4 use warnings::register;
6 use vars qw($VERSION %declared);
9 #=======================================================================
11 # Some names are evil choices.
12 my %keywords = map +($_, 1), qw{ BEGIN INIT CHECK END DESTROY AUTOLOAD };
13 $keywords{UNITCHECK}++ if $] > 5.009;
15 my %forced_into_main = map +($_, 1),
16 qw{ STDIN STDOUT STDERR ARGV ARGVOUT ENV INC SIG };
18 my %forbidden = (%keywords, %forced_into_main);
20 #=======================================================================
21 # import() - import symbols into user's namespace
23 # What we actually do is define a function in the caller's namespace
24 # which returns the value. The function we create will normally
25 # be inlined as a constant, thereby avoiding further sub calling
27 #=======================================================================
30 return unless @_; # Ignore 'use constant;'
32 my $multiple = ref $_[0];
35 my $str_end = $] >= 5.006 ? "\\z" : "\\Z";
39 $symtab = \%{$pkg . '::'};
43 if (ref $_[0] ne 'HASH') {
45 Carp::croak("Invalid reference type '".ref(shift)."' not 'HASH'");
49 $constants->{+shift} = undef;
52 foreach my $name ( keys %$constants ) {
53 unless (defined $name) {
55 Carp::croak("Can't use undef as constant name");
58 # Normal constant name
59 if ($name =~ /^_?[^\W_0-9]\w*$str_end/ and !$forbidden{$name}) {
62 # Name forced into main, but we're not in main. Fatal.
63 } elsif ($forced_into_main{$name} and $pkg ne 'main') {
65 Carp::croak("Constant name '$name' is forced into main::");
67 # Starts with double underscore. Fatal.
68 } elsif ($name =~ /^__/) {
70 Carp::croak("Constant name '$name' begins with '__'");
72 # Maybe the name is tolerable
73 } elsif ($name =~ /^[A-Za-z_]\w*$str_end/) {
74 # Then we'll warn only if you've asked for warnings
75 if (warnings::enabled()) {
76 if ($keywords{$name}) {
77 warnings::warn("Constant name '$name' is a Perl keyword");
78 } elsif ($forced_into_main{$name}) {
79 warnings::warn("Constant name '$name' is " .
80 "forced into package main::");
84 # Looks like a boolean
85 # use constant FRED == fred;
86 } elsif ($name =~ /^[01]?$str_end/) {
89 Carp::croak("Constant name '$name' is invalid");
91 Carp::croak("Constant name looks like boolean value");
95 # Must have bad characters
97 Carp::croak("Constant name '$name' has invalid characters");
102 my $full_name = "${pkg}::$name";
103 $declared{$full_name}++;
104 if ($multiple || @_ == 1) {
105 my $scalar = $multiple ? $constants->{$name} : $_[0];
106 if ($symtab && !exists $symtab->{$name}) {
107 # No typeglob yet, so we can use a reference as space-
108 # efficient proxy for a constant subroutine
109 # The check in Perl_ck_rvconst knows that inlinable
110 # constants from cv_const_sv are read only. So we have to:
111 Internals::SvREADONLY($scalar, 1);
112 $symtab->{$name} = \$scalar;
113 mro::method_changed_in($pkg);
115 *$full_name = sub () { $scalar };
119 *$full_name = sub () { @list };
121 *$full_name = sub () { };
133 constant - Perl pragma to declare constants
137 use constant PI => 4 * atan2(1, 1);
138 use constant DEBUG => 0;
140 print "Pi equals ", PI, "...\n" if DEBUG;
154 use constant WEEKDAYS => qw(
155 Sunday Monday Tuesday Wednesday Thursday Friday Saturday
158 print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";
162 This pragma allows you to declare constants at compile-time.
164 When you declare a constant such as C<PI> using the method shown
165 above, each machine your script runs upon can have as many digits
166 of accuracy as it can use. Also, your program will be easier to
167 read, more likely to be maintained (and maintained correctly), and
168 far less likely to send a space probe to the wrong planet because
169 nobody noticed the one equation in which you wrote C<3.14195>.
171 When a constant is used in an expression, Perl replaces it with its
172 value at compile time, and may then optimize the expression further.
173 In particular, any code in an C<if (CONSTANT)> block will be optimized
174 away if the constant is false.
178 As with all C<use> directives, defining a constant happens at
179 compile time. Thus, it's probably not correct to put a constant
180 declaration inside of a conditional statement (like C<if ($foo)
181 { use constant ... }>).
183 Constants defined using this module cannot be interpolated into
184 strings like variables. However, concatenation works just fine:
186 print "Pi equals PI...\n"; # WRONG: does not expand "PI"
187 print "Pi equals ".PI."...\n"; # right
189 Even though a reference may be declared as a constant, the reference may
190 point to data which may be changed, as this code shows.
192 use constant ARRAY => [ 1,2,3,4 ];
194 ARRAY->[1] = " be changed";
197 Dereferencing constant references incorrectly (such as using an array
198 subscript on a constant hash reference, or vice versa) will be trapped at
201 Constants belong to the package they are defined in. To refer to a
202 constant defined in another package, specify the full package name, as
203 in C<Some::Package::CONSTANT>. Constants may be exported by modules,
204 and may also be called as either class or instance methods, that is,
205 as C<< Some::Package->CONSTANT >> or as C<< $obj->CONSTANT >> where
206 C<$obj> is an instance of C<Some::Package>. Subclasses may define
207 their own constants to override those in their base class.
209 The use of all caps for constant names is merely a convention,
210 although it is recommended in order to make constants stand out
211 and to help avoid collisions with other barewords, keywords, and
212 subroutine names. Constant names must begin with a letter or
213 underscore. Names beginning with a double underscore are reserved. Some
214 poor choices for names will generate warnings, if warnings are enabled at
217 =head2 List constants
219 Constants may be lists of more (or less) than one value. A constant
220 with no values evaluates to C<undef> in scalar context. Note that
221 constants with more than one value do I<not> return their last value in
222 scalar context as one might expect. They currently return the number
223 of values, but B<this may change in the future>. Do not use constants
224 with multiple values in scalar context.
226 B<NOTE:> This implies that the expression defining the value of a
227 constant is evaluated in list context. This may produce surprises:
229 use constant TIMESTAMP => localtime; # WRONG!
230 use constant TIMESTAMP => scalar localtime; # right
232 The first line above defines C<TIMESTAMP> as a 9-element list, as
233 returned by C<localtime()> in list context. To set it to the string
234 returned by C<localtime()> in scalar context, an explicit C<scalar>
237 List constants are lists, not arrays. To index or slice them, they
238 must be placed in parentheses.
240 my @workdays = WEEKDAYS[1 .. 5]; # WRONG!
241 my @workdays = (WEEKDAYS)[1 .. 5]; # right
243 =head2 Defining multiple constants at once
245 Instead of writing multiple C<use constant> statements, you may define
246 multiple constants in a single statement by giving, instead of the
247 constant name, a reference to a hash where the keys are the names of
248 the constants to be defined. Obviously, all constants defined using
249 this method must have a single value.
252 FOO => "A single value",
253 BAR => "This", "won't", "work!", # Error!
256 This is a fundamental limitation of the way hashes are constructed in
257 Perl. The error messages produced when this happens will often be
258 quite cryptic -- in the worst case there may be none at all, and
259 you'll only later find that something is broken.
261 When defining multiple constants, you cannot use the values of other
262 constants defined in the same declaration. This is because the
263 calling package doesn't know about any constant within that group
264 until I<after> the C<use> statement is finished.
267 BITMASK => 0xAFBAEBA8,
268 NEGMASK => ~BITMASK, # Error!
271 =head2 Magic constants
273 Magical values and references can be made into constants at compile
274 time, allowing for way cool stuff like this. (These error numbers
275 aren't totally portable, alas.)
277 use constant E2BIG => ($! = 7);
278 print E2BIG, "\n"; # something like "Arg list too long"
279 print 0+E2BIG, "\n"; # "7"
281 You can't produce a tied constant by giving a tied scalar as the
282 value. References to tied variables, however, can be used as
283 constants without any problems.
285 =head1 TECHNICAL NOTES
287 In the current implementation, scalar constants are actually
288 inlinable subroutines. As of version 5.004 of Perl, the appropriate
289 scalar constant is inserted directly in place of some subroutine
290 calls, thereby saving the overhead of a subroutine call. See
291 L<perlsub/"Constant Functions"> for details about how and when this
294 In the rare case in which you need to discover at run time whether a
295 particular constant has been declared via this module, you may use
296 this function to examine the hash C<%constant::declared>. If the given
297 constant name does not include a package name, the current package is
301 use constant 1.01; # don't omit this!
303 $name =~ s/^::/main::/;
305 my $full_name = $name =~ /::/ ? $name : "${pkg}::$name";
306 $constant::declared{$full_name};
311 In the current version of Perl, list constants are not inlined
312 and some symbols may be redefined without generating a warning.
314 It is not possible to have a subroutine or a keyword with the same
315 name as a constant in the same package. This is probably a Good Thing.
317 A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT
318 ENV INC SIG> is not allowed anywhere but in package C<main::>, for
321 Unlike constants in some languages, these cannot be overridden
322 on the command line or via environment variables.
324 You can get into trouble if you use constants in a context which
325 automatically quotes barewords (as is true for any subroutine call).
326 For example, you can't say C<$hash{CONSTANT}> because C<CONSTANT> will
327 be interpreted as a string. Use C<$hash{CONSTANT()}> or
328 C<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from
329 kicking in. Similarly, since the C<< => >> operator quotes a bareword
330 immediately to its left, you have to say C<< CONSTANT() => 'value' >>
331 (or simply use a comma in place of the big arrow) instead of
332 C<< CONSTANT => 'value' >>.
336 L<Readonly> - Facility for creating read-only scalars, arrays, hashes.
338 L<Const> - Facility for creating read-only variables. Similar to C<Readonly>,
339 but uses C<SvREADONLY> instead of C<tie>.
341 L<Attribute::Constant> - Make read-only variables via attribute
343 L<Scalar::Readonly> - Perl extension to the C<SvREADONLY> scalar flag
345 L<Hash::Util> - A selection of general-utility hash subroutines (mostly
346 to lock/unlock keys and values)
350 Please report any bugs or feature requests via the perlbug(1) utility.
354 Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from
357 Multiple constant declarations at once added by Casey West,
358 E<lt>F<casey@geeknest.com>E<gt>.
360 Documentation mostly rewritten by Ilmari Karonen,
361 E<lt>F<perl@itz.pp.sci.fi>E<gt>.
363 This program is maintained by the Perl 5 Porters.
364 The CPAN distribution is maintained by SE<eacute>bastien Aperghis-Tramoni
365 E<lt>F<sebastien@aperghis.net>E<gt>.
367 =head1 COPYRIGHT & LICENSE
369 Copyright (C) 1997, 1999 Tom Phoenix
371 This module is free software; you can redistribute it or modify it
372 under the same terms as Perl itself.