2 Using References
d74e8afc 314X<reference, use> X<dereferencing> X<dereference>
5a964f20 315
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316That's it for creating references. By now you're probably dying to
317know how to use references to get back to your long-lost data. There
318are several basic methods.
319
320=over 4
321
322=item 1.
323
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324Anywhere you'd put an identifier (or chain of identifiers) as part
325of a variable or subroutine name, you can replace the identifier with
326a simple scalar variable containing a reference of the correct type:
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327
328 $bar = $$scalarref;
329 push(@$arrayref, $filename);
330 $$arrayref[0] = "January";
331 $$hashref{"KEY"} = "VALUE";
332 &$coderef(1,2,3);
cb1a09d0 333 print $globref "output\n";
a0d0e21e 334
19799a22 335It's important to understand that we are specifically I<not> dereferencing
a0d0e21e 336C<$arrayref[0]> or C<$hashref{"KEY"}> there. The dereference of the
19799a22 337scalar variable happens I<before> it does any key lookups. Anything more
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338complicated than a simple scalar variable must use methods 2 or 3 below.
339However, a "simple scalar" includes an identifier that itself uses method
3401 recursively. Therefore, the following prints "howdy".
341
342 $refrefref = \\\"howdy";
343 print $$$$refrefref;
344
345=item 2.
346
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347Anywhere you'd put an identifier (or chain of identifiers) as part of a
348variable or subroutine name, you can replace the identifier with a
349BLOCK returning a reference of the correct type. In other words, the
350previous examples could be written like this:
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351
352 $bar = ${$scalarref};
353 push(@{$arrayref}, $filename);
354 ${$arrayref}[0] = "January";
355 ${$hashref}{"KEY"} = "VALUE";
356 &{$coderef}(1,2,3);
36477c24 357 $globref->print("output\n"); # iff IO::Handle is loaded
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358
359Admittedly, it's a little silly to use the curlies in this case, but
360the BLOCK can contain any arbitrary expression, in particular,
361subscripted expressions:
362
54310121 363 &{ $dispatch{$index} }(1,2,3); # call correct routine
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364
365Because of being able to omit the curlies for the simple case of C<$$x>,
366people often make the mistake of viewing the dereferencing symbols as
367proper operators, and wonder about their precedence. If they were,
5f05dabc 368though, you could use parentheses instead of braces. That's not the case.
a0d0e21e 369Consider the difference below; case 0 is a short-hand version of case 1,
19799a22 370I<not> case 2:
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371
372 $$hashref{"KEY"} = "VALUE"; # CASE 0
373 ${$hashref}{"KEY"} = "VALUE"; # CASE 1
374 ${$hashref{"KEY"}} = "VALUE"; # CASE 2
375 ${$hashref->{"KEY"}} = "VALUE"; # CASE 3
376
377Case 2 is also deceptive in that you're accessing a variable
378called %hashref, not dereferencing through $hashref to the hash
379it's presumably referencing. That would be case 3.
380
381=item 3.
382
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383Subroutine calls and lookups of individual array elements arise often
384enough that it gets cumbersome to use method 2. As a form of
385syntactic sugar, the examples for method 2 may be written:
a0d0e21e 386
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387 $arrayref->[0] = "January"; # Array element
388 $hashref->{"KEY"} = "VALUE"; # Hash element
389 $coderef->(1,2,3); # Subroutine call
a0d0e21e 390
6da72b64 391The left side of the arrow can be any expression returning a reference,
19799a22 392including a previous dereference. Note that C<$array[$x]> is I<not> the
c47ff5f1 393same thing as C<< $array->[$x] >> here:
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394
395 $array[$x]->{"foo"}->[0] = "January";
396
397This is one of the cases we mentioned earlier in which references could
398spring into existence when in an lvalue context. Before this
399statement, C<$array[$x]> may have been undefined. If so, it's
400automatically defined with a hash reference so that we can look up
c47ff5f1 401C<{"foo"}> in it. Likewise C<< $array[$x]->{"foo"} >> will automatically get
a0d0e21e 402defined with an array reference so that we can look up C<[0]> in it.
5a964f20 403This process is called I<autovivification>.
a0d0e21e 404
19799a22 405One more thing here. The arrow is optional I<between> brackets
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406subscripts, so you can shrink the above down to
407
408 $array[$x]{"foo"}[0] = "January";
409
410Which, in the degenerate case of using only ordinary arrays, gives you
411multidimensional arrays just like C's:
412
413 $score[$x][$y][$z] += 42;
414
415Well, okay, not entirely like C's arrays, actually. C doesn't know how
416to grow its arrays on demand. Perl does.
417
418=item 4.
419
420If a reference happens to be a reference to an object, then there are
421probably methods to access the things referred to, and you should probably
422stick to those methods unless you're in the class package that defines the
423object's methods. In other words, be nice, and don't violate the object's
424encapsulation without a very good reason. Perl does not enforce
425encapsulation. We are not totalitarians here. We do expect some basic
426civility though.
427
428=back
429
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430Using a string or number as a reference produces a symbolic reference,
431as explained above. Using a reference as a number produces an
432integer representing its storage location in memory. The only
433useful thing to be done with this is to compare two references
434numerically to see whether they refer to the same location.
d74e8afc 435X<reference, numeric context>
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436
437 if ($ref1 == $ref2) { # cheap numeric compare of references
438 print "refs 1 and 2 refer to the same thing\n";
439 }
440
441Using a reference as a string produces both its referent's type,
442including any package blessing as described in L<perlobj>, as well
443as the numeric address expressed in hex. The ref() operator returns
444just the type of thing the reference is pointing to, without the
445address. See L<perlfunc/ref> for details and examples of its use.
d74e8afc 446X<reference, string context>
a0d0e21e 447
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448The bless() operator may be used to associate the object a reference
449points to with a package functioning as an object class. See L<perlobj>.
a0d0e21e 450
5f05dabc 451A typeglob may be dereferenced the same way a reference can, because
7c2ea1c7 452the dereference syntax always indicates the type of reference desired.
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453So C<${*foo}> and C<${\$foo}> both indicate the same scalar variable.
454
455Here's a trick for interpolating a subroutine call into a string:
456
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457 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
458
459The way it works is that when the C<@{...}> is seen in the double-quoted
460string, it's evaluated as a block. The block creates a reference to an
461anonymous array containing the results of the call to C<mysub(1,2,3)>. So
462the whole block returns a reference to an array, which is then
463dereferenced by C<@{...}> and stuck into the double-quoted string. This
464chicanery is also useful for arbitrary expressions:
a0d0e21e 465
184e9718 466 print "That yields @{[$n + 5]} widgets\n";
a0d0e21e 467
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468Similarly, an expression that returns a reference to a scalar can be
469dereferenced via C<${...}>. Thus, the above expression may be written
470as:
471
472 print "That yields ${\($n + 5)} widgets\n";
473
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474=head2 Circular References
475X<circular reference> X<reference, circular>
476
477It is possible to create a "circular reference" in Perl, which can lead
478to memory leaks. A circular reference occurs when two references
479contain a reference to each other, like this:
480
481 my $foo = {};
482 my $bar = { foo => $foo };
483 $foo->{bar} = $bar;
484
485You can also create a circular reference with a single variable:
486
487 my $foo;
488 $foo = \$foo;
489
490In this case, the reference count for the variables will never reach 0,
491and the references will never be garbage-collected. This can lead to
492memory leaks.
493
494Because objects in Perl are implemented as references, it's possible to
495have circular references with objects as well. Imagine a TreeNode class
496where each node references its parent and child nodes. Any node with a
497parent will be part of a circular reference.
498
499You can break circular references by creating a "weak reference". A
500weak reference does not increment the reference count for a variable,
501which means that the object can go out of scope and be destroyed. You
502can weaken a reference with the C<weaken> function exported by the
503L<Scalar::Util> module.
504
505Here's how we can make the first example safer:
506
507 use Scalar::Util 'weaken';
508
509 my $foo = {};
510 my $bar = { foo => $foo };
511 $foo->{bar} = $bar;
512
513 weaken $foo->{bar};
514
515The reference from C<$foo> to C<$bar> has been weakened. When the
516C<$bar> variable goes out of scope, it will be garbage-collected. The
517next time you look at the value of the C<< $foo->{bar} >> key, it will
518be C<undef>.
519
520This action at a distance can be confusing, so you should be careful
521with your use of weaken. You should weaken the reference in the
522variable that will go out of scope I<first>. That way, the longer-lived
523variable will contain the expected reference until it goes out of
524scope.
525
a0d0e21e 526=head2 Symbolic references
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527X<reference, symbolic> X<reference, soft>
528X<symbolic reference> X<soft reference>
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529
530We said that references spring into existence as necessary if they are
531undefined, but we didn't say what happens if a value used as a
19799a22 532reference is already defined, but I<isn't> a hard reference. If you
7c2ea1c7 533use it as a reference, it'll be treated as a symbolic
19799a22 534reference. That is, the value of the scalar is taken to be the I<name>
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535of a variable, rather than a direct link to a (possibly) anonymous
536value.
537
538People frequently expect it to work like this. So it does.
539
540 $name = "foo";
541 $$name = 1; # Sets $foo
542 ${$name} = 2; # Sets $foo
543 ${$name x 2} = 3; # Sets $foofoo
544 $name->[0] = 4; # Sets $foo[0]
545 @$name = (); # Clears @foo
546 &$name(); # Calls &foo() (as in Perl 4)
547 $pack = "THAT";
548 ${"${pack}::$name"} = 5; # Sets $THAT::foo without eval
549
7c2ea1c7 550This is powerful, and slightly dangerous, in that it's possible
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551to intend (with the utmost sincerity) to use a hard reference, and
552accidentally use a symbolic reference instead. To protect against
553that, you can say
554
555 use strict 'refs';
556
557and then only hard references will be allowed for the rest of the enclosing
54310121 558block. An inner block may countermand that with
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559
560 no strict 'refs';
561
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562Only package variables (globals, even if localized) are visible to
563symbolic references. Lexical variables (declared with my()) aren't in
564a symbol table, and thus are invisible to this mechanism. For example:
a0d0e21e 565
5a964f20 566 local $value = 10;
b0c35547 567 $ref = "value";
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568 {
569 my $value = 20;
570 print $$ref;
54310121 571 }
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572
573This will still print 10, not 20. Remember that local() affects package
574variables, which are all "global" to the package.
575
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576=head2 Not-so-symbolic references
577
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578Since Perl verion 5.001, brackets around a symbolic reference can simply
579serve to isolate an identifier or variable name from the rest of an
580expression, just as they always have within a string. For example,
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581
582 $push = "pop on ";
583 print "${push}over";
584
7c2ea1c7 585has always meant to print "pop on over", even though push is
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586a reserved word. In 5.001, this was generalized to work the same
587without the enclosing double quotes, so that
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588
589 print ${push} . "over";
590
591and even
592
593 print ${ push } . "over";
594
595will have the same effect. (This would have been a syntax error in
7c2ea1c7 596Perl 5.000, though Perl 4 allowed it in the spaceless form.) This
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597construct is I<not> considered to be a symbolic reference when you're
598using strict refs:
599
600 use strict 'refs';
601 ${ bareword }; # Okay, means $bareword.
602 ${ "bareword" }; # Error, symbolic reference.
603
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604Similarly, because of all the subscripting that is done using single words,
605the same rule applies to any bareword that is used for subscripting a hash.
606So now, instead of writing
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607
608 $array{ "aaa" }{ "bbb" }{ "ccc" }
609
5f05dabc 610you can write just
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611
612 $array{ aaa }{ bbb }{ ccc }
613
614and not worry about whether the subscripts are reserved words. In the
615rare event that you do wish to do something like
616
617 $array{ shift }
618
619you can force interpretation as a reserved word by adding anything that
620makes it more than a bareword:
621
622 $array{ shift() }
623 $array{ +shift }
624 $array{ shift @_ }
625
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626The C<use warnings> pragma or the B<-w> switch will warn you if it
627interprets a reserved word as a string.
5f05dabc 628But it will no longer warn you about using lowercase words, because the
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629string is effectively quoted.
630
49399b3f 631=head2 Pseudo-hashes: Using an array as a hash
d74e8afc 632X<pseudo-hash> X<pseudo hash> X<pseudohash>
49399b3f 633
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634Pseudo-hashes have been removed from Perl. The 'fields' pragma
635remains available.
e0478e5a 636
5a964f20 637=head2 Function Templates
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638X<scope, lexical> X<closure> X<lexical> X<lexical scope>
639X<subroutine, nested> X<sub, nested> X<subroutine, local> X<sub, local>
5a964f20 640
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641As explained above, an anonymous function with access to the lexical
642variables visible when that function was compiled, creates a closure. It
643retains access to those variables even though it doesn't get run until
644later, such as in a signal handler or a Tk callback.
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645
646Using a closure as a function template allows us to generate many functions
c2611fb3 647that act similarly. Suppose you wanted functions named after the colors
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648that generated HTML font changes for the various colors:
649
650 print "Be ", red("careful"), "with that ", green("light");
651
7c2ea1c7 652The red() and green() functions would be similar. To create these,
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653we'll assign a closure to a typeglob of the name of the function we're
654trying to build.
655
656 @colors = qw(red blue green yellow orange purple violet);
657 for my $name (@colors) {
658 no strict 'refs'; # allow symbol table manipulation
659 *$name = *{uc $name} = sub { "<FONT COLOR='$name'>@_</FONT>" };
660 }
661
662Now all those different functions appear to exist independently. You can
663call red(), RED(), blue(), BLUE(), green(), etc. This technique saves on
664both compile time and memory use, and is less error-prone as well, since
665syntax checks happen at compile time. It's critical that any variables in
666the anonymous subroutine be lexicals in order to create a proper closure.
667That's the reasons for the C<my> on the loop iteration variable.
668
669This is one of the only places where giving a prototype to a closure makes
670much sense. If you wanted to impose scalar context on the arguments of
671these functions (probably not a wise idea for this particular example),
672you could have written it this way instead:
673
674 *$name = sub ($) { "<FONT COLOR='$name'>$_[0]</FONT>" };
675
676However, since prototype checking happens at compile time, the assignment
677above happens too late to be of much use. You could address this by
678putting the whole loop of assignments within a BEGIN block, forcing it
679to occur during compilation.
680
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681Access to lexicals that change over time--like those in the C<for> loop
682above, basically aliases to elements from the surrounding lexical scopes--
683only works with anonymous subs, not with named subroutines. Generally
684said, named subroutines do not nest properly and should only be declared
685in the main package scope.
686
687This is because named subroutines are created at compile time so their
688lexical variables get assigned to the parent lexicals from the first
689execution of the parent block. If a parent scope is entered a second
690time, its lexicals are created again, while the nested subs still
691reference the old ones.
692
693Anonymous subroutines get to capture each time you execute the C<sub>
694operator, as they are created on the fly. If you are accustomed to using
695nested subroutines in other programming languages with their own private
696variables, you'll have to work at it a bit in Perl. The intuitive coding
697of this type of thing incurs mysterious warnings about "will not stay
698shared" due to the reasons explained above.
699For example, this won't work:
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700
701 sub outer {
702 my $x = $_[0] + 35;
703 sub inner { return $x * 19 } # WRONG
704 return $x + inner();
b432a672 705 }
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706
707A work-around is the following:
708
709 sub outer {
710 my $x = $_[0] + 35;
711 local *inner = sub { return $x * 19 };
712 return $x + inner();
b432a672 713 }
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714
715Now inner() can only be called from within outer(), because of the
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716temporary assignments of the anonymous subroutine. But when it does,
717it has normal access to the lexical variable $x from the scope of
718outer() at the time outer is invoked.
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719
720This has the interesting effect of creating a function local to another
721function, something not normally supported in Perl.
722
cb1a09d0 723=head1 WARNING
d74e8afc 724X<reference, string context> X<reference, use as hash key>
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725
726You may not (usefully) use a reference as the key to a hash. It will be
727converted into a string:
728
729 $x{ \$a } = $a;
730
54310121 731If you try to dereference the key, it won't do a hard dereference, and
184e9718 732you won't accomplish what you're attempting. You might want to do something
cb1a09d0 733more like
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735 $r = \@a;
736 $x{ $r } = $r;
737
738And then at least you can use the values(), which will be
739real refs, instead of the keys(), which won't.
740
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741The standard Tie::RefHash module provides a convenient workaround to this.
742
cb1a09d0 743=head1 SEE ALSO
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744
745Besides the obvious documents, source code can be instructive.
7c2ea1c7 746Some pathological examples of the use of references can be found
a0d0e21e 747in the F<t/op/ref.t> regression test in the Perl source directory.
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748
749See also L<perldsc> and L<perllol> for how to use references to create
82e1c0d9 750complex data structures, and L<perlootut> and L<perlobj>
5a964f20 751for how to use them to create objects.