3 perlref - Perl references and nested data structures
7 Before release 5 of Perl it was difficult to represent complex data
8 structures, because all references had to be symbolic, and even that was
9 difficult to do when you wanted to refer to a variable rather than a
10 symbol table entry. Perl 5 not only makes it easier to use symbolic
11 references to variables, but lets you have "hard" references to any piece
12 of data. Any scalar may hold a hard reference. Since arrays and hashes
13 contain scalars, you can now easily build arrays of arrays, arrays of
14 hashes, hashes of arrays, arrays of hashes of functions, and so on.
16 Hard references are smart--they keep track of reference counts for you,
17 automatically freeing the thing referred to when its reference count
18 goes to zero. (Note: The reference counts for values in self-referential
19 or cyclic data structures may not go to zero without a little help; see
20 L<perlobj/"Two-Phased Garbage Collection"> for a detailed explanation.
21 If that thing happens to be an object, the object is
22 destructed. See L<perlobj> for more about objects. (In a sense,
23 everything in Perl is an object, but we usually reserve the word for
24 references to objects that have been officially "blessed" into a class package.)
27 A symbolic reference contains the name of a variable, just as a
28 symbolic link in the filesystem merely contains the name of a file.
29 The C<*glob> notation is a kind of symbolic reference. Hard references
30 are more like hard links in the file system: merely another way
31 at getting at the same underlying object, irrespective of its name.
33 "Hard" references are easy to use in Perl. There is just one
34 overriding principle: Perl does no implicit referencing or
35 dereferencing. When a scalar is holding a reference, it always behaves
36 as a scalar. It doesn't magically start being an array or a hash
37 unless you tell it so explicitly by dereferencing it.
39 References can be constructed several ways.
45 By using the backslash operator on a variable, subroutine, or value.
46 (This works much like the & (address-of) operator works in C.) Note
47 that this typically creates I<ANOTHER> reference to a variable, since
48 there's already a reference to the variable in the symbol table. But
49 the symbol table reference might go away, and you'll still have the
50 reference that the backslash returned. Here are some examples:
58 It isn't possible to create a reference to a filehandle using the
59 backslash operator. See the explanation of the *foo{THING} syntax
64 A reference to an anonymous array can be constructed using square
67 $arrayref = [1, 2, ['a', 'b', 'c']];
69 Here we've constructed a reference to an anonymous array of three elements
70 whose final element is itself reference to another anonymous array of three
71 elements. (The multidimensional syntax described later can be used to
72 access this. For example, after the above, C<$arrayref-E<gt>[2][1]> would have
75 Note that taking a reference to an enumerated list is not the same
76 as using square brackets--instead it's the same as creating
79 @list = (\$a, \@b, \%c);
80 @list = \($a, @b, %c); # same thing!
82 As a special case, C<\(@foo)> returns a list of references to the contents
83 of C<@foo>, not a reference to C<@foo> itself. Likewise for C<%foo>.
87 A reference to an anonymous hash can be constructed using curly
95 Anonymous hash and array constructors can be intermixed freely to
96 produce as complicated a structure as you want. The multidimensional
97 syntax described below works for these too. The values above are
98 literals, but variables and expressions would work just as well, because
99 assignment operators in Perl (even within local() or my()) are executable
100 statements, not compile-time declarations.
102 Because curly brackets (braces) are used for several other things
103 including BLOCKs, you may occasionally have to disambiguate braces at the
104 beginning of a statement by putting a C<+> or a C<return> in front so
105 that Perl realizes the opening brace isn't starting a BLOCK. The economy and
106 mnemonic value of using curlies is deemed worth this occasional extra
109 For example, if you wanted a function to make a new hash and return a
110 reference to it, you have these options:
112 sub hashem { { @_ } } # silently wrong
113 sub hashem { +{ @_ } } # ok
114 sub hashem { return { @_ } } # ok
118 A reference to an anonymous subroutine can be constructed by using
119 C<sub> without a subname:
121 $coderef = sub { print "Boink!\n" };
123 Note the presence of the semicolon. Except for the fact that the code
124 inside isn't executed immediately, a C<sub {}> is not so much a
125 declaration as it is an operator, like C<do{}> or C<eval{}>. (However, no
126 matter how many times you execute that line (unless you're in an
127 C<eval("...")>), C<$coderef> will still have a reference to the I<SAME>
128 anonymous subroutine.)
130 Anonymous subroutines act as closures with respect to my() variables,
131 that is, variables visible lexically within the current scope. Closure
132 is a notion out of the Lisp world that says if you define an anonymous
133 function in a particular lexical context, it pretends to run in that
134 context even when it's called outside of the context.
136 In human terms, it's a funny way of passing arguments to a subroutine when
137 you define it as well as when you call it. It's useful for setting up
138 little bits of code to run later, such as callbacks. You can even
139 do object-oriented stuff with it, though Perl provides a different
140 mechanism to do that already--see L<perlobj>.
142 You can also think of closure as a way to write a subroutine template without
143 using eval. (In fact, in version 5.000, eval was the I<only> way to get
144 closures. You may wish to use "require 5.001" if you use closures.)
146 Here's a small example of how closures works:
150 return sub { my $y = shift; print "$x, $y!\n"; };
152 $h = newprint("Howdy");
153 $g = newprint("Greetings");
163 Greetings, earthlings!
165 Note particularly that $x continues to refer to the value passed into
166 newprint() I<despite> the fact that the "my $x" has seemingly gone out of
167 scope by the time the anonymous subroutine runs. That's what closure
170 This only applies to lexical variables, by the way. Dynamic variables
171 continue to work as they have always worked. Closure is not something
172 that most Perl programmers need trouble themselves about to begin with.
176 References are often returned by special subroutines called constructors.
177 Perl objects are just references to a special kind of object that happens to know
178 which package it's associated with. Constructors are just special
179 subroutines that know how to create that association. They do so by
180 starting with an ordinary reference, and it remains an ordinary reference
181 even while it's also being an object. Constructors are customarily
182 named new(), but don't have to be:
184 $objref = new Doggie (Tail => 'short', Ears => 'long');
188 References of the appropriate type can spring into existence if you
189 dereference them in a context that assumes they exist. Since we haven't
190 talked about dereferencing yet, we can't show you any examples yet.
194 A reference can be created by using a special syntax, lovingly known as
195 the *foo{THING} syntax. *foo{THING} returns a reference to the THING
196 slot in *foo (which is the symbol table entry which holds everything
199 $scalarref = *foo{SCALAR};
200 $arrayref = *ARGV{ARRAY};
201 $hashref = *ENV{HASH};
202 $coderef = *handler{CODE};
203 $fhref = *STDIN{FILEHANDLE};
204 $globref = *foo{GLOB};
206 Using *foo{FILEHANDLE} is the best way to pass filehandles into or out
207 of subroutines, or to store them in larger data structures.
209 splutter(*STDOUT{FILEHANDLE});
212 print $fh "her um well a hmmm\n";
215 $rec = get_rec(*STDIN{FILEHANDLE});
221 The best way to do this used to be to use the entire *foo typeglob (or a
222 reference to it), so you'll probably come accross old code which does it
227 That's it for creating references. By now you're probably dying to
228 know how to use references to get back to your long-lost data. There
229 are several basic methods.
235 Anywhere you'd put an identifier (or chain of identifiers) as part
236 of a variable or subroutine name, you can replace the identifier with
237 a simple scalar variable containing a reference of the correct type:
240 push(@$arrayref, $filename);
241 $$arrayref[0] = "January";
242 $$hashref{"KEY"} = "VALUE";
244 print $globref "output\n";
246 It's important to understand that we are specifically I<NOT> dereferencing
247 C<$arrayref[0]> or C<$hashref{"KEY"}> there. The dereference of the
248 scalar variable happens I<BEFORE> it does any key lookups. Anything more
249 complicated than a simple scalar variable must use methods 2 or 3 below.
250 However, a "simple scalar" includes an identifier that itself uses method
251 1 recursively. Therefore, the following prints "howdy".
253 $refrefref = \\\"howdy";
258 Anywhere you'd put an identifier (or chain of identifiers) as part of a
259 variable or subroutine name, you can replace the identifier with a
260 BLOCK returning a reference of the correct type. In other words, the
261 previous examples could be written like this:
263 $bar = ${$scalarref};
264 push(@{$arrayref}, $filename);
265 ${$arrayref}[0] = "January";
266 ${$hashref}{"KEY"} = "VALUE";
268 $globref->print("output\n"); # iff you use FileHandle
270 Admittedly, it's a little silly to use the curlies in this case, but
271 the BLOCK can contain any arbitrary expression, in particular,
272 subscripted expressions:
274 &{ $dispatch{$index} }(1,2,3); # call correct routine
276 Because of being able to omit the curlies for the simple case of C<$$x>,
277 people often make the mistake of viewing the dereferencing symbols as
278 proper operators, and wonder about their precedence. If they were,
279 though, you could use parens instead of braces. That's not the case.
280 Consider the difference below; case 0 is a short-hand version of case 1,
283 $$hashref{"KEY"} = "VALUE"; # CASE 0
284 ${$hashref}{"KEY"} = "VALUE"; # CASE 1
285 ${$hashref{"KEY"}} = "VALUE"; # CASE 2
286 ${$hashref->{"KEY"}} = "VALUE"; # CASE 3
288 Case 2 is also deceptive in that you're accessing a variable
289 called %hashref, not dereferencing through $hashref to the hash
290 it's presumably referencing. That would be case 3.
294 The case of individual array elements arises often enough that it gets
295 cumbersome to use method 2. As a form of syntactic sugar, the two
296 lines like that above can be written:
298 $arrayref->[0] = "January";
299 $hashref->{"KEY"} = "VALUE";
301 The left side of the array can be any expression returning a reference,
302 including a previous dereference. Note that C<$array[$x]> is I<NOT> the
303 same thing as C<$array-E<gt>[$x]> here:
305 $array[$x]->{"foo"}->[0] = "January";
307 This is one of the cases we mentioned earlier in which references could
308 spring into existence when in an lvalue context. Before this
309 statement, C<$array[$x]> may have been undefined. If so, it's
310 automatically defined with a hash reference so that we can look up
311 C<{"foo"}> in it. Likewise C<$array[$x]-E<gt>{"foo"}> will automatically get
312 defined with an array reference so that we can look up C<[0]> in it.
314 One more thing here. The arrow is optional I<BETWEEN> brackets
315 subscripts, so you can shrink the above down to
317 $array[$x]{"foo"}[0] = "January";
319 Which, in the degenerate case of using only ordinary arrays, gives you
320 multidimensional arrays just like C's:
322 $score[$x][$y][$z] += 42;
324 Well, okay, not entirely like C's arrays, actually. C doesn't know how
325 to grow its arrays on demand. Perl does.
329 If a reference happens to be a reference to an object, then there are
330 probably methods to access the things referred to, and you should probably
331 stick to those methods unless you're in the class package that defines the
332 object's methods. In other words, be nice, and don't violate the object's
333 encapsulation without a very good reason. Perl does not enforce
334 encapsulation. We are not totalitarians here. We do expect some basic
339 The ref() operator may be used to determine what type of thing the
340 reference is pointing to. See L<perlfunc>.
342 The bless() operator may be used to associate a reference with a package
343 functioning as an object class. See L<perlobj>.
345 A typeglob may be dereferenced the same way a reference can, since
346 the dereference syntax always indicates the kind of reference desired.
347 So C<${*foo}> and C<${\$foo}> both indicate the same scalar variable.
349 Here's a trick for interpolating a subroutine call into a string:
351 print "My sub returned @{[mysub(1,2,3)]} that time.\n";
353 The way it works is that when the C<@{...}> is seen in the double-quoted
354 string, it's evaluated as a block. The block creates a reference to an
355 anonymous array containing the results of the call to C<mysub(1,2,3)>. So
356 the whole block returns a reference to an array, which is then
357 dereferenced by C<@{...}> and stuck into the double-quoted string. This
358 chicanery is also useful for arbitrary expressions:
360 print "That yields @{[$n + 5]} widgets\n";
362 =head2 Symbolic references
364 We said that references spring into existence as necessary if they are
365 undefined, but we didn't say what happens if a value used as a
366 reference is already defined, but I<ISN'T> a hard reference. If you
367 use it as a reference in this case, it'll be treated as a symbolic
368 reference. That is, the value of the scalar is taken to be the I<NAME>
369 of a variable, rather than a direct link to a (possibly) anonymous
372 People frequently expect it to work like this. So it does.
375 $$name = 1; # Sets $foo
376 ${$name} = 2; # Sets $foo
377 ${$name x 2} = 3; # Sets $foofoo
378 $name->[0] = 4; # Sets $foo[0]
379 @$name = (); # Clears @foo
380 &$name(); # Calls &foo() (as in Perl 4)
382 ${"${pack}::$name"} = 5; # Sets $THAT::foo without eval
384 This is very powerful, and slightly dangerous, in that it's possible
385 to intend (with the utmost sincerity) to use a hard reference, and
386 accidentally use a symbolic reference instead. To protect against
391 and then only hard references will be allowed for the rest of the enclosing
392 block. An inner block may countermand that with
396 Only package variables are visible to symbolic references. Lexical
397 variables (declared with my()) aren't in a symbol table, and thus are
398 invisible to this mechanism. For example:
407 This will still print 10, not 20. Remember that local() affects package
408 variables, which are all "global" to the package.
410 =head2 Not-so-symbolic references
412 A new feature contributing to readability in 5.001 is that the brackets
413 around a symbolic reference behave more like quotes, just as they
414 always have within a string. That is,
419 has always meant to print "pop on over", despite the fact that push is
420 a reserved word. This has been generalized to work the same outside
423 print ${push} . "over";
427 print ${ push } . "over";
429 will have the same effect. (This would have been a syntax error in
430 5.000, though Perl 4 allowed it in the spaceless form.) Note that this
431 construct is I<not> considered to be a symbolic reference when you're
435 ${ bareword }; # Okay, means $bareword.
436 ${ "bareword" }; # Error, symbolic reference.
438 Similarly, because of all the subscripting that is done using single
439 words, we've applied the same rule to any bareword that is used for
440 subscripting a hash. So now, instead of writing
442 $array{ "aaa" }{ "bbb" }{ "ccc" }
446 $array{ aaa }{ bbb }{ ccc }
448 and not worry about whether the subscripts are reserved words. In the
449 rare event that you do wish to do something like
453 you can force interpretation as a reserved word by adding anything that
454 makes it more than a bareword:
460 The B<-w> switch will warn you if it interprets a reserved word as a string.
461 But it will no longer warn you about using lowercase words, since the
462 string is effectively quoted.
466 You may not (usefully) use a reference as the key to a hash. It will be
467 converted into a string:
471 If you try to dereference the key, it won't do a hard dereference, and
472 you won't accomplish what you're attempting. You might want to do something
478 And then at least you can use the values(), which will be
479 real refs, instead of the keys(), which won't.
483 Besides the obvious documents, source code can be instructive.
484 Some rather pathological examples of the use of references can be found
485 in the F<t/op/ref.t> regression test in the Perl source directory.
487 See also L<perldsc> and L<perllol> for how to use references to create
488 complex data structures, and L<perlobj> for how to use them to create