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1 perry 1 PERLFAQ7
2 !!!PERLFAQ7
3 NAME
4 DESCRIPTION
5 AUTHOR AND COPYRIGHT
6 ----
7 !!NAME
8
9
10 perlfaq7 - Perl Language Issues ($Revision: 1.28 $, $Date: 1999/05/23 20:36:18 $)
11 !!DESCRIPTION
12
13
14 This section deals with general Perl language issues that
15 don't clearly fit into any of the other
16 sections.
17
18
19 __Can I get a BNF/yacc/RE for the Perl
20 language?__
21
22
23 There is no BNF , but you can paw your way
24 through the yacc grammar in perly.y in the source
25 distribution if you're particularly brave. The grammar
26 relies on very smart tokenizing code, so be prepared to
27 venture into toke.c as well.
28
29
30 In the words of Chaim Frenkel: ``Perl's grammar can not be
31 reduced to BNF . The work of parsing perl is
32 distributed between yacc, the lexer, smoke and
33 mirrors.''
34
35
36 __What are all these $@%
37 __
38
39
40 They are type specifiers, as detailed in
41 perldata:
42
43
44 $ for scalar values (number, string or reference)
45 @ for arrays
46 % for hashes (associative arrays)
47 There are couple of other symbols that you're likely to encounter that aren't really type specifiers:
48
49
50
51 Note that FILE neither'' the type specifier for files nor the name of the handle. It is the operator applied to the handle FILE . It reads one line (well, record--see ``$/'' in perlvar) from the handle FILE in scalar context, or ''all'' lines in list context. When performing open, close, or any other operation besides on files, or even when talking about the handle, do ''not'' use the brackets. These are correct: eof(FH), seek(FH, 0, 2) and ``copying from STDIN to FILE ''.
52
53
54 __Do I always/never have to quote my strings or use
55 semicolons and commas?__
56
57
58 Normally, a bareword doesn't need to be quoted, but in most
59 cases probably should be (and must be under use
60 strict). But a hash key consisting of a simple word
61 (that isn't the name of a defined subroutine) and the
62 left-hand operand to the = operator both count
63 as though they were quoted:
64
65
66 This is like this
67 ------------ ---------------
68 $foo{line} $foo{
69 The final semicolon in a block is optional, as is the final comma in a list. Good style (see perlstyle) says to put them in except for one-liners:
70
71
72 if ($whoops) { exit 1 }
73 @nums = (1, 2, 3);
74 if ($whoops) {
75 exit 1;
76 }
77 @lines = (
78
79
80 __How do I skip some return values?__
81
82
83 One way is to treat the return values as a list and index
84 into it:
85
86
87 $dir = (getpwnam($user))[[7];
88 Another way is to use undef as an element on the left-hand-side:
89
90
91 ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
92
93
94 __How do I temporarily block warnings?__
95
96
97 If you are running Perl 5.6.0 or better, the use
98 warnings pragma allows fine control of what warning are
99 produced. See perllexwarn for more details.
100
101
102 {
103 no warnings; # temporarily turn off warnings
104 $a = $b + $c; # I know these might be undef
105 }
106 If you have an older version of Perl, the $^W variable (documented in perlvar) controls runtime warnings for a block:
107
108
109 {
110 local $^W = 0; # temporarily turn off warnings
111 $a = $b + $c; # I know these might be undef
112 }
113 Note that like all the punctuation variables, you cannot currently use ''my()'' on $^W, only ''local()''.
114
115
116 __What's an extension?__
117
118
119 An extension is a way of calling compiled C code from Perl.
120 Reading perlxstut is a good place to learn more about
121 extensions.
122
123
124 __Why do Perl operators have different precedence than C
125 operators?__
126
127
128 Actually, they don't. All C operators that Perl copies have
129 the same precedence in Perl as they do in C. The problem is
130 with operators that C doesn't have, especially functions
131 that give a list context to everything on their right, eg.
132 print, chmod, exec, and so on. Such functions are called
133 ``list operators'' and appear as such in the precedence
134 table in perlop.
135
136
137 A common mistake is to write:
138
139
140 unlink $file die
141 This gets interpreted as:
142
143
144 unlink ($file die
145 To avoid this problem, either put in extra parentheses or use the super low precedence or operator:
146
147
148 (unlink $file) die
149 The ``English'' operators (and, or, xor, and not) deliberately have precedence lower than that of list operators for just such situations as the one above.
150
151
152 Another operator with surprising precedence is
153 exponentiation. It binds more tightly even than unary minus,
154 making -2**2 product a negative not a positive
155 four. It is also right-associating, meaning that
156 2**3**2 is two raised to the ninth power, not eight
157 squared.
158
159
160 Although it has the same precedence as in C, Perl's
161 ?: operator produces an lvalue. This assigns
162 $x to either $a or $b, depending
163 on the trueness of $maybe:
164
165
166 ($maybe ? $a : $b) = $x;
167
168
169 __How do I declare/create a structure?__
170
171
172 In general, you don't ``declare'' a structure. Just use a
173 (probably anonymous) hash reference. See perlref and perldsc
174 for details. Here's an example:
175
176
177 $person = {}; # new anonymous hash
178 $person-
179 If you're looking for something a bit more rigorous, try perltoot.
180
181
182 __How do I create a module?__
183
184
185 A module is a package that lives in a file of the same name.
186 For example, the Hello::There module would live in
187 Hello/There.pm. For details, read perlmod. You'll also find
188 Exporter helpful. If you're writing a C or mixed-language
189 module with both C and Perl, then you should study
190 perlxstut.
191
192
193 Here's a convenient template you might wish you use when
194 starting your own module. Make sure to change the names
195 appropriately.
196
197
198 package Some::Module; # assumes Some/Module.pm
199 use strict;
200 use warnings;
201 BEGIN {
202 use Exporter ();
203 our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);
204 ## set the version for version checking; uncomment to use
205 ## $VERSION = 1.00;
206 # if using RCS/CVS, this next line may be preferred,
207 # but beware two-digit versions.
208 $VERSION = do{my@r=q$Revision: 1.28 $=~/d+/g;sprintf '%d.'.'%02d'x$#r,@r};
209 @ISA = qw(Exporter);
210 @EXPORT = qw(
211 # your exported package globals go here,
212 # as well as any optionally exported functions
213 @EXPORT_OK = qw($Var1 %Hashit);
214 }
215 our @EXPORT_OK;
216 # exported package globals go here
217 our $Var1;
218 our %Hashit;
219 # non-exported package globals go here
220 our @more;
221 our $stuff;
222 # initialize package globals, first exported ones
223 $Var1 = '';
224 %Hashit = ();
225 # then the others (which are still accessible as $Some::Module::stuff)
226 $stuff = '';
227 @more = ();
228 # all file-scoped lexicals must be created before
229 # the functions below that use them.
230 # file-private lexicals go here
231 my $priv_var = '';
232 my %secret_hash = ();
233 # here's a file-private function as a closure,
234 # callable as
235 # make all your functions, whether exported or not;
236 # remember to put something interesting in the {} stubs
237 sub func1 {} # no prototype
238 sub func2() {} # proto'd void
239 sub func3($$) {} # proto'd to 2 scalars
240 # this one isn't exported, but could be called!
241 sub func4(%) {} # proto'd to 1 hash ref
242 END { } # module clean-up code here (global destructor)
243 1; # modules must return true
244 The h2xs program will create stubs for all the important stuff for you:
245
246
247 % h2xs -XA -n My::Module
248
249
250 __How do I create a class?__
251
252
253 See perltoot for an introduction to classes and objects, as
254 well as perlobj and perlbot.
255
256
257 __How can I tell if a variable is tainted?__
258
259
260 See ``Laundering and Detecting Tainted Data'' in perlsec.
261 Here's an example (which doesn't use any system calls,
262 because the ''kill()'' is given no processes to
263 signal):
264
265
266 sub is_tainted {
267 return ! eval { join('',@_), kill 0; 1; };
268 }
269 This is not -w clean, however. There is no -w clean way to detect taintedness--take this as a hint that you should untaint all possibly-tainted data.
270
271
272 __What's a closure?__
273
274
275 Closures are documented in perlref.
276
277
278 ''Closure'' is a computer science term with a precise but
279 hard-to-explain meaning. Closures are implemented in Perl as
280 anonymous subroutines with lasting references to lexical
281 variables outside their own scopes. These lexicals magically
282 refer to the variables that were around when the subroutine
283 was defined (deep binding).
284
285
286 Closures make sense in any programming language where you
287 can have the return value of a function be itself a
288 function, as you can in Perl. Note that some languages
289 provide anonymous functions but are not capable of providing
290 proper closures: the Python language, for example. For more
291 information on closures, check out any textbook on
292 functional programming. Scheme is a language that not only
293 supports but encourages closures.
294
295
296 Here's a classic function-generating function:
297
298
299 sub add_function_generator {
300 return sub { shift + shift };
301 }
302 $add_sub = add_function_generator();
303 $sum = $add_sub-
304 The closure works as a ''function template'' with some customization slots left out to be filled later. The anonymous subroutine returned by ''add_function_generator()'' isn't technically a closure because it refers to no lexicals outside its own scope.
305
306
307 Contrast this with the following ''make_adder()''
308 function, in which the returned anonymous function contains
309 a reference to a lexical variable outside the scope of that
310 function itself. Such a reference requires that Perl return
311 a proper closure, thus locking in for all time the value
312 that the lexical had when the function was
313 created.
314
315
316 sub make_adder {
317 my $addpiece = shift;
318 return sub { shift + $addpiece };
319 }
320 $f1 = make_adder(20);
321 $f2 = make_adder(555);
322 Now is always 20 plus whatever $n you pass in, whereas is always 555 plus whatever $n you pass in. The $addpiece in the closure sticks around.
323
324
325 Closures are often used for less esoteric purposes. For
326 example, when you want to pass in a bit of code into a
327 function:
328
329
330 my $line;
331 timeout( 30, sub { $line =
332 If the code to execute had been passed in as a string, '$line = , there would have been no way for the hypothetical ''timeout()'' function to access the lexical variable $line back in its caller's scope.
333
334
335 __What is variable suicide and how can I prevent
336 it?__
337
338
339 Variable suicide is when you (temporarily or permanently)
340 lose the value of a variable. It is caused by scoping
341 through ''my()'' and ''local()'' interacting with
342 either closures or aliased ''foreach()'' iterator
343 variables and subroutine arguments. It used to be easy to
344 inadvertently lose a variable's value this way, but now it's
345 much harder. Take this code:
346
347
348 my $f =
349 The $f that has ``bar'' added to it three times should be a new $f (my $f should create a new local variable each time through the loop). It isn't, however. This was a bug, now fixed in the latest releases (tested against 5.004_05, 5.005_03, and 5.005_56).
350
351
2 perry 352 __How can I pass/return a {Function, !FileHandle, Array,
1 perry 353 Hash, Method, Regex}?__
354
355
356 With the exception of regexes, you need to pass references
357 to these objects. See ``Pass by Reference'' in perlsub for
358 this particular question, and perlref for information on
359 references.
360
361
362 See ``Passing Regexes'', below, for information on passing
363 regular expressions.
364
365
366 Passing Variables and Functions
367
368
369 Regular variables and functions are quite easy to pass: just
370 pass in a reference to an existing or anonymous variable or
371 function:
372
373
374 func( $some_scalar );
375 func( @some_array );
376 func( [[ 1 .. 10 ] );
377 func( %some_hash );
378 func( { this =
379 func(
380
381
382 Passing Filehandles
383
384
385 To pass filehandles to subroutines, use the *FH or
386 *FH notations. These are ``typeglobs''--see
387 ``Typeglobs and Filehandles'' in perldata and especially
388 ``Pass by Reference'' in perlsub for more
389 information.
390
391
392 Here's an excerpt:
393
394
395 If you're passing around filehandles, you could usually just
396 use the bare typeglob, like *STDOUT, but typeglobs
397 references would be better because they'll still work
398 properly under use strict 'refs'. For
399 example:
400
401
402 splutter(*STDOUT);
403 sub splutter {
404 my $fh = shift;
405 print $fh
406 $rec = get_rec(*STDIN);
407 sub get_rec {
408 my $fh = shift;
409 return scalar
410 If you're planning on generating new filehandles, you could do this:
411
412
413 sub openit {
414 my $path = shift;
415 local *FH;
416 return open (FH, $path) ? *FH : undef;
417 }
418 $fh = openit('
419
420
421 Passing Regexes
422
423
424 To pass regexes around, you'll need to be using a release of
425 Perl sufficiently recent as to support the qr//
426 construct, pass around strings and use an exception-trapping
427 eval, or else be very, very clever.
428
429
430 Here's an example of how to pass in a string to be regex
431 compared using qr//:
432
433
434 sub compare($$) {
435 my ($val1, $regex) = @_;
436 my $retval = $val1 =~ /$regex/;
437 return $retval;
438 }
439 $match = compare(
440 Notice how qr// allows flags at the end. That pattern was compiled at compile time, although it was executed later. The nifty qr// notation wasn't introduced until the 5.005 release. Before that, you had to approach this problem much less intuitively. For example, here it is again if you don't have qr//:
441
442
443 sub compare($$) {
444 my ($val1, $regex) = @_;
445 my $retval = eval { $val1 =~ /$regex/ };
446 die if $@;
447 return $retval;
448 }
449 $match = compare(
450 Make sure you never say something like this:
451
452
453 return eval
454 or someone can sneak shell escapes into the regex due to the double interpolation of the eval and the double-quoted string. For example:
455
456
457 $pattern_of_evil = 'danger ${ system(
458 eval
2 perry 459 Those preferring to be very, very clever might see the O'Reilly book, ''Mastering Regular Expressions'', by Jeffrey Friedl. Page 273's ''Build_!MatchMany_Function()'' is particularly interesting. A complete citation of this book is given in perlfaq2.
1 perry 460
461
462 Passing Methods
463
464
465 To pass an object method into a subroutine, you can do
466 this:
467
468
469 call_a_lot(10, $some_obj,
470 Or, you can use a closure to bundle up the object, its method call, and arguments:
471
472
473 my $whatnot = sub { $some_obj-
474 You could also investigate the ''can()'' method in the UNIVERSAL class (part of the standard perl distribution).
475
476
477 __How do I create a static variable?__
478
479
480 As with most things in Perl, TMTOWTDI . What
481 is a ``static variable'' in other languages could be either
482 a function-private variable (visible only within a single
483 function, retaining its value between calls to that
484 function), or a file-private variable (visible only to
485 functions within the file it was declared in) in
486 Perl.
487
488
489 Here's code to implement a function-private
490 variable:
491
492
493 BEGIN {
494 my $counter = 42;
495 sub prev_counter { return --$counter }
496 sub next_counter { return $counter++ }
497 }
498 Now ''prev_counter()'' and ''next_counter()'' share a private variable $counter that was initialized at compile time.
499
500
501 To declare a file-private variable, you'll still use a
502 ''my()'', putting the declaration at the outer scope
503 level at the top of the file. Assume this is in file
504 Pax.pm:
505
506
507 package Pax;
508 my $started = scalar(localtime(time()));
509 sub begun { return $started }
510 When use Pax or require Pax loads this module, the variable will be initialized. It won't get garbage-collected the way most variables going out of scope do, because the ''begun()'' function cares about it, but no one else can get it. It is not called $Pax::started because its scope is unrelated to the package. It's scoped to the file. You could conceivably have several packages in that same file all accessing the same private variable, but another file with the same package couldn't get to it.
511
512
513 See ``Persistent Private Variables'' in perlsub for
514 details.
515
516
517 __What's the difference between dynamic and lexical
518 (static) scoping? Between__ ''local()'' __and__
519 ''my()''__?__
520
521
522 local($x) saves away the old value of the global
523 variable $x and assigns a new value for the
524 duration of the subroutine ''which is visible in other
525 functions called from that subroutine''. This is done at
526 run-time, so is called dynamic scoping. ''local()''
527 always affects global variables, also called package
528 variables or dynamic variables.
529
530
531 my($x) creates a new variable that is only visible
532 in the current subroutine. This is done at compile-time, so
533 it is called lexical or static scoping. ''my()'' always
534 affects private variables, also called lexical variables or
535 (improperly) static(ly scoped) variables.
536
537
538 For instance:
539
540
541 sub visible {
542 print
543 sub dynamic {
544 local $var = 'local'; # new temporary value for the still-global
545 visible(); # variable called $var
546 }
547 sub lexical {
548 my $var = 'private'; # new private variable, $var
549 visible(); # (invisible outside of sub scope)
550 }
551 $var = 'global';
552 visible(); # prints global
553 dynamic(); # prints local
554 lexical(); # prints global
555 Notice how at no point does the value ``private'' get printed. That's because $var only has that value within the block of the ''lexical()'' function, and it is hidden from called subroutine.
556
557
558 In summary, ''local()'' doesn't make what you think of as
559 private, local variables. It gives a global variable a
560 temporary value. ''my()'' is what you're looking for if
561 you want private variables.
562
563
564 See ``Private Variables via ''my()'''' in perlsub and
565 ``Temporary Values via ''local()'''' in perlsub for
566 excruciating details.
567
568
569 __How can I access a dynamic variable while a similarly
570 named lexical is in scope?__
571
572
573 You can do this via symbolic references, provided you
574 haven't set use strict . So instead
575 of $var, use ${'var'}.
576
577
578 local $var =
579 print
580 no strict 'refs';
581 print
582 If you know your package, you can just mention it explicitly, as in $Some_Pack::var. Note that the notation $::var is ''not'' the dynamic $var in the current package, but rather the one in the main package, as though you had written $main::var. Specifying the package directly makes you hard-code its name, but it executes faster and avoids running afoul of use strict .
583
584
585 __What's the difference between deep and shallow
586 binding?__
587
588
589 In deep binding, lexical variables mentioned in anonymous
590 subroutines are the same ones that were in scope when the
591 subroutine was created. In shallow binding, they are
592 whichever variables with the same names happen to be in
593 scope when the subroutine is called. Perl always uses deep
594 binding of lexical variables (i.e., those created with
595 ''my()''). However, dynamic variables (aka global, local,
596 or package variables) are effectively shallowly bound.
597 Consider this just one more reason not to use them. See the
598 answer to ``What's a closure?''.
599
600
601 __Why doesn't ``my($foo) = __FILE
602
603
604 my() and local() give list context to the
605 right hand side of =. The FH
606 scalar()''
607 function can help. This function does nothing to the data
608 itself (contrary to popular myth) but rather tells its
609 argument to behave in whatever its scalar fashion is. If
610 that function doesn't have a defined scalar behavior, this
611 of course doesn't help you (such as with
612 ''sort()'').
613
614
615 To enforce scalar context in this particular case, however,
616 you need merely omit the parentheses:
617
618
619 local($foo) =
620 You should probably be using lexical variables anyway, although the issue is the same here:
621
622
623 my($foo) =
624
625
626 __How do I redefine a builtin function, operator, or
627 method?__
628
629
630 Why do you want to do that? :-)
631
632
633 If you want to override a predefined function, such as
634 ''open()'', then you'll have to import the new definition
635 from a different module. See ``Overriding Built-in
636 Functions'' in perlsub. There's also an example in
637 ``Class::Template'' in perltoot.
638
639
640 If you want to overload a Perl operator, such as +
641 or **, then you'll want to use the use
642 overload pragma, documented in overload.
643
644
645 If you're talking about obscuring method calls in parent
646 classes, see ``Overridden Methods'' in
647 perltoot.
648
649
650 __What's the difference between calling a function as
651 __ ''foo()''__?__
652
653
654 When you call a function as , you allow
655 that function access to your current @_ values, and
656 you bypass prototypes. The function doesn't get an empty
657 @_--it gets yours! While not strictly speaking a
658 bug (it's documented that way in perlsub), it would be hard
659 to consider this a feature in most cases.
660
661
662 When you call your function as , then you
663 ''do'' get a new @_, but prototyping is still
664 circumvented.
665
666
667 Normally, you want to call a function using foo().
668 You may only omit the parentheses if the function is already
669 known to the compiler because it already saw the definition
670 (use but not require), or via a forward
671 reference or use subs declaration. Even in this
672 case, you get a clean @_ without any of the old
673 values leaking through where they don't belong.
674
675
676 __How do I create a switch or case
677 statement?__
678
679
680 This is explained in more depth in the perlsyn. Briefly,
681 there's no official case statement, because of the variety
682 of tests possible in Perl (numeric comparison, string
683 comparison, glob comparison, regex matching, overloaded
684 comparisons, ...). Larry couldn't decide how best to do
685 this, so he left it out, even though it's been on the wish
686 list since perl1.
687
688
689 The general answer is to write a construct like
690 this:
691
692
693 for ($variable_to_test) {
694 if (/pat1/) { } # do something
695 elsif (/pat2/) { } # do something else
696 elsif (/pat3/) { } # do something else
697 else { } # default
698 }
699 Here's a simple example of a switch based on pattern matching, this time lined up in a way to make it look more like a switch statement. We'll do a multi-way conditional based on the type of reference stored in $whatchamacallit:
700
701
702 SWITCH: for (ref $whatchamacallit) {
703 /^$/
704 /SCALAR/
705 /ARRAY/
706 /HASH/
707 /CODE/
708 # DEFAULT
709 warn
710 }
711 See perlsyn/ for many other examples in this style.
712
713
714 Sometimes you should change the positions of the constant
715 and the variable. For example, let's say you wanted to test
716 which of many answers you were given, but in a
717 case-insensitive way that also allows abbreviations. You can
718 use the following technique if the strings all start with
719 different characters or if you want to arrange the matches
720 so that one takes precedence over another, as
721 has precedence over
722 here:
723
724
725 chomp($answer =
726 A totally different approach is to create a hash of function references.
727
728
729 my %commands = (
730 print
731
732
733 __How can I catch accesses to undefined
734 variables/functions/methods?__
735
736
737 The AUTOLOAD method, discussed in
738 ``Autoloading'' in perlsub and `` AUTOLOAD:
739 Proxy Methods'' in perltoot, lets you capture calls to
740 undefined functions and methods.
741
742
743 When it comes to undefined variables that would trigger a
744 warning under -w, you can use a handler to trap the
745 pseudo-signal __WARN__ like this:
746
747
748 $SIG{__WARN__} = sub {
749 for ( $_[[0] ) { # voici un switch statement
750 /Use of uninitialized value/
751 # other warning cases to catch could go here;
752 warn $_;
753 }
754 };
755
756
757 __Why can't a method included in this same file be
758 found?__
759
760
761 Some possible reasons: your inheritance is getting confused,
762 you've misspelled the method name, or the object is of the
763 wrong type. Check out perltoot for details about any of the
764 above cases. You may also use print ref($object) to
765 find out the class $object was blessed
766 into.
767
768
769 Another possible reason for problems is because you've used
770 the indirect object syntax (eg, find Guru
771 ) on a class name before Perl has seen
772 that such a package exists. It's wisest to make sure your
773 packages are all defined before you start using them, which
774 will be taken care of if you use the use statement
775 instead of require. If not, make sure to use arrow
776 notation (eg., Guru-)
777 instead. Object notation is explained in
778 perlobj.
779
780
781 Make sure to read about creating modules in perlmod and the
782 perils of indirect objects in `` WARNING ''
783 in perlobj.
784
785
786 __How can I find out my current package?__
787
788
789 If you're just a random program, you can do this to find out
790 what the currently compiled package is:
791
792
793 my $packname = __PACKAGE__;
794 But, if you're a method and you want to print an error message that includes the kind of object you were called on (which is not necessarily the same as the one in which you were compiled):
795
796
797 sub amethod {
798 my $self = shift;
799 my $class = ref($self) $self;
800 warn
801
802
803 __How can I comment out a large block of perl
804 code?__
805
806
807 Use embedded POD to discard it:
808
809
810 # program is here
811 =for nobody
812 This paragraph is commented out
813 # program continues
814 =begin comment text
815 all of this stuff
816 here will be ignored
817 by everyone
818 =end comment text
819 =cut
820 This can't go just anywhere. You have to put a pod directive where the parser is expecting a new statement, not just in the middle of an expression or some other arbitrary yacc grammar production.
821
822
823 __How do I clear a package?__
824
825
826 Use this code, provided by Mark-Jason Dominus:
827
828
829 sub scrub_package {
830 no strict 'refs';
831 my $pack = shift;
832 die
833 Or, if you're using a recent release of Perl, you can just use the ''Symbol::delete_package()'' function instead.
834
835
836 __How can I use a variable as a variable
837 name?__
838
839
840 Beginners often think they want to have a variable contain
841 the name of a variable.
842
843
844 $fred = 23;
845 $varname =
846 This works ''sometimes'', but it is a very bad idea for two reasons.
847
848
849 The first reason is that this technique ''only works on
850 global variables''. That means that if $fred is a
851 lexical variable created with ''my()'' in the above
852 example, the code wouldn't work at all: you'd accidentally
853 access the global and skip right over the private lexical
854 altogether. Global variables are bad because they can easily
855 collide accidentally and in general make for non-scalable
856 and confusing code.
857
858
859 Symbolic references are forbidden under the use
860 strict pragma. They are not true references and
861 consequently are not reference counted or garbage
862 collected.
863
864
865 The other reason why using a variable to hold the name of
866 another variable is a bad idea is that the question often
867 stems from a lack of understanding of Perl data structures,
868 particularly hashes. By using symbolic references, you are
869 just using the package's symbol-table hash (like
870 %main::) instead of a user-defined hash. The
871 solution is to use your own hash or a real reference
872 instead.
873
874
875 $fred = 23;
876 $varname =
877 There we're using the %USER_VARS hash instead of symbolic references. Sometimes this comes up in reading strings from the user with variable references and wanting to expand them to the values of your perl program's variables. This is also a bad idea because it conflates the program-addressable namespace and the user-addressable one. Instead of reading a string and expanding it to the actual contents of your program's own variables:
878
879
880 $str = 'this has a $fred and $barney in it';
881 $str =~ s/($w+)/$1/eeg; # need double eval
882 it would be better to keep a hash around like %USER_VARS and have variable references actually refer to entries in that hash:
883
884
885 $str =~ s/$(w+)/$USER_VARS{$1}/g; # no /e here at all
886 That's faster, cleaner, and safer than the previous approach. Of course, you don't need to use a dollar sign. You could use your own scheme to make it less confusing, like bracketed percent symbols, etc.
887
888
889 $str = 'this has a %fred% and %barney% in it';
890 $str =~ s/%(w+)%/$USER_VARS{$1}/g; # no /e here at all
891 Another reason that folks sometimes think they want a variable to contain the name of a variable is because they don't know how to build proper data structures using hashes. For example, let's say they wanted two hashes in their program: %fred and %barney, and that they wanted to use another scalar variable to refer to those by name.
892
893
894 $name =
895 $name =
896 This is still a symbolic reference, and is still saddled with the problems enumerated above. It would be far better to write:
897
898
899 $folks{
900 And just use a multilevel hash to start with.
901
902
903 The only times that you absolutely ''must'' use symbolic
904 references are when you really must refer to the symbol
905 table. This may be because it's something that can't take a
906 real reference to, such as a format name. Doing so may also
907 be important for method calls, since these always go through
908 the symbol table for resolution.
909
910
911 In those cases, you would turn off strict 'refs'
912 temporarily so you can play around with the symbol table.
913 For example:
914
915
916 @colors = qw(red blue green yellow orange purple violet);
917 for my $name (@colors) {
918 no strict 'refs'; # renege for the block
919 *$name = sub {
920 All those functions (''red()'', ''blue()'', ''green()'', etc.) appear to be separate, but the real code in the closure actually was compiled only once.
921
922
923 So, sometimes you might want to use symbolic references to
924 directly manipulate the symbol table. This doesn't matter
925 for formats, handles, and subroutines, because they are
926 always global--you can't use ''my()'' on them. For
927 scalars, arrays, and hashes, though--and usually for
928 subroutines-- you probably only want to use hard
929 references.
930 !!AUTHOR AND COPYRIGHT
931
932
933 Copyright (c) 1997-1999 Tom Christiansen and Nathan
934 Torkington. All rights reserved.
935
936
937 When included as part of the Standard Version of Perl, or as
938 part of its complete documentation whether printed or
939 otherwise, this work may be distributed only under the terms
940 of Perl's Artistic License. Any distribution of this file or
941 derivatives thereof ''outside'' of that package require
942 that special arrangements be made with copyright
943 holder.
944
945
946 Irrespective of its distribution, all code examples in this
947 file are hereby placed into the public domain. You are
948 permitted and encouraged to use this code in your own
949 programs for fun or for profit as you see fit. A simple
950 comment in the code giving credit would be courteous but is
951 not required.
952 ----
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