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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 |
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10 | symbol table entry. Perl not only makes it easier to use symbolic |
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11 | references to variables, but lets you have "hard" references to any piece |
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12 | of data. Any scalar may hold a hard reference. Because arrays and hashes |
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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, |
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17 | automatically freeing the thing referred to when its reference count goes |
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18 | to zero. (Note: the reference counts for values in self-referential or |
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19 | cyclic data structures may not go to zero without a little help; see |
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20 | L<perlobj/"Two-Phased Garbage Collection"> for a detailed explanation.) |
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21 | If that thing happens to be an object, the object is destructed. See |
22 | L<perlobj> for more about objects. (In a sense, everything in Perl is an |
23 | object, but we usually reserve the word for references to objects that |
24 | have been officially "blessed" into a class package.) |
25 | |
26 | Symbolic references are names of variables or other objects, just as a |
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27 | symbolic link in a Unix filesystem contains merely the name of a file. |
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28 | The C<*glob> notation is a kind of symbolic reference. (Symbolic |
29 | references are sometimes called "soft references", but please don't call |
30 | them that; references are confusing enough without useless synonyms.) |
31 | |
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32 | In contrast, hard references are more like hard links in a Unix file |
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33 | system: They are used to access an underlying object without concern for |
34 | what its (other) name is. When the word "reference" is used without an |
35 | adjective, like in the following paragraph, it usually is talking about a |
36 | hard reference. |
37 | |
38 | References are easy to use in Perl. There is just one overriding |
39 | principle: Perl does no implicit referencing or dereferencing. When a |
40 | scalar is holding a reference, it always behaves as a simple scalar. It |
41 | doesn't magically start being an array or hash or subroutine; you have to |
42 | tell it explicitly to do so, by dereferencing it. |
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43 | |
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44 | References can be constructed in several ways. |
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45 | |
46 | =over 4 |
47 | |
48 | =item 1. |
49 | |
50 | By using the backslash operator on a variable, subroutine, or value. |
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51 | (This works much like the & (address-of) operator in C.) Note |
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52 | that this typically creates I<ANOTHER> reference to a variable, because |
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53 | there's already a reference to the variable in the symbol table. But |
54 | the symbol table reference might go away, and you'll still have the |
55 | reference that the backslash returned. Here are some examples: |
56 | |
57 | $scalarref = \$foo; |
58 | $arrayref = \@ARGV; |
59 | $hashref = \%ENV; |
60 | $coderef = \&handler; |
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61 | $globref = \*foo; |
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62 | |
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63 | It isn't possible to create a true reference to an IO handle (filehandle or |
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64 | dirhandle) using the backslash operator. See the explanation of the |
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65 | *foo{THING} syntax below. (However, you're apt to find Perl code |
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66 | out there using globrefs as though they were IO handles, which is |
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67 | grandfathered into continued functioning.) |
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68 | |
69 | =item 2. |
70 | |
71 | A reference to an anonymous array can be constructed using square |
72 | brackets: |
73 | |
74 | $arrayref = [1, 2, ['a', 'b', 'c']]; |
75 | |
76 | Here we've constructed a reference to an anonymous array of three elements |
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77 | whose final element is itself a reference to another anonymous array of three |
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78 | elements. (The multidimensional syntax described later can be used to |
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79 | access this. For example, after the above, C<$arrayref-E<gt>[2][1]> would have |
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80 | the value "b".) |
81 | |
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82 | Note that taking a reference to an enumerated list is not the same |
83 | as using square brackets--instead it's the same as creating |
84 | a list of references! |
85 | |
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86 | @list = (\$a, \@b, \%c); |
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87 | @list = \($a, @b, %c); # same thing! |
88 | |
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89 | As a special case, C<\(@foo)> returns a list of references to the contents |
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90 | of C<@foo>, not a reference to C<@foo> itself. Likewise for C<%foo>. |
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91 | |
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92 | =item 3. |
93 | |
94 | A reference to an anonymous hash can be constructed using curly |
95 | brackets: |
96 | |
97 | $hashref = { |
98 | 'Adam' => 'Eve', |
99 | 'Clyde' => 'Bonnie', |
100 | }; |
101 | |
102 | Anonymous hash and array constructors can be intermixed freely to |
103 | produce as complicated a structure as you want. The multidimensional |
104 | syntax described below works for these too. The values above are |
105 | literals, but variables and expressions would work just as well, because |
106 | assignment operators in Perl (even within local() or my()) are executable |
107 | statements, not compile-time declarations. |
108 | |
109 | Because curly brackets (braces) are used for several other things |
110 | including BLOCKs, you may occasionally have to disambiguate braces at the |
111 | beginning of a statement by putting a C<+> or a C<return> in front so |
112 | that Perl realizes the opening brace isn't starting a BLOCK. The economy and |
113 | mnemonic value of using curlies is deemed worth this occasional extra |
114 | hassle. |
115 | |
116 | For example, if you wanted a function to make a new hash and return a |
117 | reference to it, you have these options: |
118 | |
119 | sub hashem { { @_ } } # silently wrong |
120 | sub hashem { +{ @_ } } # ok |
121 | sub hashem { return { @_ } } # ok |
122 | |
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123 | On the other hand, if you want the other meaning, you can do this: |
124 | |
125 | sub showem { { @_ } } # ambiguous (currently ok, but may change) |
126 | sub showem { {; @_ } } # ok |
127 | sub showem { { return @_ } } # ok |
128 | |
129 | Note how the leading C<+{> and C<{;> always serve to disambiguate |
130 | the expression to mean either the HASH reference, or the BLOCK. |
131 | |
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132 | =item 4. |
133 | |
134 | A reference to an anonymous subroutine can be constructed by using |
135 | C<sub> without a subname: |
136 | |
137 | $coderef = sub { print "Boink!\n" }; |
138 | |
139 | Note the presence of the semicolon. Except for the fact that the code |
140 | inside isn't executed immediately, a C<sub {}> is not so much a |
141 | declaration as it is an operator, like C<do{}> or C<eval{}>. (However, no |
142 | matter how many times you execute that line (unless you're in an |
143 | C<eval("...")>), C<$coderef> will still have a reference to the I<SAME> |
144 | anonymous subroutine.) |
145 | |
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146 | Anonymous subroutines act as closures with respect to my() variables, |
147 | that is, variables visible lexically within the current scope. Closure |
148 | is a notion out of the Lisp world that says if you define an anonymous |
149 | function in a particular lexical context, it pretends to run in that |
150 | context even when it's called outside of the context. |
151 | |
152 | In human terms, it's a funny way of passing arguments to a subroutine when |
153 | you define it as well as when you call it. It's useful for setting up |
154 | little bits of code to run later, such as callbacks. You can even |
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155 | do object-oriented stuff with it, though Perl already provides a different |
156 | mechanism to do that--see L<perlobj>. |
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157 | |
158 | You can also think of closure as a way to write a subroutine template without |
159 | using eval. (In fact, in version 5.000, eval was the I<only> way to get |
160 | closures. You may wish to use "require 5.001" if you use closures.) |
161 | |
162 | Here's a small example of how closures works: |
163 | |
164 | sub newprint { |
165 | my $x = shift; |
166 | return sub { my $y = shift; print "$x, $y!\n"; }; |
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167 | } |
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168 | $h = newprint("Howdy"); |
169 | $g = newprint("Greetings"); |
170 | |
171 | # Time passes... |
172 | |
173 | &$h("world"); |
174 | &$g("earthlings"); |
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175 | |
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176 | This prints |
177 | |
178 | Howdy, world! |
179 | Greetings, earthlings! |
180 | |
181 | Note particularly that $x continues to refer to the value passed into |
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182 | newprint() I<despite> the fact that the "my $x" has seemingly gone out of |
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183 | scope by the time the anonymous subroutine runs. That's what closure |
184 | is all about. |
185 | |
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186 | This applies to only lexical variables, by the way. Dynamic variables |
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187 | continue to work as they have always worked. Closure is not something |
188 | that most Perl programmers need trouble themselves about to begin with. |
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189 | |
190 | =item 5. |
191 | |
192 | References are often returned by special subroutines called constructors. |
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193 | Perl objects are just references to a special kind of object that happens to know |
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194 | which package it's associated with. Constructors are just special |
195 | subroutines that know how to create that association. They do so by |
196 | starting with an ordinary reference, and it remains an ordinary reference |
197 | even while it's also being an object. Constructors are customarily |
198 | named new(), but don't have to be: |
199 | |
200 | $objref = new Doggie (Tail => 'short', Ears => 'long'); |
201 | |
202 | =item 6. |
203 | |
204 | References of the appropriate type can spring into existence if you |
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205 | dereference them in a context that assumes they exist. Because we haven't |
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206 | talked about dereferencing yet, we can't show you any examples yet. |
207 | |
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208 | =item 7. |
209 | |
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210 | A reference can be created by using a special syntax, lovingly known as |
211 | the *foo{THING} syntax. *foo{THING} returns a reference to the THING |
212 | slot in *foo (which is the symbol table entry which holds everything |
213 | known as foo). |
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214 | |
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215 | $scalarref = *foo{SCALAR}; |
216 | $arrayref = *ARGV{ARRAY}; |
217 | $hashref = *ENV{HASH}; |
218 | $coderef = *handler{CODE}; |
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219 | $ioref = *STDIN{IO}; |
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220 | $globref = *foo{GLOB}; |
221 | |
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222 | All of these are self-explanatory except for *foo{IO}. It returns the |
223 | IO handle, used for file handles (L<perlfunc/open>), sockets |
224 | (L<perlfunc/socket> and L<perlfunc/socketpair>), and directory handles |
225 | (L<perlfunc/opendir>). For compatibility with previous versions of |
226 | Perl, *foo{FILEHANDLE} is a synonym for *foo{IO}. |
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227 | |
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228 | *foo{THING} returns undef if that particular THING hasn't been used yet, |
229 | except in the case of scalars. *foo{SCALAR} returns a reference to an |
230 | anonymous scalar if $foo hasn't been used yet. This might change in a |
231 | future release. |
232 | |
233 | The use of *foo{IO} is the best way to pass bareword filehandles into or |
234 | out of subroutines, or to store them in larger data structures. |
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235 | |
236 | splutter(*STDOUT{IO}); |
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237 | sub splutter { |
238 | my $fh = shift; |
239 | print $fh "her um well a hmmm\n"; |
240 | } |
241 | |
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242 | $rec = get_rec(*STDIN{IO}); |
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243 | sub get_rec { |
244 | my $fh = shift; |
245 | return scalar <$fh>; |
246 | } |
247 | |
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248 | Beware, though, that you can't do this with a routine which is going to |
249 | open the filehandle for you, because *HANDLE{IO} will be undef if HANDLE |
250 | hasn't been used yet. Use \*HANDLE for that sort of thing instead. |
251 | |
252 | Using \*HANDLE (or *HANDLE) is another way to use and store non-bareword |
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253 | filehandles (before perl version 5.002 it was the only way). The two |
254 | methods are largely interchangeable, you can do |
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255 | |
256 | splutter(\*STDOUT); |
257 | $rec = get_rec(\*STDIN); |
258 | |
259 | with the above subroutine definitions. |
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260 | |
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261 | =back |
262 | |
263 | That's it for creating references. By now you're probably dying to |
264 | know how to use references to get back to your long-lost data. There |
265 | are several basic methods. |
266 | |
267 | =over 4 |
268 | |
269 | =item 1. |
270 | |
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271 | Anywhere you'd put an identifier (or chain of identifiers) as part |
272 | of a variable or subroutine name, you can replace the identifier with |
273 | a simple scalar variable containing a reference of the correct type: |
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274 | |
275 | $bar = $$scalarref; |
276 | push(@$arrayref, $filename); |
277 | $$arrayref[0] = "January"; |
278 | $$hashref{"KEY"} = "VALUE"; |
279 | &$coderef(1,2,3); |
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280 | print $globref "output\n"; |
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281 | |
282 | It's important to understand that we are specifically I<NOT> dereferencing |
283 | C<$arrayref[0]> or C<$hashref{"KEY"}> there. The dereference of the |
284 | scalar variable happens I<BEFORE> it does any key lookups. Anything more |
285 | complicated than a simple scalar variable must use methods 2 or 3 below. |
286 | However, a "simple scalar" includes an identifier that itself uses method |
287 | 1 recursively. Therefore, the following prints "howdy". |
288 | |
289 | $refrefref = \\\"howdy"; |
290 | print $$$$refrefref; |
291 | |
292 | =item 2. |
293 | |
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294 | Anywhere you'd put an identifier (or chain of identifiers) as part of a |
295 | variable or subroutine name, you can replace the identifier with a |
296 | BLOCK returning a reference of the correct type. In other words, the |
297 | previous examples could be written like this: |
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298 | |
299 | $bar = ${$scalarref}; |
300 | push(@{$arrayref}, $filename); |
301 | ${$arrayref}[0] = "January"; |
302 | ${$hashref}{"KEY"} = "VALUE"; |
303 | &{$coderef}(1,2,3); |
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304 | $globref->print("output\n"); # iff IO::Handle is loaded |
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305 | |
306 | Admittedly, it's a little silly to use the curlies in this case, but |
307 | the BLOCK can contain any arbitrary expression, in particular, |
308 | subscripted expressions: |
309 | |
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310 | &{ $dispatch{$index} }(1,2,3); # call correct routine |
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311 | |
312 | Because of being able to omit the curlies for the simple case of C<$$x>, |
313 | people often make the mistake of viewing the dereferencing symbols as |
314 | proper operators, and wonder about their precedence. If they were, |
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315 | though, you could use parentheses instead of braces. That's not the case. |
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316 | Consider the difference below; case 0 is a short-hand version of case 1, |
317 | I<NOT> case 2: |
318 | |
319 | $$hashref{"KEY"} = "VALUE"; # CASE 0 |
320 | ${$hashref}{"KEY"} = "VALUE"; # CASE 1 |
321 | ${$hashref{"KEY"}} = "VALUE"; # CASE 2 |
322 | ${$hashref->{"KEY"}} = "VALUE"; # CASE 3 |
323 | |
324 | Case 2 is also deceptive in that you're accessing a variable |
325 | called %hashref, not dereferencing through $hashref to the hash |
326 | it's presumably referencing. That would be case 3. |
327 | |
328 | =item 3. |
329 | |
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330 | Subroutine calls and lookups of individual array elements arise often |
331 | enough that it gets cumbersome to use method 2. As a form of |
332 | syntactic sugar, the examples for method 2 may be written: |
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333 | |
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334 | $arrayref->[0] = "January"; # Array element |
335 | $hashref->{"KEY"} = "VALUE"; # Hash element |
336 | $coderef->(1,2,3); # Subroutine call |
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337 | |
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338 | The left side of the arrow can be any expression returning a reference, |
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339 | including a previous dereference. Note that C<$array[$x]> is I<NOT> the |
340 | same thing as C<$array-E<gt>[$x]> here: |
341 | |
342 | $array[$x]->{"foo"}->[0] = "January"; |
343 | |
344 | This is one of the cases we mentioned earlier in which references could |
345 | spring into existence when in an lvalue context. Before this |
346 | statement, C<$array[$x]> may have been undefined. If so, it's |
347 | automatically defined with a hash reference so that we can look up |
348 | C<{"foo"}> in it. Likewise C<$array[$x]-E<gt>{"foo"}> will automatically get |
349 | defined with an array reference so that we can look up C<[0]> in it. |
350 | |
351 | One more thing here. The arrow is optional I<BETWEEN> brackets |
352 | subscripts, so you can shrink the above down to |
353 | |
354 | $array[$x]{"foo"}[0] = "January"; |
355 | |
356 | Which, in the degenerate case of using only ordinary arrays, gives you |
357 | multidimensional arrays just like C's: |
358 | |
359 | $score[$x][$y][$z] += 42; |
360 | |
361 | Well, okay, not entirely like C's arrays, actually. C doesn't know how |
362 | to grow its arrays on demand. Perl does. |
363 | |
364 | =item 4. |
365 | |
366 | If a reference happens to be a reference to an object, then there are |
367 | probably methods to access the things referred to, and you should probably |
368 | stick to those methods unless you're in the class package that defines the |
369 | object's methods. In other words, be nice, and don't violate the object's |
370 | encapsulation without a very good reason. Perl does not enforce |
371 | encapsulation. We are not totalitarians here. We do expect some basic |
372 | civility though. |
373 | |
374 | =back |
375 | |
376 | The ref() operator may be used to determine what type of thing the |
377 | reference is pointing to. See L<perlfunc>. |
378 | |
379 | The bless() operator may be used to associate a reference with a package |
380 | functioning as an object class. See L<perlobj>. |
381 | |
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382 | A typeglob may be dereferenced the same way a reference can, because |
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383 | the dereference syntax always indicates the kind of reference desired. |
384 | So C<${*foo}> and C<${\$foo}> both indicate the same scalar variable. |
385 | |
386 | Here's a trick for interpolating a subroutine call into a string: |
387 | |
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388 | print "My sub returned @{[mysub(1,2,3)]} that time.\n"; |
389 | |
390 | The way it works is that when the C<@{...}> is seen in the double-quoted |
391 | string, it's evaluated as a block. The block creates a reference to an |
392 | anonymous array containing the results of the call to C<mysub(1,2,3)>. So |
393 | the whole block returns a reference to an array, which is then |
394 | dereferenced by C<@{...}> and stuck into the double-quoted string. This |
395 | chicanery is also useful for arbitrary expressions: |
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396 | |
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397 | print "That yields @{[$n + 5]} widgets\n"; |
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398 | |
399 | =head2 Symbolic references |
400 | |
401 | We said that references spring into existence as necessary if they are |
402 | undefined, but we didn't say what happens if a value used as a |
403 | reference is already defined, but I<ISN'T> a hard reference. If you |
404 | use it as a reference in this case, it'll be treated as a symbolic |
405 | reference. That is, the value of the scalar is taken to be the I<NAME> |
406 | of a variable, rather than a direct link to a (possibly) anonymous |
407 | value. |
408 | |
409 | People frequently expect it to work like this. So it does. |
410 | |
411 | $name = "foo"; |
412 | $$name = 1; # Sets $foo |
413 | ${$name} = 2; # Sets $foo |
414 | ${$name x 2} = 3; # Sets $foofoo |
415 | $name->[0] = 4; # Sets $foo[0] |
416 | @$name = (); # Clears @foo |
417 | &$name(); # Calls &foo() (as in Perl 4) |
418 | $pack = "THAT"; |
419 | ${"${pack}::$name"} = 5; # Sets $THAT::foo without eval |
420 | |
421 | This is very powerful, and slightly dangerous, in that it's possible |
422 | to intend (with the utmost sincerity) to use a hard reference, and |
423 | accidentally use a symbolic reference instead. To protect against |
424 | that, you can say |
425 | |
426 | use strict 'refs'; |
427 | |
428 | and then only hard references will be allowed for the rest of the enclosing |
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429 | block. An inner block may countermand that with |
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430 | |
431 | no strict 'refs'; |
432 | |
433 | Only package variables are visible to symbolic references. Lexical |
434 | variables (declared with my()) aren't in a symbol table, and thus are |
435 | invisible to this mechanism. For example: |
436 | |
437 | local($value) = 10; |
438 | $ref = \$value; |
439 | { |
440 | my $value = 20; |
441 | print $$ref; |
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442 | } |
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443 | |
444 | This will still print 10, not 20. Remember that local() affects package |
445 | variables, which are all "global" to the package. |
446 | |
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447 | =head2 Not-so-symbolic references |
448 | |
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449 | A new feature contributing to readability in perl version 5.001 is that the |
450 | brackets around a symbolic reference behave more like quotes, just as they |
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451 | always have within a string. That is, |
452 | |
453 | $push = "pop on "; |
454 | print "${push}over"; |
455 | |
456 | has always meant to print "pop on over", despite the fact that push is |
457 | a reserved word. This has been generalized to work the same outside |
458 | of quotes, so that |
459 | |
460 | print ${push} . "over"; |
461 | |
462 | and even |
463 | |
464 | print ${ push } . "over"; |
465 | |
466 | will have the same effect. (This would have been a syntax error in |
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467 | Perl 5.000, though Perl 4 allowed it in the spaceless form.) Note that this |
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468 | construct is I<not> considered to be a symbolic reference when you're |
469 | using strict refs: |
470 | |
471 | use strict 'refs'; |
472 | ${ bareword }; # Okay, means $bareword. |
473 | ${ "bareword" }; # Error, symbolic reference. |
474 | |
475 | Similarly, because of all the subscripting that is done using single |
476 | words, we've applied the same rule to any bareword that is used for |
477 | subscripting a hash. So now, instead of writing |
478 | |
479 | $array{ "aaa" }{ "bbb" }{ "ccc" } |
480 | |
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481 | you can write just |
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482 | |
483 | $array{ aaa }{ bbb }{ ccc } |
484 | |
485 | and not worry about whether the subscripts are reserved words. In the |
486 | rare event that you do wish to do something like |
487 | |
488 | $array{ shift } |
489 | |
490 | you can force interpretation as a reserved word by adding anything that |
491 | makes it more than a bareword: |
492 | |
493 | $array{ shift() } |
494 | $array{ +shift } |
495 | $array{ shift @_ } |
496 | |
497 | The B<-w> switch will warn you if it interprets a reserved word as a string. |
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498 | But it will no longer warn you about using lowercase words, because the |
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499 | string is effectively quoted. |
500 | |
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501 | =head1 WARNING |
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502 | |
503 | You may not (usefully) use a reference as the key to a hash. It will be |
504 | converted into a string: |
505 | |
506 | $x{ \$a } = $a; |
507 | |
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508 | If you try to dereference the key, it won't do a hard dereference, and |
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509 | you won't accomplish what you're attempting. You might want to do something |
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510 | more like |
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511 | |
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512 | $r = \@a; |
513 | $x{ $r } = $r; |
514 | |
515 | And then at least you can use the values(), which will be |
516 | real refs, instead of the keys(), which won't. |
517 | |
518 | =head1 SEE ALSO |
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519 | |
520 | Besides the obvious documents, source code can be instructive. |
521 | Some rather pathological examples of the use of references can be found |
522 | in the F<t/op/ref.t> regression test in the Perl source directory. |
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523 | |
524 | See also L<perldsc> and L<perllol> for how to use references to create |
525 | complex data structures, and L<perlobj> for how to use them to create |
526 | objects. |