3 perlfaq7 - Perl Language Issues ($Revision: 1.28 $, $Date: 1999/05/23 20:36:18 $)
7 This section deals with general Perl language issues that don't
8 clearly fit into any of the other sections.
10 =head2 Can I get a BNF/yacc/RE for the Perl language?
12 There is no BNF, but you can paw your way through the yacc grammar in
13 perly.y in the source distribution if you're particularly brave. The
14 grammar relies on very smart tokenizing code, so be prepared to
15 venture into toke.c as well.
17 In the words of Chaim Frenkel: "Perl's grammar can not be reduced to BNF.
18 The work of parsing perl is distributed between yacc, the lexer, smoke
21 =head2 What are all these $@%&* punctuation signs, and how do I know when to use them?
23 They are type specifiers, as detailed in L<perldata>:
25 $ for scalar values (number, string or reference)
27 % for hashes (associative arrays)
28 & for subroutines (aka functions, procedures, methods)
29 * for all types of that symbol name. In version 4 you used them like
30 pointers, but in modern perls you can just use references.
32 A couple of others that you're likely to encounter that aren't
33 really type specifiers are:
35 <> are used for inputting a record from a filehandle.
36 \ takes a reference to something.
38 Note that E<lt>FILEE<gt> is I<neither> the type specifier for files
39 nor the name of the handle. It is the C<E<lt>E<gt>> operator applied
40 to the handle FILE. It reads one line (well, record - see
41 L<perlvar/$/>) from the handle FILE in scalar context, or I<all> lines
42 in list context. When performing open, close, or any other operation
43 besides C<E<lt>E<gt>> on files, or even talking about the handle, do
44 I<not> use the brackets. These are correct: C<eof(FH)>, C<seek(FH, 0,
45 2)> and "copying from STDIN to FILE".
47 =head2 Do I always/never have to quote my strings or use semicolons and commas?
49 Normally, a bareword doesn't need to be quoted, but in most cases
50 probably should be (and must be under C<use strict>). But a hash key
51 consisting of a simple word (that isn't the name of a defined
52 subroutine) and the left-hand operand to the C<=E<gt>> operator both
53 count as though they were quoted:
56 ------------ ---------------
57 $foo{line} $foo{"line"}
58 bar => stuff "bar" => stuff
60 The final semicolon in a block is optional, as is the final comma in a
61 list. Good style (see L<perlstyle>) says to put them in except for
64 if ($whoops) { exit 1 }
71 "There Beren came from mountains cold",
72 "And lost he wandered under leaves",
75 =head2 How do I skip some return values?
77 One way is to treat the return values as a list and index into it:
79 $dir = (getpwnam($user))[7];
81 Another way is to use undef as an element on the left-hand-side:
83 ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
85 =head2 How do I temporarily block warnings?
87 The C<$^W> variable (documented in L<perlvar>) controls
88 runtime warnings for a block:
91 local $^W = 0; # temporarily turn off warnings
92 $a = $b + $c; # I know these might be undef
95 Note that like all the punctuation variables, you cannot currently
96 use my() on C<$^W>, only local().
98 A new C<use warnings> pragma is in the works to provide finer control
99 over all this. The curious should check the perl5-porters mailing list
100 archives for details.
102 =head2 What's an extension?
104 A way of calling compiled C code from Perl. Reading L<perlxstut>
105 is a good place to learn more about extensions.
107 =head2 Why do Perl operators have different precedence than C operators?
109 Actually, they don't. All C operators that Perl copies have the same
110 precedence in Perl as they do in C. The problem is with operators that C
111 doesn't have, especially functions that give a list context to everything
112 on their right, eg print, chmod, exec, and so on. Such functions are
113 called "list operators" and appear as such in the precedence table in
116 A common mistake is to write:
118 unlink $file || die "snafu";
120 This gets interpreted as:
122 unlink ($file || die "snafu");
124 To avoid this problem, either put in extra parentheses or use the
125 super low precedence C<or> operator:
127 (unlink $file) || die "snafu";
128 unlink $file or die "snafu";
130 The "English" operators (C<and>, C<or>, C<xor>, and C<not>)
131 deliberately have precedence lower than that of list operators for
132 just such situations as the one above.
134 Another operator with surprising precedence is exponentiation. It
135 binds more tightly even than unary minus, making C<-2**2> product a
136 negative not a positive four. It is also right-associating, meaning
137 that C<2**3**2> is two raised to the ninth power, not eight squared.
139 Although it has the same precedence as in C, Perl's C<?:> operator
140 produces an lvalue. This assigns $x to either $a or $b, depending
141 on the trueness of $maybe:
143 ($maybe ? $a : $b) = $x;
145 =head2 How do I declare/create a structure?
147 In general, you don't "declare" a structure. Just use a (probably
148 anonymous) hash reference. See L<perlref> and L<perldsc> for details.
151 $person = {}; # new anonymous hash
152 $person->{AGE} = 24; # set field AGE to 24
153 $person->{NAME} = "Nat"; # set field NAME to "Nat"
155 If you're looking for something a bit more rigorous, try L<perltoot>.
157 =head2 How do I create a module?
159 A module is a package that lives in a file of the same name. For
160 example, the Hello::There module would live in Hello/There.pm. For
161 details, read L<perlmod>. You'll also find L<Exporter> helpful. If
162 you're writing a C or mixed-language module with both C and Perl, then
163 you should study L<perlxstut>.
165 Here's a convenient template you might wish you use when starting your
166 own module. Make sure to change the names appropriately.
168 package Some::Module; # assumes Some/Module.pm
174 use vars qw($VERSION @ISA @EXPORT @EXPORT_OK %EXPORT_TAGS);
176 ## set the version for version checking; uncomment to use
179 # if using RCS/CVS, this next line may be preferred,
180 # but beware two-digit versions.
181 $VERSION = do{my@r=q$Revision: 1.28 $=~/\d+/g;sprintf '%d.'.'%02d'x$#r,@r};
184 @EXPORT = qw(&func1 &func2 &func3);
185 %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ],
187 # your exported package globals go here,
188 # as well as any optionally exported functions
189 @EXPORT_OK = qw($Var1 %Hashit);
193 # non-exported package globals go here
194 use vars qw( @more $stuff );
196 # initialize package globals, first exported ones
200 # then the others (which are still accessible as $Some::Module::stuff)
204 # all file-scoped lexicals must be created before
205 # the functions below that use them.
207 # file-private lexicals go here
209 my %secret_hash = ();
211 # here's a file-private function as a closure,
212 # callable as &$priv_func; it cannot be prototyped.
213 my $priv_func = sub {
217 # make all your functions, whether exported or not;
218 # remember to put something interesting in the {} stubs
219 sub func1 {} # no prototype
220 sub func2() {} # proto'd void
221 sub func3($$) {} # proto'd to 2 scalars
223 # this one isn't exported, but could be called!
224 sub func4(\%) {} # proto'd to 1 hash ref
226 END { } # module clean-up code here (global destructor)
228 1; # modules must return true
230 The h2xs program will create stubs for all the important stuff for you:
232 % h2xs -XA -n My::Module
234 =head2 How do I create a class?
236 See L<perltoot> for an introduction to classes and objects, as well as
237 L<perlobj> and L<perlbot>.
239 =head2 How can I tell if a variable is tainted?
241 See L<perlsec/"Laundering and Detecting Tainted Data">. Here's an
242 example (which doesn't use any system calls, because the kill()
243 is given no processes to signal):
246 return ! eval { join('',@_), kill 0; 1; };
249 This is not C<-w> clean, however. There is no C<-w> clean way to
250 detect taintedness - take this as a hint that you should untaint
251 all possibly-tainted data.
253 =head2 What's a closure?
255 Closures are documented in L<perlref>.
257 I<Closure> is a computer science term with a precise but
258 hard-to-explain meaning. Closures are implemented in Perl as anonymous
259 subroutines with lasting references to lexical variables outside their
260 own scopes. These lexicals magically refer to the variables that were
261 around when the subroutine was defined (deep binding).
263 Closures make sense in any programming language where you can have the
264 return value of a function be itself a function, as you can in Perl.
265 Note that some languages provide anonymous functions but are not
266 capable of providing proper closures; the Python language, for
267 example. For more information on closures, check out any textbook on
268 functional programming. Scheme is a language that not only supports
269 but encourages closures.
271 Here's a classic function-generating function:
273 sub add_function_generator {
274 return sub { shift + shift };
277 $add_sub = add_function_generator();
278 $sum = $add_sub->(4,5); # $sum is 9 now.
280 The closure works as a I<function template> with some customization
281 slots left out to be filled later. The anonymous subroutine returned
282 by add_function_generator() isn't technically a closure because it
283 refers to no lexicals outside its own scope.
285 Contrast this with the following make_adder() function, in which the
286 returned anonymous function contains a reference to a lexical variable
287 outside the scope of that function itself. Such a reference requires
288 that Perl return a proper closure, thus locking in for all time the
289 value that the lexical had when the function was created.
292 my $addpiece = shift;
293 return sub { shift + $addpiece };
296 $f1 = make_adder(20);
297 $f2 = make_adder(555);
299 Now C<&$f1($n)> is always 20 plus whatever $n you pass in, whereas
300 C<&$f2($n)> is always 555 plus whatever $n you pass in. The $addpiece
301 in the closure sticks around.
303 Closures are often used for less esoteric purposes. For example, when
304 you want to pass in a bit of code into a function:
307 timeout( 30, sub { $line = <STDIN> } );
309 If the code to execute had been passed in as a string, C<'$line =
310 E<lt>STDINE<gt>'>, there would have been no way for the hypothetical
311 timeout() function to access the lexical variable $line back in its
314 =head2 What is variable suicide and how can I prevent it?
316 Variable suicide is when you (temporarily or permanently) lose the
317 value of a variable. It is caused by scoping through my() and local()
318 interacting with either closures or aliased foreach() iterator
319 variables and subroutine arguments. It used to be easy to
320 inadvertently lose a variable's value this way, but now it's much
321 harder. Take this code:
325 while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" }
328 print "Finally $f\n";
330 The $f that has "bar" added to it three times should be a new C<$f>
331 (C<my $f> should create a new local variable each time through the loop).
332 It isn't, however. This was a bug, now fixed in the latest releases
333 (tested against 5.004_05, 5.005_03, and 5.005_56).
335 =head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?
337 With the exception of regexes, you need to pass references to these
338 objects. See L<perlsub/"Pass by Reference"> for this particular
339 question, and L<perlref> for information on references.
343 =item Passing Variables and Functions
345 Regular variables and functions are quite easy: just pass in a
346 reference to an existing or anonymous variable or function:
348 func( \$some_scalar );
350 func( \@some_array );
354 func( { this => 10, that => 20 } );
357 func( sub { $_[0] ** $_[1] } );
359 =item Passing Filehandles
361 To pass filehandles to subroutines, use the C<*FH> or C<\*FH> notations.
362 These are "typeglobs" - see L<perldata/"Typeglobs and Filehandles">
363 and especially L<perlsub/"Pass by Reference"> for more information.
367 If you're passing around filehandles, you could usually just use the bare
368 typeglob, like *STDOUT, but typeglobs references would be better because
369 they'll still work properly under C<use strict 'refs'>. For example:
374 print $fh "her um well a hmmm\n";
377 $rec = get_rec(\*STDIN);
383 If you're planning on generating new filehandles, you could do this:
388 return open (FH, $path) ? *FH : undef;
390 $fh = openit('< /etc/motd');
393 =item Passing Regexes
395 To pass regexes around, you'll need to be using a release of Perl
396 sufficiently recent as to support the C<qr//> construct, pass around
397 strings and use an exception-trapping eval, or else be very, very clever.
399 Here's an example of how to pass in a string to be regex compared
403 my ($val1, $regex) = @_;
404 my $retval = $val1 =~ /$regex/;
407 $match = compare("old McDonald", qr/d.*D/i);
409 Notice how C<qr//> allows flags at the end. That pattern was compiled
410 at compile time, although it was executed later. The nifty C<qr//>
411 notation wasn't introduced until the 5.005 release. Before that, you
412 had to approach this problem much less intuitively. For example, here
413 it is again if you don't have C<qr//>:
416 my ($val1, $regex) = @_;
417 my $retval = eval { $val1 =~ /$regex/ };
422 $match = compare("old McDonald", q/($?i)d.*D/);
424 Make sure you never say something like this:
426 return eval "\$val =~ /$regex/"; # WRONG
428 or someone can sneak shell escapes into the regex due to the double
429 interpolation of the eval and the double-quoted string. For example:
431 $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger';
433 eval "\$string =~ /$pattern_of_evil/";
435 Those preferring to be very, very clever might see the O'Reilly book,
436 I<Mastering Regular Expressions>, by Jeffrey Friedl. Page 273's
437 Build_MatchMany_Function() is particularly interesting. A complete
438 citation of this book is given in L<perlfaq2>.
440 =item Passing Methods
442 To pass an object method into a subroutine, you can do this:
444 call_a_lot(10, $some_obj, "methname")
446 my ($count, $widget, $trick) = @_;
447 for (my $i = 0; $i < $count; $i++) {
452 Or you can use a closure to bundle up the object and its method call
455 my $whatnot = sub { $some_obj->obfuscate(@args) };
462 You could also investigate the can() method in the UNIVERSAL class
463 (part of the standard perl distribution).
467 =head2 How do I create a static variable?
469 As with most things in Perl, TMTOWTDI. What is a "static variable" in
470 other languages could be either a function-private variable (visible
471 only within a single function, retaining its value between calls to
472 that function), or a file-private variable (visible only to functions
473 within the file it was declared in) in Perl.
475 Here's code to implement a function-private variable:
479 sub prev_counter { return --$counter }
480 sub next_counter { return $counter++ }
483 Now prev_counter() and next_counter() share a private variable $counter
484 that was initialized at compile time.
486 To declare a file-private variable, you'll still use a my(), putting
487 it at the outer scope level at the top of the file. Assume this is in
491 my $started = scalar(localtime(time()));
493 sub begun { return $started }
495 When C<use Pax> or C<require Pax> loads this module, the variable will
496 be initialized. It won't get garbage-collected the way most variables
497 going out of scope do, because the begun() function cares about it,
498 but no one else can get it. It is not called $Pax::started because
499 its scope is unrelated to the package. It's scoped to the file. You
500 could conceivably have several packages in that same file all
501 accessing the same private variable, but another file with the same
502 package couldn't get to it.
504 See L<perlsub/"Persistent Private Variables"> for details.
506 =head2 What's the difference between dynamic and lexical (static) scoping? Between local() and my()?
508 C<local($x)> saves away the old value of the global variable C<$x>,
509 and assigns a new value for the duration of the subroutine, I<which is
510 visible in other functions called from that subroutine>. This is done
511 at run-time, so is called dynamic scoping. local() always affects global
512 variables, also called package variables or dynamic variables.
514 C<my($x)> creates a new variable that is only visible in the current
515 subroutine. This is done at compile-time, so is called lexical or
516 static scoping. my() always affects private variables, also called
517 lexical variables or (improperly) static(ly scoped) variables.
522 print "var has value $var\n";
526 local $var = 'local'; # new temporary value for the still-global
527 visible(); # variable called $var
531 my $var = 'private'; # new private variable, $var
532 visible(); # (invisible outside of sub scope)
537 visible(); # prints global
538 dynamic(); # prints local
539 lexical(); # prints global
541 Notice how at no point does the value "private" get printed. That's
542 because $var only has that value within the block of the lexical()
543 function, and it is hidden from called subroutine.
545 In summary, local() doesn't make what you think of as private, local
546 variables. It gives a global variable a temporary value. my() is
547 what you're looking for if you want private variables.
549 See L<perlsub/"Private Variables via my()"> and L<perlsub/"Temporary
550 Values via local()"> for excruciating details.
552 =head2 How can I access a dynamic variable while a similarly named lexical is in scope?
554 You can do this via symbolic references, provided you haven't set
555 C<use strict "refs">. So instead of $var, use C<${'var'}>.
557 local $var = "global";
560 print "lexical is $var\n";
563 print "global is ${'var'}\n";
565 If you know your package, you can just mention it explicitly, as in
566 $Some_Pack::var. Note that the notation $::var is I<not> the dynamic
567 $var in the current package, but rather the one in the C<main>
568 package, as though you had written $main::var. Specifying the package
569 directly makes you hard-code its name, but it executes faster and
570 avoids running afoul of C<use strict "refs">.
572 =head2 What's the difference between deep and shallow binding?
574 In deep binding, lexical variables mentioned in anonymous subroutines
575 are the same ones that were in scope when the subroutine was created.
576 In shallow binding, they are whichever variables with the same names
577 happen to be in scope when the subroutine is called. Perl always uses
578 deep binding of lexical variables (i.e., those created with my()).
579 However, dynamic variables (aka global, local, or package variables)
580 are effectively shallowly bound. Consider this just one more reason
581 not to use them. See the answer to L<"What's a closure?">.
583 =head2 Why doesn't "my($foo) = E<lt>FILEE<gt>;" work right?
585 C<my()> and C<local()> give list context to the right hand side
586 of C<=>. The E<lt>FHE<gt> read operation, like so many of Perl's
587 functions and operators, can tell which context it was called in and
588 behaves appropriately. In general, the scalar() function can help.
589 This function does nothing to the data itself (contrary to popular myth)
590 but rather tells its argument to behave in whatever its scalar fashion is.
591 If that function doesn't have a defined scalar behavior, this of course
592 doesn't help you (such as with sort()).
594 To enforce scalar context in this particular case, however, you need
595 merely omit the parentheses:
597 local($foo) = <FILE>; # WRONG
598 local($foo) = scalar(<FILE>); # ok
599 local $foo = <FILE>; # right
601 You should probably be using lexical variables anyway, although the
602 issue is the same here:
604 my($foo) = <FILE>; # WRONG
605 my $foo = <FILE>; # right
607 =head2 How do I redefine a builtin function, operator, or method?
609 Why do you want to do that? :-)
611 If you want to override a predefined function, such as open(),
612 then you'll have to import the new definition from a different
613 module. See L<perlsub/"Overriding Builtin Functions">. There's
614 also an example in L<perltoot/"Class::Template">.
616 If you want to overload a Perl operator, such as C<+> or C<**>,
617 then you'll want to use the C<use overload> pragma, documented
620 If you're talking about obscuring method calls in parent classes,
621 see L<perltoot/"Overridden Methods">.
623 =head2 What's the difference between calling a function as &foo and foo()?
625 When you call a function as C<&foo>, you allow that function access to
626 your current @_ values, and you by-pass prototypes. That means that
627 the function doesn't get an empty @_, it gets yours! While not
628 strictly speaking a bug (it's documented that way in L<perlsub>), it
629 would be hard to consider this a feature in most cases.
631 When you call your function as C<&foo()>, then you I<do> get a new @_,
632 but prototyping is still circumvented.
634 Normally, you want to call a function using C<foo()>. You may only
635 omit the parentheses if the function is already known to the compiler
636 because it already saw the definition (C<use> but not C<require>),
637 or via a forward reference or C<use subs> declaration. Even in this
638 case, you get a clean @_ without any of the old values leaking through
639 where they don't belong.
641 =head2 How do I create a switch or case statement?
643 This is explained in more depth in the L<perlsyn>. Briefly, there's
644 no official case statement, because of the variety of tests possible
645 in Perl (numeric comparison, string comparison, glob comparison,
646 regex matching, overloaded comparisons, ...). Larry couldn't decide
647 how best to do this, so he left it out, even though it's been on the
648 wish list since perl1.
650 The general answer is to write a construct like this:
652 for ($variable_to_test) {
653 if (/pat1/) { } # do something
654 elsif (/pat2/) { } # do something else
655 elsif (/pat3/) { } # do something else
659 Here's a simple example of a switch based on pattern matching, this
660 time lined up in a way to make it look more like a switch statement.
661 We'll do a multi-way conditional based on the type of reference stored
664 SWITCH: for (ref $whatchamacallit) {
666 /^$/ && die "not a reference";
684 warn "can't print function ref";
690 warn "User defined type skipped";
694 See C<perlsyn/"Basic BLOCKs and Switch Statements"> for many other
695 examples in this style.
697 Sometimes you should change the positions of the constant and the variable.
698 For example, let's say you wanted to test which of many answers you were
699 given, but in a case-insensitive way that also allows abbreviations.
700 You can use the following technique if the strings all start with
701 different characters, or if you want to arrange the matches so that
702 one takes precedence over another, as C<"SEND"> has precedence over
706 if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" }
707 elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" }
708 elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" }
709 elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" }
710 elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" }
712 A totally different approach is to create a hash of function references.
717 "done" => sub { die "See ya!" },
721 print "How are you? ";
722 chomp($string = <STDIN>);
723 if ($commands{$string}) {
724 $commands{$string}->();
726 print "No such command: $string\n";
729 =head2 How can I catch accesses to undefined variables/functions/methods?
731 The AUTOLOAD method, discussed in L<perlsub/"Autoloading"> and
732 L<perltoot/"AUTOLOAD: Proxy Methods">, lets you capture calls to
733 undefined functions and methods.
735 When it comes to undefined variables that would trigger a warning
736 under C<-w>, you can use a handler to trap the pseudo-signal
737 C<__WARN__> like this:
739 $SIG{__WARN__} = sub {
741 for ( $_[0] ) { # voici un switch statement
743 /Use of uninitialized value/ && do {
744 # promote warning to a fatal
748 # other warning cases to catch could go here;
755 =head2 Why can't a method included in this same file be found?
757 Some possible reasons: your inheritance is getting confused, you've
758 misspelled the method name, or the object is of the wrong type. Check
759 out L<perltoot> for details on these. You may also use C<print
760 ref($object)> to find out the class C<$object> was blessed into.
762 Another possible reason for problems is because you've used the
763 indirect object syntax (eg, C<find Guru "Samy">) on a class name
764 before Perl has seen that such a package exists. It's wisest to make
765 sure your packages are all defined before you start using them, which
766 will be taken care of if you use the C<use> statement instead of
767 C<require>. If not, make sure to use arrow notation (eg,
768 C<Guru-E<gt>find("Samy")>) instead. Object notation is explained in
771 Make sure to read about creating modules in L<perlmod> and
772 the perils of indirect objects in L<perlobj/"WARNING">.
774 =head2 How can I find out my current package?
776 If you're just a random program, you can do this to find
777 out what the currently compiled package is:
779 my $packname = __PACKAGE__;
781 But if you're a method and you want to print an error message
782 that includes the kind of object you were called on (which is
783 not necessarily the same as the one in which you were compiled):
787 my $class = ref($self) || $self;
788 warn "called me from a $class object";
791 =head2 How can I comment out a large block of perl code?
793 Use embedded POD to discard it:
798 This paragraph is commented out
813 This can't go just anywhere. You have to put a pod directive where
814 the parser is expecting a new statement, not just in the middle
815 of an expression or some other arbitrary yacc grammar production.
817 =head2 How do I clear a package?
819 Use this code, provided by Mark-Jason Dominus:
824 die "Shouldn't delete main package"
825 if $pack eq "" || $pack eq "main";
826 my $stash = *{$pack . '::'}{HASH};
828 foreach $name (keys %$stash) {
829 my $fullname = $pack . '::' . $name;
830 # Get rid of everything with that name.
839 Or, if you're using a recent release of Perl, you can
840 just use the Symbol::delete_package() function instead.
842 =head2 How can I use a variable as a variable name?
844 Beginners often think they want to have a variable contain the name
849 ++$$varname; # $fred now 24
851 This works I<sometimes>, but it is a very bad idea for two reasons.
853 The first reason is that they I<only work on global variables>.
854 That means above that if $fred is a lexical variable created with my(),
855 that the code won't work at all: you'll accidentally access the global
856 and skip right over the private lexical altogether. Global variables
857 are bad because they can easily collide accidentally and in general make
858 for non-scalable and confusing code.
860 Symbolic references are forbidden under the C<use strict> pragma.
861 They are not true references and consequently are not reference counted
862 or garbage collected.
864 The other reason why using a variable to hold the name of another
865 variable a bad idea is that the question often stems from a lack of
866 understanding of Perl data structures, particularly hashes. By using
867 symbolic references, you are just using the package's symbol-table hash
868 (like C<%main::>) instead of a user-defined hash. The solution is to
869 use your own hash or a real reference instead.
873 $USER_VARS{$varname}++; # not $$varname++
875 There we're using the %USER_VARS hash instead of symbolic references.
876 Sometimes this comes up in reading strings from the user with variable
877 references and wanting to expand them to the values of your perl
878 program's variables. This is also a bad idea because it conflates the
879 program-addressable namespace and the user-addressable one. Instead of
880 reading a string and expanding it to the actual contents of your program's
883 $str = 'this has a $fred and $barney in it';
884 $str =~ s/(\$\w+)/$1/eeg; # need double eval
886 Instead, it would be better to keep a hash around like %USER_VARS and have
887 variable references actually refer to entries in that hash:
889 $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all
891 That's faster, cleaner, and safer than the previous approach. Of course,
892 you don't need to use a dollar sign. You could use your own scheme to
893 make it less confusing, like bracketed percent symbols, etc.
895 $str = 'this has a %fred% and %barney% in it';
896 $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all
898 Another reason that folks sometimes think they want a variable to contain
899 the name of a variable is because they don't know how to build proper
900 data structures using hashes. For example, let's say they wanted two
901 hashes in their program: %fred and %barney, and to use another scalar
902 variable to refer to those by name.
905 $$name{WIFE} = "wilma"; # set %fred
908 $$name{WIFE} = "betty"; # set %barney
910 This is still a symbolic reference, and is still saddled with the
911 problems enumerated above. It would be far better to write:
913 $folks{"fred"}{WIFE} = "wilma";
914 $folks{"barney"}{WIFE} = "betty";
916 And just use a multilevel hash to start with.
918 The only times that you absolutely I<must> use symbolic references are
919 when you really must refer to the symbol table. This may be because it's
920 something that can't take a real reference to, such as a format name.
921 Doing so may also be important for method calls, since these always go
922 through the symbol table for resolution.
924 In those cases, you would turn off C<strict 'refs'> temporarily so you
925 can play around with the symbol table. For example:
927 @colors = qw(red blue green yellow orange purple violet);
928 for my $name (@colors) {
929 no strict 'refs'; # renege for the block
930 *$name = sub { "<FONT COLOR='$name'>@_</FONT>" };
933 All those functions (red(), blue(), green(), etc.) appear to be separate,
934 but the real code in the closure actually was compiled only once.
936 So, sometimes you might want to use symbolic references to directly
937 manipulate the symbol table. This doesn't matter for formats, handles, and
938 subroutines, because they are always global -- you can't use my() on them.
939 But for scalars, arrays, and hashes -- and usually for subroutines --
940 you probably want to use hard references only.
942 =head1 AUTHOR AND COPYRIGHT
944 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
947 When included as part of the Standard Version of Perl, or as part of
948 its complete documentation whether printed or otherwise, this work
949 may be distributed only under the terms of Perl's Artistic License.
950 Any distribution of this file or derivatives thereof I<outside>
951 of that package require that special arrangements be made with
954 Irrespective of its distribution, all code examples in this file
955 are hereby placed into the public domain. You are permitted and
956 encouraged to use this code in your own programs for fun
957 or for profit as you see fit. A simple comment in the code giving
958 credit would be courteous but is not required.