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1 | =head1 NAME |
2 | X<reference> X<pointer> X<data structure> X<structure> X<struct> | |
3 | ||
4 | perlref - Perl references and nested data structures | |
5 | ||
6 | =head1 NOTE | |
7 | ||
8 | This is complete documentation about all aspects of references. | |
9 | For a shorter, tutorial introduction to just the essential features, | |
10 | see L<perlreftut>. | |
11 | ||
12 | =head1 DESCRIPTION | |
13 | ||
14 | Before release 5 of Perl it was difficult to represent complex data | |
15 | structures, because all references had to be symbolic--and even then | |
16 | it was difficult to refer to a variable instead of a symbol table entry. | |
17 | Perl now not only makes it easier to use symbolic references to variables, | |
18 | but also lets you have "hard" references to any piece of data or code. | |
19 | Any scalar may hold a hard reference. Because arrays and hashes contain | |
20 | scalars, you can now easily build arrays of arrays, arrays of hashes, | |
21 | hashes of arrays, arrays of hashes of functions, and so on. | |
22 | ||
23 | Hard references are smart--they keep track of reference counts for you, | |
24 | automatically freeing the thing referred to when its reference count goes | |
25 | to zero. (Reference counts for values in self-referential or | |
26 | cyclic data structures may not go to zero without a little help; see | |
27 | L<perlobj/"Two-Phased Garbage Collection"> for a detailed explanation.) | |
28 | If that thing happens to be an object, the object is destructed. See | |
29 | L<perlobj> for more about objects. (In a sense, everything in Perl is an | |
30 | object, but we usually reserve the word for references to objects that | |
31 | have been officially "blessed" into a class package.) | |
32 | ||
33 | Symbolic references are names of variables or other objects, just as a | |
34 | symbolic link in a Unix filesystem contains merely the name of a file. | |
35 | The C<*glob> notation is something of a symbolic reference. (Symbolic | |
36 | references are sometimes called "soft references", but please don't call | |
37 | them that; references are confusing enough without useless synonyms.) | |
38 | X<reference, symbolic> X<reference, soft> | |
39 | X<symbolic reference> X<soft reference> | |
40 | ||
41 | In contrast, hard references are more like hard links in a Unix file | |
42 | system: They are used to access an underlying object without concern for | |
43 | what its (other) name is. When the word "reference" is used without an | |
44 | adjective, as in the following paragraph, it is usually talking about a | |
45 | hard reference. | |
46 | X<reference, hard> X<hard reference> | |
47 | ||
48 | References are easy to use in Perl. There is just one overriding | |
49 | principle: Perl does no implicit referencing or dereferencing. When a | |
50 | scalar is holding a reference, it always behaves as a simple scalar. It | |
51 | doesn't magically start being an array or hash or subroutine; you have to | |
52 | tell it explicitly to do so, by dereferencing it. | |
53 | ||
54 | =head2 Making References | |
55 | X<reference, creation> X<referencing> | |
56 | ||
57 | References can be created in several ways. | |
58 | ||
59 | =over 4 | |
60 | ||
61 | =item 1. | |
62 | X<\> X<backslash> | |
63 | ||
64 | By using the backslash operator on a variable, subroutine, or value. | |
65 | (This works much like the & (address-of) operator in C.) | |
66 | This typically creates I<another> reference to a variable, because | |
67 | there's already a reference to the variable in the symbol table. But | |
68 | the symbol table reference might go away, and you'll still have the | |
69 | reference that the backslash returned. Here are some examples: | |
70 | ||
71 | $scalarref = \$foo; | |
72 | $arrayref = \@ARGV; | |
73 | $hashref = \%ENV; | |
74 | $coderef = \&handler; | |
75 | $globref = \*foo; | |
76 | ||
77 | It isn't possible to create a true reference to an IO handle (filehandle | |
78 | or dirhandle) using the backslash operator. The most you can get is a | |
79 | reference to a typeglob, which is actually a complete symbol table entry. | |
80 | But see the explanation of the C<*foo{THING}> syntax below. However, | |
81 | you can still use type globs and globrefs as though they were IO handles. | |
82 | ||
83 | =item 2. | |
84 | X<array, anonymous> X<[> X<[]> X<square bracket> | |
85 | X<bracket, square> X<arrayref> X<array reference> X<reference, array> | |
86 | ||
87 | A reference to an anonymous array can be created using square | |
88 | brackets: | |
89 | ||
90 | $arrayref = [1, 2, ['a', 'b', 'c']]; | |
91 | ||
92 | Here we've created a reference to an anonymous array of three elements | |
93 | whose final element is itself a reference to another anonymous array of three | |
94 | elements. (The multidimensional syntax described later can be used to | |
95 | access this. For example, after the above, C<< $arrayref->[2][1] >> would have | |
96 | the value "b".) | |
97 | ||
98 | Taking a reference to an enumerated list is not the same | |
99 | as using square brackets--instead it's the same as creating | |
100 | a list of references! | |
101 | ||
102 | @list = (\$a, \@b, \%c); | |
103 | @list = \($a, @b, %c); # same thing! | |
104 | ||
105 | As a special case, C<\(@foo)> returns a list of references to the contents | |
106 | of C<@foo>, not a reference to C<@foo> itself. Likewise for C<%foo>, | |
107 | except that the key references are to copies (since the keys are just | |
108 | strings rather than full-fledged scalars). | |
109 | ||
110 | =item 3. | |
111 | X<hash, anonymous> X<{> X<{}> X<curly bracket> | |
112 | X<bracket, curly> X<brace> X<hashref> X<hash reference> X<reference, hash> | |
113 | ||
114 | A reference to an anonymous hash can be created using curly | |
115 | brackets: | |
116 | ||
117 | $hashref = { | |
118 | 'Adam' => 'Eve', | |
119 | 'Clyde' => 'Bonnie', | |
120 | }; | |
121 | ||
122 | Anonymous hash and array composers like these can be intermixed freely to | |
123 | produce as complicated a structure as you want. The multidimensional | |
124 | syntax described below works for these too. The values above are | |
125 | literals, but variables and expressions would work just as well, because | |
126 | assignment operators in Perl (even within local() or my()) are executable | |
127 | statements, not compile-time declarations. | |
128 | ||
129 | Because curly brackets (braces) are used for several other things | |
130 | including BLOCKs, you may occasionally have to disambiguate braces at the | |
131 | beginning of a statement by putting a C<+> or a C<return> in front so | |
132 | that Perl realizes the opening brace isn't starting a BLOCK. The economy and | |
133 | mnemonic value of using curlies is deemed worth this occasional extra | |
134 | hassle. | |
135 | ||
136 | For example, if you wanted a function to make a new hash and return a | |
137 | reference to it, you have these options: | |
138 | ||
139 | sub hashem { { @_ } } # silently wrong | |
140 | sub hashem { +{ @_ } } # ok | |
141 | sub hashem { return { @_ } } # ok | |
142 | ||
143 | On 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 | ||
149 | The leading C<+{> and C<{;> always serve to disambiguate | |
150 | the expression to mean either the HASH reference, or the BLOCK. | |
151 | ||
152 | =item 4. | |
153 | X<subroutine, anonymous> X<subroutine, reference> X<reference, subroutine> | |
154 | X<scope, lexical> X<closure> X<lexical> X<lexical scope> | |
155 | ||
156 | A reference to an anonymous subroutine can be created by using | |
157 | C<sub> without a subname: | |
158 | ||
159 | $coderef = sub { print "Boink!\n" }; | |
160 | ||
161 | Note the semicolon. Except for the code | |
162 | inside not being immediately executed, a C<sub {}> is not so much a | |
163 | declaration as it is an operator, like C<do{}> or C<eval{}>. (However, no | |
164 | matter how many times you execute that particular line (unless you're in an | |
165 | C<eval("...")>), $coderef will still have a reference to the I<same> | |
166 | anonymous subroutine.) | |
167 | ||
168 | Anonymous subroutines act as closures with respect to my() variables, | |
169 | that is, variables lexically visible within the current scope. Closure | |
170 | is a notion out of the Lisp world that says if you define an anonymous | |
171 | function in a particular lexical context, it pretends to run in that | |
172 | context even when it's called outside the context. | |
173 | ||
174 | In human terms, it's a funny way of passing arguments to a subroutine when | |
175 | you define it as well as when you call it. It's useful for setting up | |
176 | little bits of code to run later, such as callbacks. You can even | |
177 | do object-oriented stuff with it, though Perl already provides a different | |
178 | mechanism to do that--see L<perlobj>. | |
179 | ||
180 | You might also think of closure as a way to write a subroutine | |
181 | template without using eval(). Here's a small example of how | |
182 | closures work: | |
183 | ||
184 | sub newprint { | |
185 | my $x = shift; | |
186 | return sub { my $y = shift; print "$x, $y!\n"; }; | |
187 | } | |
188 | $h = newprint("Howdy"); | |
189 | $g = newprint("Greetings"); | |
190 | ||
191 | # Time passes... | |
192 | ||
193 | &$h("world"); | |
194 | &$g("earthlings"); | |
195 | ||
196 | This prints | |
197 | ||
198 | Howdy, world! | |
199 | Greetings, earthlings! | |
200 | ||
201 | Note particularly that $x continues to refer to the value passed | |
202 | into newprint() I<despite> "my $x" having gone out of scope by the | |
203 | time the anonymous subroutine runs. That's what a closure is all | |
204 | about. | |
205 | ||
206 | This applies only to lexical variables, by the way. Dynamic variables | |
207 | continue to work as they have always worked. Closure is not something | |
208 | that most Perl programmers need trouble themselves about to begin with. | |
209 | ||
210 | =item 5. | |
211 | X<constructor> X<new> | |
212 | ||
213 | References are often returned by special subroutines called constructors. | |
214 | Perl objects are just references to a special type of object that happens to know | |
215 | which package it's associated with. Constructors are just special | |
216 | subroutines that know how to create that association. They do so by | |
217 | starting with an ordinary reference, and it remains an ordinary reference | |
218 | even while it's also being an object. Constructors are often | |
219 | named new() and called indirectly: | |
220 | ||
221 | $objref = new Doggie (Tail => 'short', Ears => 'long'); | |
222 | ||
223 | But 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 | ||
235 | =item 6. | |
236 | X<autovivification> | |
237 | ||
238 | References of the appropriate type can spring into existence if you | |
239 | dereference them in a context that assumes they exist. Because we haven't | |
240 | talked about dereferencing yet, we can't show you any examples yet. | |
241 | ||
242 | =item 7. | |
243 | X<*foo{THING}> X<*> | |
244 | ||
245 | A reference can be created by using a special syntax, lovingly known as | |
246 | the *foo{THING} syntax. *foo{THING} returns a reference to the THING | |
247 | slot in *foo (which is the symbol table entry which holds everything | |
248 | known as foo). | |
249 | ||
250 | $scalarref = *foo{SCALAR}; | |
251 | $arrayref = *ARGV{ARRAY}; | |
252 | $hashref = *ENV{HASH}; | |
253 | $coderef = *handler{CODE}; | |
254 | $ioref = *STDIN{IO}; | |
255 | $globref = *foo{GLOB}; | |
256 | $formatref = *foo{FORMAT}; | |
257 | ||
258 | All of these are self-explanatory except for C<*foo{IO}>. It returns | |
259 | the IO handle, used for file handles (L<perlfunc/open>), sockets | |
260 | (L<perlfunc/socket> and L<perlfunc/socketpair>), and directory | |
261 | handles (L<perlfunc/opendir>). For compatibility with previous | |
262 | versions of Perl, C<*foo{FILEHANDLE}> is a synonym for C<*foo{IO}>, though it | |
263 | is deprecated as of 5.8.0. If deprecation warnings are in effect, it will warn | |
264 | of its use. | |
265 | ||
266 | C<*foo{THING}> returns undef if that particular THING hasn't been used yet, | |
267 | except in the case of scalars. C<*foo{SCALAR}> returns a reference to an | |
268 | anonymous scalar if $foo hasn't been used yet. This might change in a | |
269 | future release. | |
270 | ||
271 | C<*foo{IO}> is an alternative to the C<*HANDLE> mechanism given in | |
272 | L<perldata/"Typeglobs and Filehandles"> for passing filehandles | |
273 | into or out of subroutines, or storing into larger data structures. | |
274 | Its disadvantage is that it won't create a new filehandle for you. | |
275 | Its advantage is that you have less risk of clobbering more than | |
276 | you want to with a typeglob assignment. (It still conflates file | |
277 | and directory handles, though.) However, if you assign the incoming | |
278 | value to a scalar instead of a typeglob as we do in the examples | |
279 | below, there's no risk of that happening. | |
280 | ||
281 | splutter(*STDOUT); # pass the whole glob | |
282 | splutter(*STDOUT{IO}); # pass both file and dir handles | |
283 | ||
284 | sub splutter { | |
285 | my $fh = shift; | |
286 | print $fh "her um well a hmmm\n"; | |
287 | } | |
288 | ||
289 | $rec = get_rec(*STDIN); # pass the whole glob | |
290 | $rec = get_rec(*STDIN{IO}); # pass both file and dir handles | |
291 | ||
292 | sub get_rec { | |
293 | my $fh = shift; | |
294 | return scalar <$fh>; | |
295 | } | |
296 | ||
297 | =back | |
298 | ||
299 | =head2 Using References | |
300 | X<reference, use> X<dereferencing> X<dereference> | |
301 | ||
302 | That's it for creating references. By now you're probably dying to | |
303 | know how to use references to get back to your long-lost data. There | |
304 | are several basic methods. | |
305 | ||
306 | =over 4 | |
307 | ||
308 | =item 1. | |
309 | ||
310 | Anywhere you'd put an identifier (or chain of identifiers) as part | |
311 | of a variable or subroutine name, you can replace the identifier with | |
312 | a simple scalar variable containing a reference of the correct type: | |
313 | ||
314 | $bar = $$scalarref; | |
315 | push(@$arrayref, $filename); | |
316 | $$arrayref[0] = "January"; | |
317 | $$hashref{"KEY"} = "VALUE"; | |
318 | &$coderef(1,2,3); | |
319 | print $globref "output\n"; | |
320 | ||
321 | It's important to understand that we are specifically I<not> dereferencing | |
322 | C<$arrayref[0]> or C<$hashref{"KEY"}> there. The dereference of the | |
323 | scalar variable happens I<before> it does any key lookups. Anything more | |
324 | complicated than a simple scalar variable must use methods 2 or 3 below. | |
325 | However, a "simple scalar" includes an identifier that itself uses method | |
326 | 1 recursively. Therefore, the following prints "howdy". | |
327 | ||
328 | $refrefref = \\\"howdy"; | |
329 | print $$$$refrefref; | |
330 | ||
331 | =item 2. | |
332 | X<${}> X<@{}> X<%{}> | |
333 | ||
334 | Anywhere you'd put an identifier (or chain of identifiers) as part of a | |
335 | variable or subroutine name, you can replace the identifier with a | |
336 | BLOCK returning a reference of the correct type. In other words, the | |
337 | previous examples could be written like this: | |
338 | ||
339 | $bar = ${$scalarref}; | |
340 | push(@{$arrayref}, $filename); | |
341 | ${$arrayref}[0] = "January"; | |
342 | ${$hashref}{"KEY"} = "VALUE"; | |
343 | &{$coderef}(1,2,3); | |
344 | $globref->print("output\n"); # iff IO::Handle is loaded | |
345 | ||
346 | Admittedly, it's a little silly to use the curlies in this case, but | |
347 | the BLOCK can contain any arbitrary expression, in particular, | |
348 | subscripted expressions: | |
349 | ||
350 | &{ $dispatch{$index} }(1,2,3); # call correct routine | |
351 | ||
352 | Because of being able to omit the curlies for the simple case of C<$$x>, | |
353 | people often make the mistake of viewing the dereferencing symbols as | |
354 | proper operators, and wonder about their precedence. If they were, | |
355 | though, you could use parentheses instead of braces. That's not the case. | |
356 | Consider the difference below; case 0 is a short-hand version of case 1, | |
357 | I<not> case 2: | |
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 | ||
364 | Case 2 is also deceptive in that you're accessing a variable | |
365 | called %hashref, not dereferencing through $hashref to the hash | |
366 | it's presumably referencing. That would be case 3. | |
367 | ||
368 | =item 3. | |
369 | X<autovivification> X<< -> >> X<arrow> | |
370 | ||
371 | Subroutine calls and lookups of individual array elements arise often | |
372 | enough that it gets cumbersome to use method 2. As a form of | |
373 | syntactic sugar, the examples for method 2 may be written: | |
374 | ||
375 | $arrayref->[0] = "January"; # Array element | |
376 | $hashref->{"KEY"} = "VALUE"; # Hash element | |
377 | $coderef->(1,2,3); # Subroutine call | |
378 | ||
379 | The left side of the arrow can be any expression returning a reference, | |
380 | including a previous dereference. Note that C<$array[$x]> is I<not> the | |
381 | same thing as C<< $array->[$x] >> here: | |
382 | ||
383 | $array[$x]->{"foo"}->[0] = "January"; | |
384 | ||
385 | This is one of the cases we mentioned earlier in which references could | |
386 | spring into existence when in an lvalue context. Before this | |
387 | statement, C<$array[$x]> may have been undefined. If so, it's | |
388 | automatically defined with a hash reference so that we can look up | |
389 | C<{"foo"}> in it. Likewise C<< $array[$x]->{"foo"} >> will automatically get | |
390 | defined with an array reference so that we can look up C<[0]> in it. | |
391 | This process is called I<autovivification>. | |
392 | ||
393 | One more thing here. The arrow is optional I<between> brackets | |
394 | subscripts, so you can shrink the above down to | |
395 | ||
396 | $array[$x]{"foo"}[0] = "January"; | |
397 | ||
398 | Which, in the degenerate case of using only ordinary arrays, gives you | |
399 | multidimensional arrays just like C's: | |
400 | ||
401 | $score[$x][$y][$z] += 42; | |
402 | ||
403 | Well, okay, not entirely like C's arrays, actually. C doesn't know how | |
404 | to grow its arrays on demand. Perl does. | |
405 | ||
406 | =item 4. | |
407 | X<encapsulation> | |
408 | ||
409 | If a reference happens to be a reference to an object, then there are | |
410 | probably methods to access the things referred to, and you should probably | |
411 | stick to those methods unless you're in the class package that defines the | |
412 | object's methods. In other words, be nice, and don't violate the object's | |
413 | encapsulation without a very good reason. Perl does not enforce | |
414 | encapsulation. We are not totalitarians here. We do expect some basic | |
415 | civility though. | |
416 | ||
417 | =back | |
418 | ||
419 | Using a string or number as a reference produces a symbolic reference, | |
420 | as explained above. Using a reference as a number produces an | |
421 | integer representing its storage location in memory. The only | |
422 | useful thing to be done with this is to compare two references | |
423 | numerically to see whether they refer to the same location. | |
424 | X<reference, numeric context> | |
425 | ||
426 | if ($ref1 == $ref2) { # cheap numeric compare of references | |
427 | print "refs 1 and 2 refer to the same thing\n"; | |
428 | } | |
429 | ||
430 | Using a reference as a string produces both its referent's type, | |
431 | including any package blessing as described in L<perlobj>, as well | |
432 | as the numeric address expressed in hex. The ref() operator returns | |
433 | just the type of thing the reference is pointing to, without the | |
434 | address. See L<perlfunc/ref> for details and examples of its use. | |
435 | X<reference, string context> | |
436 | ||
437 | The bless() operator may be used to associate the object a reference | |
438 | points to with a package functioning as an object class. See L<perlobj>. | |
439 | ||
440 | A typeglob may be dereferenced the same way a reference can, because | |
441 | the dereference syntax always indicates the type of reference desired. | |
442 | So C<${*foo}> and C<${\$foo}> both indicate the same scalar variable. | |
443 | ||
444 | Here's a trick for interpolating a subroutine call into a string: | |
445 | ||
446 | print "My sub returned @{[mysub(1,2,3)]} that time.\n"; | |
447 | ||
448 | The way it works is that when the C<@{...}> is seen in the double-quoted | |
449 | string, it's evaluated as a block. The block creates a reference to an | |
450 | anonymous array containing the results of the call to C<mysub(1,2,3)>. So | |
451 | the whole block returns a reference to an array, which is then | |
452 | dereferenced by C<@{...}> and stuck into the double-quoted string. This | |
453 | chicanery is also useful for arbitrary expressions: | |
454 | ||
455 | print "That yields @{[$n + 5]} widgets\n"; | |
456 | ||
457 | =head2 Symbolic references | |
458 | X<reference, symbolic> X<reference, soft> | |
459 | X<symbolic reference> X<soft reference> | |
460 | ||
461 | We said that references spring into existence as necessary if they are | |
462 | undefined, but we didn't say what happens if a value used as a | |
463 | reference is already defined, but I<isn't> a hard reference. If you | |
464 | use it as a reference, it'll be treated as a symbolic | |
465 | reference. That is, the value of the scalar is taken to be the I<name> | |
466 | of a variable, rather than a direct link to a (possibly) anonymous | |
467 | value. | |
468 | ||
469 | People 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 | ||
481 | This is powerful, and slightly dangerous, in that it's possible | |
482 | to intend (with the utmost sincerity) to use a hard reference, and | |
483 | accidentally use a symbolic reference instead. To protect against | |
484 | that, you can say | |
485 | ||
486 | use strict 'refs'; | |
487 | ||
488 | and then only hard references will be allowed for the rest of the enclosing | |
489 | block. An inner block may countermand that with | |
490 | ||
491 | no strict 'refs'; | |
492 | ||
493 | Only package variables (globals, even if localized) are visible to | |
494 | symbolic references. Lexical variables (declared with my()) aren't in | |
495 | a symbol table, and thus are invisible to this mechanism. For example: | |
496 | ||
497 | local $value = 10; | |
498 | $ref = "value"; | |
499 | { | |
500 | my $value = 20; | |
501 | print $$ref; | |
502 | } | |
503 | ||
504 | This will still print 10, not 20. Remember that local() affects package | |
505 | variables, which are all "global" to the package. | |
506 | ||
507 | =head2 Not-so-symbolic references | |
508 | ||
509 | A new feature contributing to readability in perl version 5.001 is that the | |
510 | brackets around a symbolic reference behave more like quotes, just as they | |
511 | always have within a string. That is, | |
512 | ||
513 | $push = "pop on "; | |
514 | print "${push}over"; | |
515 | ||
516 | has always meant to print "pop on over", even though push is | |
517 | a reserved word. This has been generalized to work the same outside | |
518 | of quotes, so that | |
519 | ||
520 | print ${push} . "over"; | |
521 | ||
522 | and even | |
523 | ||
524 | print ${ push } . "over"; | |
525 | ||
526 | will have the same effect. (This would have been a syntax error in | |
527 | Perl 5.000, though Perl 4 allowed it in the spaceless form.) This | |
528 | construct is I<not> considered to be a symbolic reference when you're | |
529 | using strict refs: | |
530 | ||
531 | use strict 'refs'; | |
532 | ${ bareword }; # Okay, means $bareword. | |
533 | ${ "bareword" }; # Error, symbolic reference. | |
534 | ||
535 | Similarly, because of all the subscripting that is done using single | |
536 | words, we've applied the same rule to any bareword that is used for | |
537 | subscripting a hash. So now, instead of writing | |
538 | ||
539 | $array{ "aaa" }{ "bbb" }{ "ccc" } | |
540 | ||
541 | you can write just | |
542 | ||
543 | $array{ aaa }{ bbb }{ ccc } | |
544 | ||
545 | and not worry about whether the subscripts are reserved words. In the | |
546 | rare event that you do wish to do something like | |
547 | ||
548 | $array{ shift } | |
549 | ||
550 | you can force interpretation as a reserved word by adding anything that | |
551 | makes it more than a bareword: | |
552 | ||
553 | $array{ shift() } | |
554 | $array{ +shift } | |
555 | $array{ shift @_ } | |
556 | ||
557 | The C<use warnings> pragma or the B<-w> switch will warn you if it | |
558 | interprets a reserved word as a string. | |
559 | But it will no longer warn you about using lowercase words, because the | |
560 | string is effectively quoted. | |
561 | ||
562 | =head2 Pseudo-hashes: Using an array as a hash | |
563 | X<pseudo-hash> X<pseudo hash> X<pseudohash> | |
564 | ||
565 | B<WARNING>: This section describes an experimental feature. Details may | |
566 | change without notice in future versions. | |
567 | ||
568 | B<NOTE>: The current user-visible implementation of pseudo-hashes | |
569 | (the weird use of the first array element) is deprecated starting from | |
570 | Perl 5.8.0 and will be removed in Perl 5.10.0, and the feature will be | |
571 | implemented differently. Not only is the current interface rather ugly, | |
572 | but the current implementation slows down normal array and hash use quite | |
573 | noticeably. The 'fields' pragma interface will remain available. | |
574 | ||
575 | Beginning with release 5.005 of Perl, you may use an array reference | |
576 | in some contexts that would normally require a hash reference. This | |
577 | allows you to access array elements using symbolic names, as if they | |
578 | were fields in a structure. | |
579 | ||
580 | For this to work, the array must contain extra information. The first | |
581 | element of the array has to be a hash reference that maps field names | |
582 | to array indices. Here is an example: | |
583 | ||
584 | $struct = [{foo => 1, bar => 2}, "FOO", "BAR"]; | |
585 | ||
586 | $struct->{foo}; # same as $struct->[1], i.e. "FOO" | |
587 | $struct->{bar}; # same as $struct->[2], i.e. "BAR" | |
588 | ||
589 | keys %$struct; # will return ("foo", "bar") in some order | |
590 | values %$struct; # will return ("FOO", "BAR") in same some order | |
591 | ||
592 | while (my($k,$v) = each %$struct) { | |
593 | print "$k => $v\n"; | |
594 | } | |
595 | ||
596 | Perl will raise an exception if you try to access nonexistent fields. | |
597 | To avoid inconsistencies, always use the fields::phash() function | |
598 | provided by the C<fields> pragma. | |
599 | ||
600 | use fields; | |
601 | $pseudohash = fields::phash(foo => "FOO", bar => "BAR"); | |
602 | ||
603 | For better performance, Perl can also do the translation from field | |
604 | names to array indices at compile time for typed object references. | |
605 | See L<fields>. | |
606 | ||
607 | There are two ways to check for the existence of a key in a | |
608 | pseudo-hash. The first is to use exists(). This checks to see if the | |
609 | given field has ever been set. It acts this way to match the behavior | |
610 | of a regular hash. For instance: | |
611 | ||
612 | use fields; | |
613 | $phash = fields::phash([qw(foo bar pants)], ['FOO']); | |
614 | $phash->{pants} = undef; | |
615 | ||
616 | print exists $phash->{foo}; # true, 'foo' was set in the declaration | |
617 | print exists $phash->{bar}; # false, 'bar' has not been used. | |
618 | print exists $phash->{pants}; # true, your 'pants' have been touched | |
619 | ||
620 | The second is to use exists() on the hash reference sitting in the | |
621 | first array element. This checks to see if the given key is a valid | |
622 | field in the pseudo-hash. | |
623 | ||
624 | print exists $phash->[0]{bar}; # true, 'bar' is a valid field | |
625 | print exists $phash->[0]{shoes};# false, 'shoes' can't be used | |
626 | ||
627 | delete() on a pseudo-hash element only deletes the value corresponding | |
628 | to the key, not the key itself. To delete the key, you'll have to | |
629 | explicitly delete it from the first hash element. | |
630 | ||
631 | print delete $phash->{foo}; # prints $phash->[1], "FOO" | |
632 | print exists $phash->{foo}; # false | |
633 | print exists $phash->[0]{foo}; # true, key still exists | |
634 | print delete $phash->[0]{foo}; # now key is gone | |
635 | print $phash->{foo}; # runtime exception | |
636 | ||
637 | =head2 Function Templates | |
638 | X<scope, lexical> X<closure> X<lexical> X<lexical scope> | |
639 | X<subroutine, nested> X<sub, nested> X<subroutine, local> X<sub, local> | |
640 | ||
641 | As explained above, an anonymous function with access to the lexical | |
642 | variables visible when that function was compiled, creates a closure. It | |
643 | retains access to those variables even though it doesn't get run until | |
644 | later, such as in a signal handler or a Tk callback. | |
645 | ||
646 | Using a closure as a function template allows us to generate many functions | |
647 | that act similarly. Suppose you wanted functions named after the colors | |
648 | that generated HTML font changes for the various colors: | |
649 | ||
650 | print "Be ", red("careful"), "with that ", green("light"); | |
651 | ||
652 | The red() and green() functions would be similar. To create these, | |
653 | we'll assign a closure to a typeglob of the name of the function we're | |
654 | trying 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 | ||
662 | Now all those different functions appear to exist independently. You can | |
663 | call red(), RED(), blue(), BLUE(), green(), etc. This technique saves on | |
664 | both compile time and memory use, and is less error-prone as well, since | |
665 | syntax checks happen at compile time. It's critical that any variables in | |
666 | the anonymous subroutine be lexicals in order to create a proper closure. | |
667 | That's the reasons for the C<my> on the loop iteration variable. | |
668 | ||
669 | This is one of the only places where giving a prototype to a closure makes | |
670 | much sense. If you wanted to impose scalar context on the arguments of | |
671 | these functions (probably not a wise idea for this particular example), | |
672 | you could have written it this way instead: | |
673 | ||
674 | *$name = sub ($) { "<FONT COLOR='$name'>$_[0]</FONT>" }; | |
675 | ||
676 | However, since prototype checking happens at compile time, the assignment | |
677 | above happens too late to be of much use. You could address this by | |
678 | putting the whole loop of assignments within a BEGIN block, forcing it | |
679 | to occur during compilation. | |
680 | ||
681 | Access to lexicals that change over type--like those in the C<for> loop | |
682 | above--only works with closures, not general subroutines. In the general | |
683 | case, then, named subroutines do not nest properly, although anonymous | |
684 | ones do. Thus is because named subroutines are created (and capture any | |
685 | outer lexicals) only once at compile time, whereas anonymous subroutines | |
686 | get to capture each time you execute the 'sub' operator. If you are | |
687 | accustomed to using nested subroutines in other programming languages with | |
688 | their own private variables, you'll have to work at it a bit in Perl. The | |
689 | intuitive coding of this type of thing incurs mysterious warnings about | |
690 | "will not stay shared". For example, this won't work: | |
691 | ||
692 | sub outer { | |
693 | my $x = $_[0] + 35; | |
694 | sub inner { return $x * 19 } # WRONG | |
695 | return $x + inner(); | |
696 | } | |
697 | ||
698 | A work-around is the following: | |
699 | ||
700 | sub outer { | |
701 | my $x = $_[0] + 35; | |
702 | local *inner = sub { return $x * 19 }; | |
703 | return $x + inner(); | |
704 | } | |
705 | ||
706 | Now inner() can only be called from within outer(), because of the | |
707 | temporary assignments of the closure (anonymous subroutine). But when | |
708 | it does, it has normal access to the lexical variable $x from the scope | |
709 | of outer(). | |
710 | ||
711 | This has the interesting effect of creating a function local to another | |
712 | function, something not normally supported in Perl. | |
713 | ||
714 | =head1 WARNING | |
715 | X<reference, string context> X<reference, use as hash key> | |
716 | ||
717 | You may not (usefully) use a reference as the key to a hash. It will be | |
718 | converted into a string: | |
719 | ||
720 | $x{ \$a } = $a; | |
721 | ||
722 | If you try to dereference the key, it won't do a hard dereference, and | |
723 | you won't accomplish what you're attempting. You might want to do something | |
724 | more like | |
725 | ||
726 | $r = \@a; | |
727 | $x{ $r } = $r; | |
728 | ||
729 | And then at least you can use the values(), which will be | |
730 | real refs, instead of the keys(), which won't. | |
731 | ||
732 | The standard Tie::RefHash module provides a convenient workaround to this. | |
733 | ||
734 | =head1 SEE ALSO | |
735 | ||
736 | Besides the obvious documents, source code can be instructive. | |
737 | Some pathological examples of the use of references can be found | |
738 | in the F<t/op/ref.t> regression test in the Perl source directory. | |
739 | ||
740 | See also L<perldsc> and L<perllol> for how to use references to create | |
741 | complex data structures, and L<perltoot>, L<perlobj>, and L<perlbot> | |
742 | for how to use them to create objects. |