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1 | =head1 NAME |
2 | |
3 | perlref - Perl references and nested data structures |
4 | |
5 | =head1 DESCRIPTION |
6 | |
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7 | Before release 5 of Perl it was difficult to represent complex data |
8 | structures, because all references had to be symbolic, and even that was |
9 | difficult to do when you wanted to refer to a variable rather than a |
10 | symbol table entry. Perl 5 not only makes it easier to use symbolic |
11 | references to variables, but lets you have "hard" references to any piece |
12 | of data. Any scalar may hold a hard reference. Since arrays and hashes |
13 | contain scalars, you can now easily build arrays of arrays, arrays of |
14 | hashes, hashes of arrays, arrays of hashes of functions, and so on. |
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15 | |
16 | Hard references are smart--they keep track of reference counts for you, |
17 | automatically freeing the thing referred to when its reference count |
18 | goes to zero. If that thing happens to be an object, the object is |
19 | destructed. See L<perlobj> for more about objects. (In a sense, |
20 | everything in Perl is an object, but we usually reserve the word for |
21 | references to objects that have been officially "blessed" into a class package.) |
22 | |
23 | A symbolic reference contains the name of a variable, just as a |
24 | symbolic link in the filesystem merely contains the name of a file. |
25 | The C<*glob> notation is a kind of symbolic reference. Hard references |
26 | are more like hard links in the file system: merely another way |
27 | at getting at the same underlying object, irrespective of its name. |
28 | |
29 | "Hard" references are easy to use in Perl. There is just one |
30 | overriding principle: Perl does no implicit referencing or |
31 | dereferencing. When a scalar is holding a reference, it always behaves |
32 | as a scalar. It doesn't magically start being an array or a hash |
33 | unless you tell it so explicitly by dereferencing it. |
34 | |
35 | References can be constructed several ways. |
36 | |
37 | =over 4 |
38 | |
39 | =item 1. |
40 | |
41 | By using the backslash operator on a variable, subroutine, or value. |
42 | (This works much like the & (address-of) operator works in C.) Note |
43 | that this typically creates I<ANOTHER> reference to a variable, since |
44 | there's already a reference to the variable in the symbol table. But |
45 | the symbol table reference might go away, and you'll still have the |
46 | reference that the backslash returned. Here are some examples: |
47 | |
48 | $scalarref = \$foo; |
49 | $arrayref = \@ARGV; |
50 | $hashref = \%ENV; |
51 | $coderef = \&handler; |
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52 | $globref = \*STDOUT; |
53 | |
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54 | |
55 | =item 2. |
56 | |
57 | A reference to an anonymous array can be constructed using square |
58 | brackets: |
59 | |
60 | $arrayref = [1, 2, ['a', 'b', 'c']]; |
61 | |
62 | Here we've constructed a reference to an anonymous array of three elements |
63 | whose final element is itself reference to another anonymous array of three |
64 | elements. (The multidimensional syntax described later can be used to |
65 | access this. For example, after the above, $arrayref->[2][1] would have |
66 | the value "b".) |
67 | |
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68 | Note that taking a reference to an enumerated list is not the same |
69 | as using square brackets--instead it's the same as creating |
70 | a list of references! |
71 | |
72 | @list = (\$a, \$b, \$c); |
73 | @list = \($a, $b, $c); # same thing! |
74 | |
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75 | =item 3. |
76 | |
77 | A reference to an anonymous hash can be constructed using curly |
78 | brackets: |
79 | |
80 | $hashref = { |
81 | 'Adam' => 'Eve', |
82 | 'Clyde' => 'Bonnie', |
83 | }; |
84 | |
85 | Anonymous hash and array constructors can be intermixed freely to |
86 | produce as complicated a structure as you want. The multidimensional |
87 | syntax described below works for these too. The values above are |
88 | literals, but variables and expressions would work just as well, because |
89 | assignment operators in Perl (even within local() or my()) are executable |
90 | statements, not compile-time declarations. |
91 | |
92 | Because curly brackets (braces) are used for several other things |
93 | including BLOCKs, you may occasionally have to disambiguate braces at the |
94 | beginning of a statement by putting a C<+> or a C<return> in front so |
95 | that Perl realizes the opening brace isn't starting a BLOCK. The economy and |
96 | mnemonic value of using curlies is deemed worth this occasional extra |
97 | hassle. |
98 | |
99 | For example, if you wanted a function to make a new hash and return a |
100 | reference to it, you have these options: |
101 | |
102 | sub hashem { { @_ } } # silently wrong |
103 | sub hashem { +{ @_ } } # ok |
104 | sub hashem { return { @_ } } # ok |
105 | |
106 | =item 4. |
107 | |
108 | A reference to an anonymous subroutine can be constructed by using |
109 | C<sub> without a subname: |
110 | |
111 | $coderef = sub { print "Boink!\n" }; |
112 | |
113 | Note the presence of the semicolon. Except for the fact that the code |
114 | inside isn't executed immediately, a C<sub {}> is not so much a |
115 | declaration as it is an operator, like C<do{}> or C<eval{}>. (However, no |
116 | matter how many times you execute that line (unless you're in an |
117 | C<eval("...")>), C<$coderef> will still have a reference to the I<SAME> |
118 | anonymous subroutine.) |
119 | |
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120 | Anonymous subroutines act as closures with respect to my() variables, |
121 | that is, variables visible lexically within the current scope. Closure |
122 | is a notion out of the Lisp world that says if you define an anonymous |
123 | function in a particular lexical context, it pretends to run in that |
124 | context even when it's called outside of the context. |
125 | |
126 | In human terms, it's a funny way of passing arguments to a subroutine when |
127 | you define it as well as when you call it. It's useful for setting up |
128 | little bits of code to run later, such as callbacks. You can even |
129 | do object-oriented stuff with it, though Perl provides a different |
130 | mechanism to do that already--see L<perlobj>. |
131 | |
132 | You can also think of closure as a way to write a subroutine template without |
133 | using eval. (In fact, in version 5.000, eval was the I<only> way to get |
134 | closures. You may wish to use "require 5.001" if you use closures.) |
135 | |
136 | Here's a small example of how closures works: |
137 | |
138 | sub newprint { |
139 | my $x = shift; |
140 | return sub { my $y = shift; print "$x, $y!\n"; }; |
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141 | } |
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142 | $h = newprint("Howdy"); |
143 | $g = newprint("Greetings"); |
144 | |
145 | # Time passes... |
146 | |
147 | &$h("world"); |
148 | &$g("earthlings"); |
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149 | |
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150 | This prints |
151 | |
152 | Howdy, world! |
153 | Greetings, earthlings! |
154 | |
155 | Note particularly that $x continues to refer to the value passed into |
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156 | newprint() I<despite> the fact that the "my $x" has seemingly gone out of |
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157 | scope by the time the anonymous subroutine runs. That's what closure |
158 | is all about. |
159 | |
160 | This only applies to lexical variables, by the way. Dynamic variables |
161 | continue to work as they have always worked. Closure is not something |
162 | that most Perl programmers need trouble themselves about to begin with. |
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163 | |
164 | =item 5. |
165 | |
166 | References are often returned by special subroutines called constructors. |
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167 | Perl objects are just references to a special kind of object that happens to know |
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168 | which package it's associated with. Constructors are just special |
169 | subroutines that know how to create that association. They do so by |
170 | starting with an ordinary reference, and it remains an ordinary reference |
171 | even while it's also being an object. Constructors are customarily |
172 | named new(), but don't have to be: |
173 | |
174 | $objref = new Doggie (Tail => 'short', Ears => 'long'); |
175 | |
176 | =item 6. |
177 | |
178 | References of the appropriate type can spring into existence if you |
179 | dereference them in a context that assumes they exist. Since we haven't |
180 | talked about dereferencing yet, we can't show you any examples yet. |
181 | |
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182 | =item 7. |
183 | |
184 | References to filehandles can be created by taking a reference to |
185 | a typeglob. This is currently the best way to pass filehandles into or |
186 | out of subroutines, or to store them in larger data structures. |
187 | |
188 | splutter(\*STDOUT); |
189 | sub splutter { |
190 | my $fh = shift; |
191 | print $fh "her um well a hmmm\n"; |
192 | } |
193 | |
194 | $rec = get_rec(\*STDIN); |
195 | sub get_rec { |
196 | my $fh = shift; |
197 | return scalar <$fh>; |
198 | } |
199 | |
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200 | =back |
201 | |
202 | That's it for creating references. By now you're probably dying to |
203 | know how to use references to get back to your long-lost data. There |
204 | are several basic methods. |
205 | |
206 | =over 4 |
207 | |
208 | =item 1. |
209 | |
210 | Anywhere you'd put an identifier as part of a variable or subroutine |
211 | name, you can replace the identifier with a simple scalar variable |
212 | containing a reference of the correct type: |
213 | |
214 | $bar = $$scalarref; |
215 | push(@$arrayref, $filename); |
216 | $$arrayref[0] = "January"; |
217 | $$hashref{"KEY"} = "VALUE"; |
218 | &$coderef(1,2,3); |
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219 | print $globref "output\n"; |
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220 | |
221 | It's important to understand that we are specifically I<NOT> dereferencing |
222 | C<$arrayref[0]> or C<$hashref{"KEY"}> there. The dereference of the |
223 | scalar variable happens I<BEFORE> it does any key lookups. Anything more |
224 | complicated than a simple scalar variable must use methods 2 or 3 below. |
225 | However, a "simple scalar" includes an identifier that itself uses method |
226 | 1 recursively. Therefore, the following prints "howdy". |
227 | |
228 | $refrefref = \\\"howdy"; |
229 | print $$$$refrefref; |
230 | |
231 | =item 2. |
232 | |
233 | Anywhere you'd put an identifier as part of a variable or subroutine |
234 | name, you can replace the identifier with a BLOCK returning a reference |
235 | of the correct type. In other words, the previous examples could be |
236 | written like this: |
237 | |
238 | $bar = ${$scalarref}; |
239 | push(@{$arrayref}, $filename); |
240 | ${$arrayref}[0] = "January"; |
241 | ${$hashref}{"KEY"} = "VALUE"; |
242 | &{$coderef}(1,2,3); |
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243 | $globref->print("output\n"); # iff you use FileHandle |
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244 | |
245 | Admittedly, it's a little silly to use the curlies in this case, but |
246 | the BLOCK can contain any arbitrary expression, in particular, |
247 | subscripted expressions: |
248 | |
249 | &{ $dispatch{$index} }(1,2,3); # call correct routine |
250 | |
251 | Because of being able to omit the curlies for the simple case of C<$$x>, |
252 | people often make the mistake of viewing the dereferencing symbols as |
253 | proper operators, and wonder about their precedence. If they were, |
254 | though, you could use parens instead of braces. That's not the case. |
255 | Consider the difference below; case 0 is a short-hand version of case 1, |
256 | I<NOT> case 2: |
257 | |
258 | $$hashref{"KEY"} = "VALUE"; # CASE 0 |
259 | ${$hashref}{"KEY"} = "VALUE"; # CASE 1 |
260 | ${$hashref{"KEY"}} = "VALUE"; # CASE 2 |
261 | ${$hashref->{"KEY"}} = "VALUE"; # CASE 3 |
262 | |
263 | Case 2 is also deceptive in that you're accessing a variable |
264 | called %hashref, not dereferencing through $hashref to the hash |
265 | it's presumably referencing. That would be case 3. |
266 | |
267 | =item 3. |
268 | |
269 | The case of individual array elements arises often enough that it gets |
270 | cumbersome to use method 2. As a form of syntactic sugar, the two |
271 | lines like that above can be written: |
272 | |
273 | $arrayref->[0] = "January"; |
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274 | $hashref->{"KEY"} = "VALUE"; |
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275 | |
276 | The left side of the array can be any expression returning a reference, |
277 | including a previous dereference. Note that C<$array[$x]> is I<NOT> the |
278 | same thing as C<$array-E<gt>[$x]> here: |
279 | |
280 | $array[$x]->{"foo"}->[0] = "January"; |
281 | |
282 | This is one of the cases we mentioned earlier in which references could |
283 | spring into existence when in an lvalue context. Before this |
284 | statement, C<$array[$x]> may have been undefined. If so, it's |
285 | automatically defined with a hash reference so that we can look up |
286 | C<{"foo"}> in it. Likewise C<$array[$x]-E<gt>{"foo"}> will automatically get |
287 | defined with an array reference so that we can look up C<[0]> in it. |
288 | |
289 | One more thing here. The arrow is optional I<BETWEEN> brackets |
290 | subscripts, so you can shrink the above down to |
291 | |
292 | $array[$x]{"foo"}[0] = "January"; |
293 | |
294 | Which, in the degenerate case of using only ordinary arrays, gives you |
295 | multidimensional arrays just like C's: |
296 | |
297 | $score[$x][$y][$z] += 42; |
298 | |
299 | Well, okay, not entirely like C's arrays, actually. C doesn't know how |
300 | to grow its arrays on demand. Perl does. |
301 | |
302 | =item 4. |
303 | |
304 | If a reference happens to be a reference to an object, then there are |
305 | probably methods to access the things referred to, and you should probably |
306 | stick to those methods unless you're in the class package that defines the |
307 | object's methods. In other words, be nice, and don't violate the object's |
308 | encapsulation without a very good reason. Perl does not enforce |
309 | encapsulation. We are not totalitarians here. We do expect some basic |
310 | civility though. |
311 | |
312 | =back |
313 | |
314 | The ref() operator may be used to determine what type of thing the |
315 | reference is pointing to. See L<perlfunc>. |
316 | |
317 | The bless() operator may be used to associate a reference with a package |
318 | functioning as an object class. See L<perlobj>. |
319 | |
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320 | A typeglob may be dereferenced the same way a reference can, since |
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321 | the dereference syntax always indicates the kind of reference desired. |
322 | So C<${*foo}> and C<${\$foo}> both indicate the same scalar variable. |
323 | |
324 | Here's a trick for interpolating a subroutine call into a string: |
325 | |
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326 | print "My sub returned @{[mysub(1,2,3)]} that time.\n"; |
327 | |
328 | The way it works is that when the C<@{...}> is seen in the double-quoted |
329 | string, it's evaluated as a block. The block creates a reference to an |
330 | anonymous array containing the results of the call to C<mysub(1,2,3)>. So |
331 | the whole block returns a reference to an array, which is then |
332 | dereferenced by C<@{...}> and stuck into the double-quoted string. This |
333 | chicanery is also useful for arbitrary expressions: |
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334 | |
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335 | print "That yeilds @{[$n + 5]} widgets\n"; |
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336 | |
337 | =head2 Symbolic references |
338 | |
339 | We said that references spring into existence as necessary if they are |
340 | undefined, but we didn't say what happens if a value used as a |
341 | reference is already defined, but I<ISN'T> a hard reference. If you |
342 | use it as a reference in this case, it'll be treated as a symbolic |
343 | reference. That is, the value of the scalar is taken to be the I<NAME> |
344 | of a variable, rather than a direct link to a (possibly) anonymous |
345 | value. |
346 | |
347 | People frequently expect it to work like this. So it does. |
348 | |
349 | $name = "foo"; |
350 | $$name = 1; # Sets $foo |
351 | ${$name} = 2; # Sets $foo |
352 | ${$name x 2} = 3; # Sets $foofoo |
353 | $name->[0] = 4; # Sets $foo[0] |
354 | @$name = (); # Clears @foo |
355 | &$name(); # Calls &foo() (as in Perl 4) |
356 | $pack = "THAT"; |
357 | ${"${pack}::$name"} = 5; # Sets $THAT::foo without eval |
358 | |
359 | This is very powerful, and slightly dangerous, in that it's possible |
360 | to intend (with the utmost sincerity) to use a hard reference, and |
361 | accidentally use a symbolic reference instead. To protect against |
362 | that, you can say |
363 | |
364 | use strict 'refs'; |
365 | |
366 | and then only hard references will be allowed for the rest of the enclosing |
367 | block. An inner block may countermand that with |
368 | |
369 | no strict 'refs'; |
370 | |
371 | Only package variables are visible to symbolic references. Lexical |
372 | variables (declared with my()) aren't in a symbol table, and thus are |
373 | invisible to this mechanism. For example: |
374 | |
375 | local($value) = 10; |
376 | $ref = \$value; |
377 | { |
378 | my $value = 20; |
379 | print $$ref; |
380 | } |
381 | |
382 | This will still print 10, not 20. Remember that local() affects package |
383 | variables, which are all "global" to the package. |
384 | |
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385 | =head2 Not-so-symbolic references |
386 | |
387 | A new feature contributing to readability in 5.001 is that the brackets |
388 | around a symbolic reference behave more like quotes, just as they |
389 | always have within a string. That is, |
390 | |
391 | $push = "pop on "; |
392 | print "${push}over"; |
393 | |
394 | has always meant to print "pop on over", despite the fact that push is |
395 | a reserved word. This has been generalized to work the same outside |
396 | of quotes, so that |
397 | |
398 | print ${push} . "over"; |
399 | |
400 | and even |
401 | |
402 | print ${ push } . "over"; |
403 | |
404 | will have the same effect. (This would have been a syntax error in |
405 | 5.000, though Perl 4 allowed it in the spaceless form.) Note that this |
406 | construct is I<not> considered to be a symbolic reference when you're |
407 | using strict refs: |
408 | |
409 | use strict 'refs'; |
410 | ${ bareword }; # Okay, means $bareword. |
411 | ${ "bareword" }; # Error, symbolic reference. |
412 | |
413 | Similarly, because of all the subscripting that is done using single |
414 | words, we've applied the same rule to any bareword that is used for |
415 | subscripting a hash. So now, instead of writing |
416 | |
417 | $array{ "aaa" }{ "bbb" }{ "ccc" } |
418 | |
419 | you can just write |
420 | |
421 | $array{ aaa }{ bbb }{ ccc } |
422 | |
423 | and not worry about whether the subscripts are reserved words. In the |
424 | rare event that you do wish to do something like |
425 | |
426 | $array{ shift } |
427 | |
428 | you can force interpretation as a reserved word by adding anything that |
429 | makes it more than a bareword: |
430 | |
431 | $array{ shift() } |
432 | $array{ +shift } |
433 | $array{ shift @_ } |
434 | |
435 | The B<-w> switch will warn you if it interprets a reserved word as a string. |
436 | But it will no longer warn you about using lowercase words, since the |
437 | string is effectively quoted. |
438 | |
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439 | =head1 WARNING |
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440 | |
441 | You may not (usefully) use a reference as the key to a hash. It will be |
442 | converted into a string: |
443 | |
444 | $x{ \$a } = $a; |
445 | |
446 | If you try to dereference the key, it won't do a hard dereference, and |
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447 | you won't accomplish what you're attemping. You might want to do something |
448 | more like |
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449 | |
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450 | $r = \@a; |
451 | $x{ $r } = $r; |
452 | |
453 | And then at least you can use the values(), which will be |
454 | real refs, instead of the keys(), which won't. |
455 | |
456 | =head1 SEE ALSO |
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457 | |
458 | Besides the obvious documents, source code can be instructive. |
459 | Some rather pathological examples of the use of references can be found |
460 | in the F<t/op/ref.t> regression test in the Perl source directory. |
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461 | |
462 | See also L<perldsc> and L<perllol> for how to use references to create |
463 | complex data structures, and L<perlobj> for how to use them to create |
464 | objects. |