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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 |
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8 | structures, because all references had to be symbolic--and even then |
9 | it was difficult to refer to a variable instead of a symbol table entry. |
10 | Perl now not only makes it easier to use symbolic references to variables, |
11 | but also lets you have "hard" references to any piece of data or code. |
12 | Any scalar may hold a hard reference. Because arrays and hashes contain |
13 | scalars, you can now easily build arrays of arrays, arrays of hashes, |
14 | 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 |
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35 | adjective, as in the following paragraph, it is usually talking about a |
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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 | =head2 Making References |
45 | |
46 | References can be created in several ways. |
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47 | |
48 | =over 4 |
49 | |
50 | =item 1. |
51 | |
52 | By using the backslash operator on a variable, subroutine, or value. |
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53 | (This works much like the & (address-of) operator in C.) Note |
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54 | that this typically creates I<ANOTHER> reference to a variable, because |
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55 | there's already a reference to the variable in the symbol table. But |
56 | the symbol table reference might go away, and you'll still have the |
57 | reference that the backslash returned. Here are some examples: |
58 | |
59 | $scalarref = \$foo; |
60 | $arrayref = \@ARGV; |
61 | $hashref = \%ENV; |
62 | $coderef = \&handler; |
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63 | $globref = \*foo; |
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64 | |
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65 | It isn't possible to create a true reference to an IO handle (filehandle |
66 | or dirhandle) using the backslash operator. The most you can get is a |
67 | reference to a typeglob, which is actually a complete symbol table entry. |
68 | But see the explanation of the C<*foo{THING}> syntax below. However, |
69 | you can still use type globs and globrefs as though they were IO handles. |
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70 | |
71 | =item 2. |
72 | |
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73 | A reference to an anonymous array can be created using square |
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74 | brackets: |
75 | |
76 | $arrayref = [1, 2, ['a', 'b', 'c']]; |
77 | |
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78 | Here we've created a reference to an anonymous array of three elements |
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79 | whose final element is itself a reference to another anonymous array of three |
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80 | elements. (The multidimensional syntax described later can be used to |
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81 | access this. For example, after the above, C<$arrayref-E<gt>[2][1]> would have |
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82 | the value "b".) |
83 | |
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84 | Note that taking a reference to an enumerated list is not the same |
85 | as using square brackets--instead it's the same as creating |
86 | a list of references! |
87 | |
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88 | @list = (\$a, \@b, \%c); |
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89 | @list = \($a, @b, %c); # same thing! |
90 | |
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91 | As a special case, C<\(@foo)> returns a list of references to the contents |
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92 | of C<@foo>, not a reference to C<@foo> itself. Likewise for C<%foo>, |
93 | except that the key references are to copies (since the keys are just |
94 | strings rather than full-fledged scalars). |
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95 | |
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96 | =item 3. |
97 | |
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98 | A reference to an anonymous hash can be created using curly |
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99 | brackets: |
100 | |
101 | $hashref = { |
102 | 'Adam' => 'Eve', |
103 | 'Clyde' => 'Bonnie', |
104 | }; |
105 | |
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106 | Anonymous hash and array composers like these can be intermixed freely to |
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107 | produce as complicated a structure as you want. The multidimensional |
108 | syntax described below works for these too. The values above are |
109 | literals, but variables and expressions would work just as well, because |
110 | assignment operators in Perl (even within local() or my()) are executable |
111 | statements, not compile-time declarations. |
112 | |
113 | Because curly brackets (braces) are used for several other things |
114 | including BLOCKs, you may occasionally have to disambiguate braces at the |
115 | beginning of a statement by putting a C<+> or a C<return> in front so |
116 | that Perl realizes the opening brace isn't starting a BLOCK. The economy and |
117 | mnemonic value of using curlies is deemed worth this occasional extra |
118 | hassle. |
119 | |
120 | For example, if you wanted a function to make a new hash and return a |
121 | reference to it, you have these options: |
122 | |
123 | sub hashem { { @_ } } # silently wrong |
124 | sub hashem { +{ @_ } } # ok |
125 | sub hashem { return { @_ } } # ok |
126 | |
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127 | On the other hand, if you want the other meaning, you can do this: |
128 | |
129 | sub showem { { @_ } } # ambiguous (currently ok, but may change) |
130 | sub showem { {; @_ } } # ok |
131 | sub showem { { return @_ } } # ok |
132 | |
133 | Note how the leading C<+{> and C<{;> always serve to disambiguate |
134 | the expression to mean either the HASH reference, or the BLOCK. |
135 | |
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136 | =item 4. |
137 | |
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138 | A reference to an anonymous subroutine can be created by using |
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139 | C<sub> without a subname: |
140 | |
141 | $coderef = sub { print "Boink!\n" }; |
142 | |
143 | Note the presence of the semicolon. Except for the fact that the code |
144 | inside isn't executed immediately, a C<sub {}> is not so much a |
145 | declaration as it is an operator, like C<do{}> or C<eval{}>. (However, no |
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146 | matter how many times you execute that particular line (unless you're in an |
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147 | C<eval("...")>), C<$coderef> will still have a reference to the I<SAME> |
148 | anonymous subroutine.) |
149 | |
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150 | Anonymous subroutines act as closures with respect to my() variables, |
151 | that is, variables visible lexically within the current scope. Closure |
152 | is a notion out of the Lisp world that says if you define an anonymous |
153 | function in a particular lexical context, it pretends to run in that |
154 | context even when it's called outside of the context. |
155 | |
156 | In human terms, it's a funny way of passing arguments to a subroutine when |
157 | you define it as well as when you call it. It's useful for setting up |
158 | little bits of code to run later, such as callbacks. You can even |
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159 | do object-oriented stuff with it, though Perl already provides a different |
160 | mechanism to do that--see L<perlobj>. |
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161 | |
162 | You can also think of closure as a way to write a subroutine template without |
163 | using eval. (In fact, in version 5.000, eval was the I<only> way to get |
164 | closures. You may wish to use "require 5.001" if you use closures.) |
165 | |
166 | Here's a small example of how closures works: |
167 | |
168 | sub newprint { |
169 | my $x = shift; |
170 | return sub { my $y = shift; print "$x, $y!\n"; }; |
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171 | } |
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172 | $h = newprint("Howdy"); |
173 | $g = newprint("Greetings"); |
174 | |
175 | # Time passes... |
176 | |
177 | &$h("world"); |
178 | &$g("earthlings"); |
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179 | |
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180 | This prints |
181 | |
182 | Howdy, world! |
183 | Greetings, earthlings! |
184 | |
185 | Note particularly that $x continues to refer to the value passed into |
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186 | newprint() I<despite> the fact that the "my $x" has seemingly gone out of |
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187 | scope by the time the anonymous subroutine runs. That's what closure |
188 | is all about. |
189 | |
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190 | This applies only to lexical variables, by the way. Dynamic variables |
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191 | continue to work as they have always worked. Closure is not something |
192 | that most Perl programmers need trouble themselves about to begin with. |
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193 | |
194 | =item 5. |
195 | |
196 | References are often returned by special subroutines called constructors. |
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197 | Perl objects are just references to a special kind of object that happens to know |
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198 | which package it's associated with. Constructors are just special |
199 | subroutines that know how to create that association. They do so by |
200 | starting with an ordinary reference, and it remains an ordinary reference |
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201 | even while it's also being an object. Constructors are often |
202 | named new() and called indirectly: |
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203 | |
204 | $objref = new Doggie (Tail => 'short', Ears => 'long'); |
205 | |
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206 | But don't have to be: |
207 | |
208 | $objref = Doggie->new(Tail => 'short', Ears => 'long'); |
209 | |
210 | use Term::Cap; |
211 | $terminal = Term::Cap->Tgetent( { OSPEED => 9600 }); |
212 | |
213 | use Tk; |
214 | $main = MainWindow->new(); |
215 | $menubar = $main->Frame(-relief => "raised", |
216 | -borderwidth => 2) |
217 | |
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218 | =item 6. |
219 | |
220 | References of the appropriate type can spring into existence if you |
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221 | dereference them in a context that assumes they exist. Because we haven't |
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222 | talked about dereferencing yet, we can't show you any examples yet. |
223 | |
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224 | =item 7. |
225 | |
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226 | A reference can be created by using a special syntax, lovingly known as |
227 | the *foo{THING} syntax. *foo{THING} returns a reference to the THING |
228 | slot in *foo (which is the symbol table entry which holds everything |
229 | known as foo). |
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230 | |
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231 | $scalarref = *foo{SCALAR}; |
232 | $arrayref = *ARGV{ARRAY}; |
233 | $hashref = *ENV{HASH}; |
234 | $coderef = *handler{CODE}; |
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235 | $ioref = *STDIN{IO}; |
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236 | $globref = *foo{GLOB}; |
237 | |
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238 | All of these are self-explanatory except for *foo{IO}. It returns the |
239 | IO handle, used for file handles (L<perlfunc/open>), sockets |
240 | (L<perlfunc/socket> and L<perlfunc/socketpair>), and directory handles |
241 | (L<perlfunc/opendir>). For compatibility with previous versions of |
242 | Perl, *foo{FILEHANDLE} is a synonym for *foo{IO}. |
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243 | |
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244 | *foo{THING} returns undef if that particular THING hasn't been used yet, |
245 | except in the case of scalars. *foo{SCALAR} returns a reference to an |
246 | anonymous scalar if $foo hasn't been used yet. This might change in a |
247 | future release. |
248 | |
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249 | *foo{IO} is an alternative to the \*HANDLE mechanism given in |
250 | L<perldata/"Typeglobs and Filehandles"> for passing filehandles |
251 | into or out of subroutines, or storing into larger data structures. |
252 | Its disadvantage is that it won't create a new filehandle for you. |
253 | Its advantage is that you have no risk of clobbering more than you want |
254 | to with a typeglob assignment, although if you assign to a scalar instead |
255 | of a typeglob, you're ok. |
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256 | |
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257 | splutter(*STDOUT); |
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258 | splutter(*STDOUT{IO}); |
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259 | |
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260 | sub splutter { |
261 | my $fh = shift; |
262 | print $fh "her um well a hmmm\n"; |
263 | } |
264 | |
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265 | $rec = get_rec(*STDIN); |
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266 | $rec = get_rec(*STDIN{IO}); |
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267 | |
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268 | sub get_rec { |
269 | my $fh = shift; |
270 | return scalar <$fh>; |
271 | } |
272 | |
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273 | =back |
274 | |
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275 | =head2 Using References |
276 | |
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277 | That's it for creating references. By now you're probably dying to |
278 | know how to use references to get back to your long-lost data. There |
279 | are several basic methods. |
280 | |
281 | =over 4 |
282 | |
283 | =item 1. |
284 | |
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285 | Anywhere you'd put an identifier (or chain of identifiers) as part |
286 | of a variable or subroutine name, you can replace the identifier with |
287 | a simple scalar variable containing a reference of the correct type: |
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288 | |
289 | $bar = $$scalarref; |
290 | push(@$arrayref, $filename); |
291 | $$arrayref[0] = "January"; |
292 | $$hashref{"KEY"} = "VALUE"; |
293 | &$coderef(1,2,3); |
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294 | print $globref "output\n"; |
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295 | |
296 | It's important to understand that we are specifically I<NOT> dereferencing |
297 | C<$arrayref[0]> or C<$hashref{"KEY"}> there. The dereference of the |
298 | scalar variable happens I<BEFORE> it does any key lookups. Anything more |
299 | complicated than a simple scalar variable must use methods 2 or 3 below. |
300 | However, a "simple scalar" includes an identifier that itself uses method |
301 | 1 recursively. Therefore, the following prints "howdy". |
302 | |
303 | $refrefref = \\\"howdy"; |
304 | print $$$$refrefref; |
305 | |
306 | =item 2. |
307 | |
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308 | Anywhere you'd put an identifier (or chain of identifiers) as part of a |
309 | variable or subroutine name, you can replace the identifier with a |
310 | BLOCK returning a reference of the correct type. In other words, the |
311 | previous examples could be written like this: |
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312 | |
313 | $bar = ${$scalarref}; |
314 | push(@{$arrayref}, $filename); |
315 | ${$arrayref}[0] = "January"; |
316 | ${$hashref}{"KEY"} = "VALUE"; |
317 | &{$coderef}(1,2,3); |
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318 | $globref->print("output\n"); # iff IO::Handle is loaded |
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319 | |
320 | Admittedly, it's a little silly to use the curlies in this case, but |
321 | the BLOCK can contain any arbitrary expression, in particular, |
322 | subscripted expressions: |
323 | |
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324 | &{ $dispatch{$index} }(1,2,3); # call correct routine |
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325 | |
326 | Because of being able to omit the curlies for the simple case of C<$$x>, |
327 | people often make the mistake of viewing the dereferencing symbols as |
328 | proper operators, and wonder about their precedence. If they were, |
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329 | though, you could use parentheses instead of braces. That's not the case. |
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330 | Consider the difference below; case 0 is a short-hand version of case 1, |
331 | I<NOT> case 2: |
332 | |
333 | $$hashref{"KEY"} = "VALUE"; # CASE 0 |
334 | ${$hashref}{"KEY"} = "VALUE"; # CASE 1 |
335 | ${$hashref{"KEY"}} = "VALUE"; # CASE 2 |
336 | ${$hashref->{"KEY"}} = "VALUE"; # CASE 3 |
337 | |
338 | Case 2 is also deceptive in that you're accessing a variable |
339 | called %hashref, not dereferencing through $hashref to the hash |
340 | it's presumably referencing. That would be case 3. |
341 | |
342 | =item 3. |
343 | |
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344 | Subroutine calls and lookups of individual array elements arise often |
345 | enough that it gets cumbersome to use method 2. As a form of |
346 | syntactic sugar, the examples for method 2 may be written: |
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347 | |
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348 | $arrayref->[0] = "January"; # Array element |
349 | $hashref->{"KEY"} = "VALUE"; # Hash element |
350 | $coderef->(1,2,3); # Subroutine call |
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351 | |
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352 | The left side of the arrow can be any expression returning a reference, |
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353 | including a previous dereference. Note that C<$array[$x]> is I<NOT> the |
354 | same thing as C<$array-E<gt>[$x]> here: |
355 | |
356 | $array[$x]->{"foo"}->[0] = "January"; |
357 | |
358 | This is one of the cases we mentioned earlier in which references could |
359 | spring into existence when in an lvalue context. Before this |
360 | statement, C<$array[$x]> may have been undefined. If so, it's |
361 | automatically defined with a hash reference so that we can look up |
362 | C<{"foo"}> in it. Likewise C<$array[$x]-E<gt>{"foo"}> will automatically get |
363 | defined with an array reference so that we can look up C<[0]> in it. |
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364 | This process is called I<autovivification>. |
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365 | |
366 | One more thing here. The arrow is optional I<BETWEEN> brackets |
367 | subscripts, so you can shrink the above down to |
368 | |
369 | $array[$x]{"foo"}[0] = "January"; |
370 | |
371 | Which, in the degenerate case of using only ordinary arrays, gives you |
372 | multidimensional arrays just like C's: |
373 | |
374 | $score[$x][$y][$z] += 42; |
375 | |
376 | Well, okay, not entirely like C's arrays, actually. C doesn't know how |
377 | to grow its arrays on demand. Perl does. |
378 | |
379 | =item 4. |
380 | |
381 | If a reference happens to be a reference to an object, then there are |
382 | probably methods to access the things referred to, and you should probably |
383 | stick to those methods unless you're in the class package that defines the |
384 | object's methods. In other words, be nice, and don't violate the object's |
385 | encapsulation without a very good reason. Perl does not enforce |
386 | encapsulation. We are not totalitarians here. We do expect some basic |
387 | civility though. |
388 | |
389 | =back |
390 | |
391 | The ref() operator may be used to determine what type of thing the |
392 | reference is pointing to. See L<perlfunc>. |
393 | |
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394 | The bless() operator may be used to associate the object a reference |
395 | points to with a package functioning as an object class. See L<perlobj>. |
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396 | |
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397 | A typeglob may be dereferenced the same way a reference can, because |
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398 | the dereference syntax always indicates the kind of reference desired. |
399 | So C<${*foo}> and C<${\$foo}> both indicate the same scalar variable. |
400 | |
401 | Here's a trick for interpolating a subroutine call into a string: |
402 | |
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403 | print "My sub returned @{[mysub(1,2,3)]} that time.\n"; |
404 | |
405 | The way it works is that when the C<@{...}> is seen in the double-quoted |
406 | string, it's evaluated as a block. The block creates a reference to an |
407 | anonymous array containing the results of the call to C<mysub(1,2,3)>. So |
408 | the whole block returns a reference to an array, which is then |
409 | dereferenced by C<@{...}> and stuck into the double-quoted string. This |
410 | chicanery is also useful for arbitrary expressions: |
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411 | |
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412 | print "That yields @{[$n + 5]} widgets\n"; |
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413 | |
414 | =head2 Symbolic references |
415 | |
416 | We said that references spring into existence as necessary if they are |
417 | undefined, but we didn't say what happens if a value used as a |
418 | reference is already defined, but I<ISN'T> a hard reference. If you |
419 | use it as a reference in this case, it'll be treated as a symbolic |
420 | reference. That is, the value of the scalar is taken to be the I<NAME> |
421 | of a variable, rather than a direct link to a (possibly) anonymous |
422 | value. |
423 | |
424 | People frequently expect it to work like this. So it does. |
425 | |
426 | $name = "foo"; |
427 | $$name = 1; # Sets $foo |
428 | ${$name} = 2; # Sets $foo |
429 | ${$name x 2} = 3; # Sets $foofoo |
430 | $name->[0] = 4; # Sets $foo[0] |
431 | @$name = (); # Clears @foo |
432 | &$name(); # Calls &foo() (as in Perl 4) |
433 | $pack = "THAT"; |
434 | ${"${pack}::$name"} = 5; # Sets $THAT::foo without eval |
435 | |
436 | This is very powerful, and slightly dangerous, in that it's possible |
437 | to intend (with the utmost sincerity) to use a hard reference, and |
438 | accidentally use a symbolic reference instead. To protect against |
439 | that, you can say |
440 | |
441 | use strict 'refs'; |
442 | |
443 | and then only hard references will be allowed for the rest of the enclosing |
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444 | block. An inner block may countermand that with |
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445 | |
446 | no strict 'refs'; |
447 | |
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448 | Only package variables (globals, even if localized) are visible to |
449 | symbolic references. Lexical variables (declared with my()) aren't in |
450 | a symbol table, and thus are invisible to this mechanism. For example: |
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451 | |
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452 | local $value = 10; |
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453 | $ref = \$value; |
454 | { |
455 | my $value = 20; |
456 | print $$ref; |
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457 | } |
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458 | |
459 | This will still print 10, not 20. Remember that local() affects package |
460 | variables, which are all "global" to the package. |
461 | |
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462 | =head2 Not-so-symbolic references |
463 | |
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464 | A new feature contributing to readability in perl version 5.001 is that the |
465 | brackets around a symbolic reference behave more like quotes, just as they |
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466 | always have within a string. That is, |
467 | |
468 | $push = "pop on "; |
469 | print "${push}over"; |
470 | |
471 | has always meant to print "pop on over", despite the fact that push is |
472 | a reserved word. This has been generalized to work the same outside |
473 | of quotes, so that |
474 | |
475 | print ${push} . "over"; |
476 | |
477 | and even |
478 | |
479 | print ${ push } . "over"; |
480 | |
481 | will have the same effect. (This would have been a syntax error in |
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482 | Perl 5.000, though Perl 4 allowed it in the spaceless form.) Note that this |
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483 | construct is I<not> considered to be a symbolic reference when you're |
484 | using strict refs: |
485 | |
486 | use strict 'refs'; |
487 | ${ bareword }; # Okay, means $bareword. |
488 | ${ "bareword" }; # Error, symbolic reference. |
489 | |
490 | Similarly, because of all the subscripting that is done using single |
491 | words, we've applied the same rule to any bareword that is used for |
492 | subscripting a hash. So now, instead of writing |
493 | |
494 | $array{ "aaa" }{ "bbb" }{ "ccc" } |
495 | |
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496 | you can write just |
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497 | |
498 | $array{ aaa }{ bbb }{ ccc } |
499 | |
500 | and not worry about whether the subscripts are reserved words. In the |
501 | rare event that you do wish to do something like |
502 | |
503 | $array{ shift } |
504 | |
505 | you can force interpretation as a reserved word by adding anything that |
506 | makes it more than a bareword: |
507 | |
508 | $array{ shift() } |
509 | $array{ +shift } |
510 | $array{ shift @_ } |
511 | |
512 | The B<-w> switch will warn you if it interprets a reserved word as a string. |
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513 | But it will no longer warn you about using lowercase words, because the |
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514 | string is effectively quoted. |
515 | |
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516 | =head2 Pseudo-hashes: Using an array as a hash |
517 | |
518 | WARNING: This section describes an experimental feature. Details may |
519 | change without notice in future versions. |
520 | |
521 | Beginning with release 5.005 of Perl you can use an array reference |
522 | in some contexts that would normally require a hash reference. This |
523 | allows you to access array elements using symbolic names, as if they |
524 | were fields in a structure. |
525 | |
526 | For this to work, the array must contain extra information. The first |
527 | element of the array has to be a hash reference that maps field names |
528 | to array indices. Here is an example: |
529 | |
530 | $struct = [{foo => 1, bar => 2}, "FOO", "BAR"]; |
531 | |
532 | $struct->{foo}; # same as $struct->[1], i.e. "FOO" |
533 | $struct->{bar}; # same as $struct->[2], i.e. "BAR" |
534 | |
535 | keys %$struct; # will return ("foo", "bar") in some order |
536 | values %$struct; # will return ("FOO", "BAR") in same some order |
537 | |
538 | while (my($k,$v) = each %$struct) { |
539 | print "$k => $v\n"; |
540 | } |
541 | |
542 | Perl will raise an exception if you try to delete keys from a pseudo-hash |
543 | or try to access nonexistent fields. For better performance, Perl can also |
544 | do the translation from field names to array indices at compile time for |
545 | typed object references. See L<fields>. |
546 | |
547 | |
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548 | =head2 Function Templates |
549 | |
550 | As explained above, a closure is an anonymous function with access to the |
551 | lexical variables visible when that function was compiled. It retains |
552 | access to those variables even though it doesn't get run until later, |
553 | such as in a signal handler or a Tk callback. |
554 | |
555 | Using a closure as a function template allows us to generate many functions |
556 | that act similarly. Suppopose you wanted functions named after the colors |
557 | that generated HTML font changes for the various colors: |
558 | |
559 | print "Be ", red("careful"), "with that ", green("light"); |
560 | |
561 | The red() and green() functions would be very similar. To create these, |
562 | we'll assign a closure to a typeglob of the name of the function we're |
563 | trying to build. |
564 | |
565 | @colors = qw(red blue green yellow orange purple violet); |
566 | for my $name (@colors) { |
567 | no strict 'refs'; # allow symbol table manipulation |
568 | *$name = *{uc $name} = sub { "<FONT COLOR='$name'>@_</FONT>" }; |
569 | } |
570 | |
571 | Now all those different functions appear to exist independently. You can |
572 | call red(), RED(), blue(), BLUE(), green(), etc. This technique saves on |
573 | both compile time and memory use, and is less error-prone as well, since |
574 | syntax checks happen at compile time. It's critical that any variables in |
575 | the anonymous subroutine be lexicals in order to create a proper closure. |
576 | That's the reasons for the C<my> on the loop iteration variable. |
577 | |
578 | This is one of the only places where giving a prototype to a closure makes |
579 | much sense. If you wanted to impose scalar context on the arguments of |
580 | these functions (probably not a wise idea for this particular example), |
581 | you could have written it this way instead: |
582 | |
583 | *$name = sub ($) { "<FONT COLOR='$name'>$_[0]</FONT>" }; |
584 | |
585 | However, since prototype checking happens at compile time, the assignment |
586 | above happens too late to be of much use. You could address this by |
587 | putting the whole loop of assignments within a BEGIN block, forcing it |
588 | to occur during compilation. |
589 | |
590 | Access to lexicals that change over type--like those in the C<for> loop |
591 | above--only works with closures, not general subroutines. In the general |
592 | case, then, named subroutines do not nest properly, although anonymous |
593 | ones do. If you are accustomed to using nested subroutines in other |
594 | programming languages with their own private variables, you'll have to |
595 | work at it a bit in Perl. The intuitive coding of this kind of thing |
596 | incurs mysterious warnings about ``will not stay shared''. For example, |
597 | this won't work: |
598 | |
599 | sub outer { |
600 | my $x = $_[0] + 35; |
601 | sub inner { return $x * 19 } # WRONG |
602 | return $x + inner(); |
603 | } |
604 | |
605 | A work-around is the following: |
606 | |
607 | sub outer { |
608 | my $x = $_[0] + 35; |
609 | local *inner = sub { return $x * 19 }; |
610 | return $x + inner(); |
611 | } |
612 | |
613 | Now inner() can only be called from within outer(), because of the |
614 | temporary assignments of the closure (anonymous subroutine). But when |
615 | it does, it has normal access to the lexical variable $x from the scope |
616 | of outer(). |
617 | |
618 | This has the interesting effect of creating a function local to another |
619 | function, something not normally supported in Perl. |
620 | |
cb1a09d0 |
621 | =head1 WARNING |
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622 | |
623 | You may not (usefully) use a reference as the key to a hash. It will be |
624 | converted into a string: |
625 | |
626 | $x{ \$a } = $a; |
627 | |
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628 | If you try to dereference the key, it won't do a hard dereference, and |
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629 | you won't accomplish what you're attempting. You might want to do something |
cb1a09d0 |
630 | more like |
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631 | |
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632 | $r = \@a; |
633 | $x{ $r } = $r; |
634 | |
635 | And then at least you can use the values(), which will be |
636 | real refs, instead of the keys(), which won't. |
637 | |
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638 | The standard Tie::RefHash module provides a convenient workaround to this. |
639 | |
cb1a09d0 |
640 | =head1 SEE ALSO |
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641 | |
642 | Besides the obvious documents, source code can be instructive. |
643 | Some rather pathological examples of the use of references can be found |
644 | in the F<t/op/ref.t> regression test in the Perl source directory. |
cb1a09d0 |
645 | |
646 | See also L<perldsc> and L<perllol> for how to use references to create |
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647 | complex data structures, and L<perltoot>, L<perlobj>, and L<perlbot> |
648 | for how to use them to create objects. |