Commit | Line | Data |
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a0d0e21e | 1 | =head1 NAME |
d74e8afc | 2 | X<reference> X<pointer> X<data structure> X<structure> X<struct> |
a0d0e21e LW |
3 | |
4 | perlref - Perl references and nested data structures | |
5 | ||
a1e2a320 GS |
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 | ||
a0d0e21e LW |
12 | =head1 DESCRIPTION |
13 | ||
cb1a09d0 | 14 | Before release 5 of Perl it was difficult to represent complex data |
5a964f20 TC |
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. | |
a0d0e21e LW |
22 | |
23 | Hard references are smart--they keep track of reference counts for you, | |
2d24ed35 | 24 | automatically freeing the thing referred to when its reference count goes |
7c2ea1c7 | 25 | to zero. (Reference counts for values in self-referential or |
2d24ed35 | 26 | cyclic data structures may not go to zero without a little help; see |
7b8d334a | 27 | L<perlobj/"Two-Phased Garbage Collection"> for a detailed explanation.) |
2d24ed35 CS |
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 | |
54310121 | 34 | symbolic link in a Unix filesystem contains merely the name of a file. |
d1be9408 | 35 | The C<*glob> notation is something of a symbolic reference. (Symbolic |
2d24ed35 CS |
36 | references are sometimes called "soft references", but please don't call |
37 | them that; references are confusing enough without useless synonyms.) | |
d74e8afc ITB |
38 | X<reference, symbolic> X<reference, soft> |
39 | X<symbolic reference> X<soft reference> | |
2d24ed35 | 40 | |
54310121 | 41 | In contrast, hard references are more like hard links in a Unix file |
2d24ed35 CS |
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 | |
5a964f20 | 44 | adjective, as in the following paragraph, it is usually talking about a |
2d24ed35 | 45 | hard reference. |
d74e8afc | 46 | X<reference, hard> X<hard reference> |
2d24ed35 CS |
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. | |
a0d0e21e | 53 | |
5a964f20 | 54 | =head2 Making References |
d74e8afc | 55 | X<reference, creation> X<referencing> |
5a964f20 TC |
56 | |
57 | References can be created in several ways. | |
a0d0e21e LW |
58 | |
59 | =over 4 | |
60 | ||
61 | =item 1. | |
d74e8afc | 62 | X<\> X<backslash> |
a0d0e21e LW |
63 | |
64 | By using the backslash operator on a variable, subroutine, or value. | |
7c2ea1c7 GS |
65 | (This works much like the & (address-of) operator in C.) |
66 | This typically creates I<another> reference to a variable, because | |
a0d0e21e LW |
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; | |
55497cff | 75 | $globref = \*foo; |
cb1a09d0 | 76 | |
5a964f20 TC |
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. | |
a0d0e21e LW |
82 | |
83 | =item 2. | |
d74e8afc ITB |
84 | X<array, anonymous> X<[> X<[]> X<square bracket> |
85 | X<bracket, square> X<arrayref> X<array reference> X<reference, array> | |
a0d0e21e | 86 | |
5a964f20 | 87 | A reference to an anonymous array can be created using square |
a0d0e21e LW |
88 | brackets: |
89 | ||
90 | $arrayref = [1, 2, ['a', 'b', 'c']]; | |
91 | ||
5a964f20 | 92 | Here we've created a reference to an anonymous array of three elements |
54310121 | 93 | whose final element is itself a reference to another anonymous array of three |
a0d0e21e | 94 | elements. (The multidimensional syntax described later can be used to |
c47ff5f1 | 95 | access this. For example, after the above, C<< $arrayref->[2][1] >> would have |
a0d0e21e LW |
96 | the value "b".) |
97 | ||
7c2ea1c7 | 98 | Taking a reference to an enumerated list is not the same |
cb1a09d0 AD |
99 | as using square brackets--instead it's the same as creating |
100 | a list of references! | |
101 | ||
54310121 | 102 | @list = (\$a, \@b, \%c); |
58e0a6ae GS |
103 | @list = \($a, @b, %c); # same thing! |
104 | ||
54310121 | 105 | As a special case, C<\(@foo)> returns a list of references to the contents |
b6429b1b GS |
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). | |
cb1a09d0 | 109 | |
a0d0e21e | 110 | =item 3. |
d74e8afc ITB |
111 | X<hash, anonymous> X<{> X<{}> X<curly bracket> |
112 | X<bracket, curly> X<brace> X<hashref> X<hash reference> X<reference, hash> | |
a0d0e21e | 113 | |
5a964f20 | 114 | A reference to an anonymous hash can be created using curly |
a0d0e21e LW |
115 | brackets: |
116 | ||
117 | $hashref = { | |
118 | 'Adam' => 'Eve', | |
119 | 'Clyde' => 'Bonnie', | |
120 | }; | |
121 | ||
5a964f20 | 122 | Anonymous hash and array composers like these can be intermixed freely to |
a0d0e21e LW |
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 | ||
ebc58f1a GS |
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 | ||
7c2ea1c7 | 149 | The leading C<+{> and C<{;> always serve to disambiguate |
ebc58f1a GS |
150 | the expression to mean either the HASH reference, or the BLOCK. |
151 | ||
a0d0e21e | 152 | =item 4. |
d74e8afc ITB |
153 | X<subroutine, anonymous> X<subroutine, reference> X<reference, subroutine> |
154 | X<scope, lexical> X<closure> X<lexical> X<lexical scope> | |
a0d0e21e | 155 | |
5a964f20 | 156 | A reference to an anonymous subroutine can be created by using |
a0d0e21e LW |
157 | C<sub> without a subname: |
158 | ||
159 | $coderef = sub { print "Boink!\n" }; | |
160 | ||
7c2ea1c7 GS |
161 | Note the semicolon. Except for the code |
162 | inside not being immediately executed, a C<sub {}> is not so much a | |
a0d0e21e | 163 | declaration as it is an operator, like C<do{}> or C<eval{}>. (However, no |
5a964f20 | 164 | matter how many times you execute that particular line (unless you're in an |
19799a22 | 165 | C<eval("...")>), $coderef will still have a reference to the I<same> |
a0d0e21e LW |
166 | anonymous subroutine.) |
167 | ||
748a9306 | 168 | Anonymous subroutines act as closures with respect to my() variables, |
7c2ea1c7 | 169 | that is, variables lexically visible within the current scope. Closure |
748a9306 LW |
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 | |
7c2ea1c7 | 172 | context even when it's called outside the context. |
748a9306 LW |
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 | |
54310121 | 177 | do object-oriented stuff with it, though Perl already provides a different |
178 | mechanism to do that--see L<perlobj>. | |
748a9306 | 179 | |
7c2ea1c7 GS |
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: | |
748a9306 LW |
183 | |
184 | sub newprint { | |
185 | my $x = shift; | |
186 | return sub { my $y = shift; print "$x, $y!\n"; }; | |
a0d0e21e | 187 | } |
748a9306 LW |
188 | $h = newprint("Howdy"); |
189 | $g = newprint("Greetings"); | |
190 | ||
191 | # Time passes... | |
192 | ||
193 | &$h("world"); | |
194 | &$g("earthlings"); | |
a0d0e21e | 195 | |
748a9306 LW |
196 | This prints |
197 | ||
198 | Howdy, world! | |
199 | Greetings, earthlings! | |
200 | ||
7c2ea1c7 GS |
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. | |
748a9306 | 205 | |
5a964f20 | 206 | This applies only to lexical variables, by the way. Dynamic variables |
748a9306 LW |
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. | |
a0d0e21e LW |
209 | |
210 | =item 5. | |
d74e8afc | 211 | X<constructor> X<new> |
a0d0e21e | 212 | |
63acfd00 | 213 | References are often returned by special subroutines called constructors. Perl |
214 | 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 subroutines | |
216 | that know how to create that association. They do so by starting with an | |
217 | ordinary reference, and it remains an ordinary reference even while it's also | |
218 | being an object. Constructors are often named C<new()>. You I<can> call them | |
219 | indirectly: | |
220 | ||
221 | $objref = new Doggie( Tail => 'short', Ears => 'long' ); | |
222 | ||
223 | But that can produce ambiguous syntax in certain cases, so it's often | |
224 | better to use the direct method invocation approach: | |
5a964f20 TC |
225 | |
226 | $objref = Doggie->new(Tail => 'short', Ears => 'long'); | |
227 | ||
228 | use Term::Cap; | |
229 | $terminal = Term::Cap->Tgetent( { OSPEED => 9600 }); | |
230 | ||
231 | use Tk; | |
232 | $main = MainWindow->new(); | |
233 | $menubar = $main->Frame(-relief => "raised", | |
234 | -borderwidth => 2) | |
235 | ||
a0d0e21e | 236 | =item 6. |
d74e8afc | 237 | X<autovivification> |
a0d0e21e LW |
238 | |
239 | References of the appropriate type can spring into existence if you | |
5f05dabc | 240 | dereference them in a context that assumes they exist. Because we haven't |
a0d0e21e LW |
241 | talked about dereferencing yet, we can't show you any examples yet. |
242 | ||
cb1a09d0 | 243 | =item 7. |
d74e8afc | 244 | X<*foo{THING}> X<*> |
cb1a09d0 | 245 | |
55497cff | 246 | A reference can be created by using a special syntax, lovingly known as |
247 | the *foo{THING} syntax. *foo{THING} returns a reference to the THING | |
248 | slot in *foo (which is the symbol table entry which holds everything | |
249 | known as foo). | |
cb1a09d0 | 250 | |
55497cff | 251 | $scalarref = *foo{SCALAR}; |
252 | $arrayref = *ARGV{ARRAY}; | |
253 | $hashref = *ENV{HASH}; | |
254 | $coderef = *handler{CODE}; | |
36477c24 | 255 | $ioref = *STDIN{IO}; |
55497cff | 256 | $globref = *foo{GLOB}; |
c0bd1adc | 257 | $formatref = *foo{FORMAT}; |
55497cff | 258 | |
7c2ea1c7 GS |
259 | All of these are self-explanatory except for C<*foo{IO}>. It returns |
260 | the IO handle, used for file handles (L<perlfunc/open>), sockets | |
261 | (L<perlfunc/socket> and L<perlfunc/socketpair>), and directory | |
262 | handles (L<perlfunc/opendir>). For compatibility with previous | |
39b99f21 | 263 | versions of Perl, C<*foo{FILEHANDLE}> is a synonym for C<*foo{IO}>, though it |
264 | is deprecated as of 5.8.0. If deprecation warnings are in effect, it will warn | |
265 | of its use. | |
55497cff | 266 | |
7c2ea1c7 GS |
267 | C<*foo{THING}> returns undef if that particular THING hasn't been used yet, |
268 | except in the case of scalars. C<*foo{SCALAR}> returns a reference to an | |
5f05dabc | 269 | anonymous scalar if $foo hasn't been used yet. This might change in a |
270 | future release. | |
271 | ||
7c2ea1c7 | 272 | C<*foo{IO}> is an alternative to the C<*HANDLE> mechanism given in |
5a964f20 TC |
273 | L<perldata/"Typeglobs and Filehandles"> for passing filehandles |
274 | into or out of subroutines, or storing into larger data structures. | |
275 | Its disadvantage is that it won't create a new filehandle for you. | |
7c2ea1c7 GS |
276 | Its advantage is that you have less risk of clobbering more than |
277 | you want to with a typeglob assignment. (It still conflates file | |
278 | and directory handles, though.) However, if you assign the incoming | |
279 | value to a scalar instead of a typeglob as we do in the examples | |
280 | below, there's no risk of that happening. | |
36477c24 | 281 | |
7c2ea1c7 GS |
282 | splutter(*STDOUT); # pass the whole glob |
283 | splutter(*STDOUT{IO}); # pass both file and dir handles | |
5a964f20 | 284 | |
cb1a09d0 AD |
285 | sub splutter { |
286 | my $fh = shift; | |
287 | print $fh "her um well a hmmm\n"; | |
288 | } | |
289 | ||
7c2ea1c7 GS |
290 | $rec = get_rec(*STDIN); # pass the whole glob |
291 | $rec = get_rec(*STDIN{IO}); # pass both file and dir handles | |
5a964f20 | 292 | |
cb1a09d0 AD |
293 | sub get_rec { |
294 | my $fh = shift; | |
295 | return scalar <$fh>; | |
296 | } | |
297 | ||
a0d0e21e LW |
298 | =back |
299 | ||
5a964f20 | 300 | =head2 Using References |
d74e8afc | 301 | X<reference, use> X<dereferencing> X<dereference> |
5a964f20 | 302 | |
a0d0e21e LW |
303 | That's it for creating references. By now you're probably dying to |
304 | know how to use references to get back to your long-lost data. There | |
305 | are several basic methods. | |
306 | ||
307 | =over 4 | |
308 | ||
309 | =item 1. | |
310 | ||
6309d9d9 | 311 | Anywhere you'd put an identifier (or chain of identifiers) as part |
312 | of a variable or subroutine name, you can replace the identifier with | |
313 | a simple scalar variable containing a reference of the correct type: | |
a0d0e21e LW |
314 | |
315 | $bar = $$scalarref; | |
316 | push(@$arrayref, $filename); | |
317 | $$arrayref[0] = "January"; | |
318 | $$hashref{"KEY"} = "VALUE"; | |
319 | &$coderef(1,2,3); | |
cb1a09d0 | 320 | print $globref "output\n"; |
a0d0e21e | 321 | |
19799a22 | 322 | It's important to understand that we are specifically I<not> dereferencing |
a0d0e21e | 323 | C<$arrayref[0]> or C<$hashref{"KEY"}> there. The dereference of the |
19799a22 | 324 | scalar variable happens I<before> it does any key lookups. Anything more |
a0d0e21e LW |
325 | complicated than a simple scalar variable must use methods 2 or 3 below. |
326 | However, a "simple scalar" includes an identifier that itself uses method | |
327 | 1 recursively. Therefore, the following prints "howdy". | |
328 | ||
329 | $refrefref = \\\"howdy"; | |
330 | print $$$$refrefref; | |
331 | ||
332 | =item 2. | |
333 | ||
6309d9d9 | 334 | Anywhere you'd put an identifier (or chain of identifiers) as part of a |
335 | variable or subroutine name, you can replace the identifier with a | |
336 | BLOCK returning a reference of the correct type. In other words, the | |
337 | previous examples could be written like this: | |
a0d0e21e LW |
338 | |
339 | $bar = ${$scalarref}; | |
340 | push(@{$arrayref}, $filename); | |
341 | ${$arrayref}[0] = "January"; | |
342 | ${$hashref}{"KEY"} = "VALUE"; | |
343 | &{$coderef}(1,2,3); | |
36477c24 | 344 | $globref->print("output\n"); # iff IO::Handle is loaded |
a0d0e21e LW |
345 | |
346 | Admittedly, it's a little silly to use the curlies in this case, but | |
347 | the BLOCK can contain any arbitrary expression, in particular, | |
348 | subscripted expressions: | |
349 | ||
54310121 | 350 | &{ $dispatch{$index} }(1,2,3); # call correct routine |
a0d0e21e LW |
351 | |
352 | Because of being able to omit the curlies for the simple case of C<$$x>, | |
353 | people often make the mistake of viewing the dereferencing symbols as | |
354 | proper operators, and wonder about their precedence. If they were, | |
5f05dabc | 355 | though, you could use parentheses instead of braces. That's not the case. |
a0d0e21e | 356 | Consider the difference below; case 0 is a short-hand version of case 1, |
19799a22 | 357 | I<not> case 2: |
a0d0e21e LW |
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 | ||
6da72b64 CS |
370 | Subroutine calls and lookups of individual array elements arise often |
371 | enough that it gets cumbersome to use method 2. As a form of | |
372 | syntactic sugar, the examples for method 2 may be written: | |
a0d0e21e | 373 | |
6da72b64 CS |
374 | $arrayref->[0] = "January"; # Array element |
375 | $hashref->{"KEY"} = "VALUE"; # Hash element | |
376 | $coderef->(1,2,3); # Subroutine call | |
a0d0e21e | 377 | |
6da72b64 | 378 | The left side of the arrow can be any expression returning a reference, |
19799a22 | 379 | including a previous dereference. Note that C<$array[$x]> is I<not> the |
c47ff5f1 | 380 | same thing as C<< $array->[$x] >> here: |
a0d0e21e LW |
381 | |
382 | $array[$x]->{"foo"}->[0] = "January"; | |
383 | ||
384 | This is one of the cases we mentioned earlier in which references could | |
385 | spring into existence when in an lvalue context. Before this | |
386 | statement, C<$array[$x]> may have been undefined. If so, it's | |
387 | automatically defined with a hash reference so that we can look up | |
c47ff5f1 | 388 | C<{"foo"}> in it. Likewise C<< $array[$x]->{"foo"} >> will automatically get |
a0d0e21e | 389 | defined with an array reference so that we can look up C<[0]> in it. |
5a964f20 | 390 | This process is called I<autovivification>. |
a0d0e21e | 391 | |
19799a22 | 392 | One more thing here. The arrow is optional I<between> brackets |
a0d0e21e LW |
393 | subscripts, so you can shrink the above down to |
394 | ||
395 | $array[$x]{"foo"}[0] = "January"; | |
396 | ||
397 | Which, in the degenerate case of using only ordinary arrays, gives you | |
398 | multidimensional arrays just like C's: | |
399 | ||
400 | $score[$x][$y][$z] += 42; | |
401 | ||
402 | Well, okay, not entirely like C's arrays, actually. C doesn't know how | |
403 | to grow its arrays on demand. Perl does. | |
404 | ||
405 | =item 4. | |
406 | ||
407 | If a reference happens to be a reference to an object, then there are | |
408 | probably methods to access the things referred to, and you should probably | |
409 | stick to those methods unless you're in the class package that defines the | |
410 | object's methods. In other words, be nice, and don't violate the object's | |
411 | encapsulation without a very good reason. Perl does not enforce | |
412 | encapsulation. We are not totalitarians here. We do expect some basic | |
413 | civility though. | |
414 | ||
415 | =back | |
416 | ||
7c2ea1c7 GS |
417 | Using a string or number as a reference produces a symbolic reference, |
418 | as explained above. Using a reference as a number produces an | |
419 | integer representing its storage location in memory. The only | |
420 | useful thing to be done with this is to compare two references | |
421 | numerically to see whether they refer to the same location. | |
d74e8afc | 422 | X<reference, numeric context> |
7c2ea1c7 GS |
423 | |
424 | if ($ref1 == $ref2) { # cheap numeric compare of references | |
425 | print "refs 1 and 2 refer to the same thing\n"; | |
426 | } | |
427 | ||
428 | Using a reference as a string produces both its referent's type, | |
429 | including any package blessing as described in L<perlobj>, as well | |
430 | as the numeric address expressed in hex. The ref() operator returns | |
431 | just the type of thing the reference is pointing to, without the | |
432 | address. See L<perlfunc/ref> for details and examples of its use. | |
d74e8afc | 433 | X<reference, string context> |
a0d0e21e | 434 | |
5a964f20 TC |
435 | The bless() operator may be used to associate the object a reference |
436 | points to with a package functioning as an object class. See L<perlobj>. | |
a0d0e21e | 437 | |
5f05dabc | 438 | A typeglob may be dereferenced the same way a reference can, because |
7c2ea1c7 | 439 | the dereference syntax always indicates the type of reference desired. |
a0d0e21e LW |
440 | So C<${*foo}> and C<${\$foo}> both indicate the same scalar variable. |
441 | ||
442 | Here's a trick for interpolating a subroutine call into a string: | |
443 | ||
cb1a09d0 AD |
444 | print "My sub returned @{[mysub(1,2,3)]} that time.\n"; |
445 | ||
446 | The way it works is that when the C<@{...}> is seen in the double-quoted | |
447 | string, it's evaluated as a block. The block creates a reference to an | |
448 | anonymous array containing the results of the call to C<mysub(1,2,3)>. So | |
449 | the whole block returns a reference to an array, which is then | |
450 | dereferenced by C<@{...}> and stuck into the double-quoted string. This | |
451 | chicanery is also useful for arbitrary expressions: | |
a0d0e21e | 452 | |
184e9718 | 453 | print "That yields @{[$n + 5]} widgets\n"; |
a0d0e21e | 454 | |
35efdb20 DL |
455 | Similarly, an expression that returns a reference to a scalar can be |
456 | dereferenced via C<${...}>. Thus, the above expression may be written | |
457 | as: | |
458 | ||
459 | print "That yields ${\($n + 5)} widgets\n"; | |
460 | ||
a0d0e21e | 461 | =head2 Symbolic references |
d74e8afc ITB |
462 | X<reference, symbolic> X<reference, soft> |
463 | X<symbolic reference> X<soft reference> | |
a0d0e21e LW |
464 | |
465 | We said that references spring into existence as necessary if they are | |
466 | undefined, but we didn't say what happens if a value used as a | |
19799a22 | 467 | reference is already defined, but I<isn't> a hard reference. If you |
7c2ea1c7 | 468 | use it as a reference, it'll be treated as a symbolic |
19799a22 | 469 | reference. That is, the value of the scalar is taken to be the I<name> |
a0d0e21e LW |
470 | of a variable, rather than a direct link to a (possibly) anonymous |
471 | value. | |
472 | ||
473 | People frequently expect it to work like this. So it does. | |
474 | ||
475 | $name = "foo"; | |
476 | $$name = 1; # Sets $foo | |
477 | ${$name} = 2; # Sets $foo | |
478 | ${$name x 2} = 3; # Sets $foofoo | |
479 | $name->[0] = 4; # Sets $foo[0] | |
480 | @$name = (); # Clears @foo | |
481 | &$name(); # Calls &foo() (as in Perl 4) | |
482 | $pack = "THAT"; | |
483 | ${"${pack}::$name"} = 5; # Sets $THAT::foo without eval | |
484 | ||
7c2ea1c7 | 485 | This is powerful, and slightly dangerous, in that it's possible |
a0d0e21e LW |
486 | to intend (with the utmost sincerity) to use a hard reference, and |
487 | accidentally use a symbolic reference instead. To protect against | |
488 | that, you can say | |
489 | ||
490 | use strict 'refs'; | |
491 | ||
492 | and then only hard references will be allowed for the rest of the enclosing | |
54310121 | 493 | block. An inner block may countermand that with |
a0d0e21e LW |
494 | |
495 | no strict 'refs'; | |
496 | ||
5a964f20 TC |
497 | Only package variables (globals, even if localized) are visible to |
498 | symbolic references. Lexical variables (declared with my()) aren't in | |
499 | a symbol table, and thus are invisible to this mechanism. For example: | |
a0d0e21e | 500 | |
5a964f20 | 501 | local $value = 10; |
b0c35547 | 502 | $ref = "value"; |
a0d0e21e LW |
503 | { |
504 | my $value = 20; | |
505 | print $$ref; | |
54310121 | 506 | } |
a0d0e21e LW |
507 | |
508 | This will still print 10, not 20. Remember that local() affects package | |
509 | variables, which are all "global" to the package. | |
510 | ||
748a9306 LW |
511 | =head2 Not-so-symbolic references |
512 | ||
a6006777 | 513 | A new feature contributing to readability in perl version 5.001 is that the |
514 | brackets around a symbolic reference behave more like quotes, just as they | |
748a9306 LW |
515 | always have within a string. That is, |
516 | ||
517 | $push = "pop on "; | |
518 | print "${push}over"; | |
519 | ||
7c2ea1c7 | 520 | has always meant to print "pop on over", even though push is |
748a9306 LW |
521 | a reserved word. This has been generalized to work the same outside |
522 | of quotes, so that | |
523 | ||
524 | print ${push} . "over"; | |
525 | ||
526 | and even | |
527 | ||
528 | print ${ push } . "over"; | |
529 | ||
530 | will have the same effect. (This would have been a syntax error in | |
7c2ea1c7 | 531 | Perl 5.000, though Perl 4 allowed it in the spaceless form.) This |
748a9306 LW |
532 | construct is I<not> considered to be a symbolic reference when you're |
533 | using strict refs: | |
534 | ||
535 | use strict 'refs'; | |
536 | ${ bareword }; # Okay, means $bareword. | |
537 | ${ "bareword" }; # Error, symbolic reference. | |
538 | ||
539 | Similarly, because of all the subscripting that is done using single | |
540 | words, we've applied the same rule to any bareword that is used for | |
541 | subscripting a hash. So now, instead of writing | |
542 | ||
543 | $array{ "aaa" }{ "bbb" }{ "ccc" } | |
544 | ||
5f05dabc | 545 | you can write just |
748a9306 LW |
546 | |
547 | $array{ aaa }{ bbb }{ ccc } | |
548 | ||
549 | and not worry about whether the subscripts are reserved words. In the | |
550 | rare event that you do wish to do something like | |
551 | ||
552 | $array{ shift } | |
553 | ||
554 | you can force interpretation as a reserved word by adding anything that | |
555 | makes it more than a bareword: | |
556 | ||
557 | $array{ shift() } | |
558 | $array{ +shift } | |
559 | $array{ shift @_ } | |
560 | ||
9f1b1f2d GS |
561 | The C<use warnings> pragma or the B<-w> switch will warn you if it |
562 | interprets a reserved word as a string. | |
5f05dabc | 563 | But it will no longer warn you about using lowercase words, because the |
748a9306 LW |
564 | string is effectively quoted. |
565 | ||
49399b3f | 566 | =head2 Pseudo-hashes: Using an array as a hash |
d74e8afc | 567 | X<pseudo-hash> X<pseudo hash> X<pseudohash> |
49399b3f | 568 | |
6d822dc4 MS |
569 | Pseudo-hashes have been removed from Perl. The 'fields' pragma |
570 | remains available. | |
e0478e5a | 571 | |
5a964f20 | 572 | =head2 Function Templates |
d74e8afc ITB |
573 | X<scope, lexical> X<closure> X<lexical> X<lexical scope> |
574 | X<subroutine, nested> X<sub, nested> X<subroutine, local> X<sub, local> | |
5a964f20 | 575 | |
b5c19bd7 DM |
576 | As explained above, an anonymous function with access to the lexical |
577 | variables visible when that function was compiled, creates a closure. It | |
578 | retains access to those variables even though it doesn't get run until | |
579 | later, such as in a signal handler or a Tk callback. | |
5a964f20 TC |
580 | |
581 | Using a closure as a function template allows us to generate many functions | |
c2611fb3 | 582 | that act similarly. Suppose you wanted functions named after the colors |
5a964f20 TC |
583 | that generated HTML font changes for the various colors: |
584 | ||
585 | print "Be ", red("careful"), "with that ", green("light"); | |
586 | ||
7c2ea1c7 | 587 | The red() and green() functions would be similar. To create these, |
5a964f20 TC |
588 | we'll assign a closure to a typeglob of the name of the function we're |
589 | trying to build. | |
590 | ||
591 | @colors = qw(red blue green yellow orange purple violet); | |
592 | for my $name (@colors) { | |
593 | no strict 'refs'; # allow symbol table manipulation | |
594 | *$name = *{uc $name} = sub { "<FONT COLOR='$name'>@_</FONT>" }; | |
595 | } | |
596 | ||
597 | Now all those different functions appear to exist independently. You can | |
598 | call red(), RED(), blue(), BLUE(), green(), etc. This technique saves on | |
599 | both compile time and memory use, and is less error-prone as well, since | |
600 | syntax checks happen at compile time. It's critical that any variables in | |
601 | the anonymous subroutine be lexicals in order to create a proper closure. | |
602 | That's the reasons for the C<my> on the loop iteration variable. | |
603 | ||
604 | This is one of the only places where giving a prototype to a closure makes | |
605 | much sense. If you wanted to impose scalar context on the arguments of | |
606 | these functions (probably not a wise idea for this particular example), | |
607 | you could have written it this way instead: | |
608 | ||
609 | *$name = sub ($) { "<FONT COLOR='$name'>$_[0]</FONT>" }; | |
610 | ||
611 | However, since prototype checking happens at compile time, the assignment | |
612 | above happens too late to be of much use. You could address this by | |
613 | putting the whole loop of assignments within a BEGIN block, forcing it | |
614 | to occur during compilation. | |
615 | ||
58e2a187 CW |
616 | Access to lexicals that change over time--like those in the C<for> loop |
617 | above, basically aliases to elements from the surrounding lexical scopes-- | |
618 | only works with anonymous subs, not with named subroutines. Generally | |
619 | said, named subroutines do not nest properly and should only be declared | |
620 | in the main package scope. | |
621 | ||
622 | This is because named subroutines are created at compile time so their | |
623 | lexical variables get assigned to the parent lexicals from the first | |
624 | execution of the parent block. If a parent scope is entered a second | |
625 | time, its lexicals are created again, while the nested subs still | |
626 | reference the old ones. | |
627 | ||
628 | Anonymous subroutines get to capture each time you execute the C<sub> | |
629 | operator, as they are created on the fly. If you are accustomed to using | |
630 | nested subroutines in other programming languages with their own private | |
631 | variables, you'll have to work at it a bit in Perl. The intuitive coding | |
632 | of this type of thing incurs mysterious warnings about "will not stay | |
633 | shared" due to the reasons explained above. | |
634 | For example, this won't work: | |
5a964f20 TC |
635 | |
636 | sub outer { | |
637 | my $x = $_[0] + 35; | |
638 | sub inner { return $x * 19 } # WRONG | |
639 | return $x + inner(); | |
b432a672 | 640 | } |
5a964f20 TC |
641 | |
642 | A work-around is the following: | |
643 | ||
644 | sub outer { | |
645 | my $x = $_[0] + 35; | |
646 | local *inner = sub { return $x * 19 }; | |
647 | return $x + inner(); | |
b432a672 | 648 | } |
5a964f20 TC |
649 | |
650 | Now inner() can only be called from within outer(), because of the | |
58e2a187 CW |
651 | temporary assignments of the anonymous subroutine. But when it does, |
652 | it has normal access to the lexical variable $x from the scope of | |
653 | outer() at the time outer is invoked. | |
5a964f20 TC |
654 | |
655 | This has the interesting effect of creating a function local to another | |
656 | function, something not normally supported in Perl. | |
657 | ||
cb1a09d0 | 658 | =head1 WARNING |
d74e8afc | 659 | X<reference, string context> X<reference, use as hash key> |
748a9306 LW |
660 | |
661 | You may not (usefully) use a reference as the key to a hash. It will be | |
662 | converted into a string: | |
663 | ||
664 | $x{ \$a } = $a; | |
665 | ||
54310121 | 666 | If you try to dereference the key, it won't do a hard dereference, and |
184e9718 | 667 | you won't accomplish what you're attempting. You might want to do something |
cb1a09d0 | 668 | more like |
748a9306 | 669 | |
cb1a09d0 AD |
670 | $r = \@a; |
671 | $x{ $r } = $r; | |
672 | ||
673 | And then at least you can use the values(), which will be | |
674 | real refs, instead of the keys(), which won't. | |
675 | ||
5a964f20 TC |
676 | The standard Tie::RefHash module provides a convenient workaround to this. |
677 | ||
cb1a09d0 | 678 | =head1 SEE ALSO |
a0d0e21e LW |
679 | |
680 | Besides the obvious documents, source code can be instructive. | |
7c2ea1c7 | 681 | Some pathological examples of the use of references can be found |
a0d0e21e | 682 | in the F<t/op/ref.t> regression test in the Perl source directory. |
cb1a09d0 AD |
683 | |
684 | See also L<perldsc> and L<perllol> for how to use references to create | |
5a964f20 TC |
685 | complex data structures, and L<perltoot>, L<perlobj>, and L<perlbot> |
686 | for how to use them to create objects. |