1constant(3)           User Contributed Perl Documentation          constant(3)
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NAME

6       constant - Perl pragma to declare constants
7

SYNOPSIS

9           use constant PI    => 4 * atan2(1, 1);
10           use constant DEBUG => 0;
11
12           print "Pi equals ", PI, "...\n" if DEBUG;
13
14           use constant {
15               SEC   => 0,
16               MIN   => 1,
17               HOUR  => 2,
18               MDAY  => 3,
19               MON   => 4,
20               YEAR  => 5,
21               WDAY  => 6,
22               YDAY  => 7,
23               ISDST => 8,
24           };
25
26           use constant WEEKDAYS => qw(
27               Sunday Monday Tuesday Wednesday Thursday Friday Saturday
28           );
29
30           print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";
31

DESCRIPTION

33       This pragma allows you to declare constants at compile-time.
34
35       When you declare a constant such as "PI" using the method shown above,
36       each machine your script runs upon can have as many digits of accuracy
37       as it can use.  Also, your program will be easier to read, more likely
38       to be maintained (and maintained correctly), and far less likely to
39       send a space probe to the wrong planet because nobody noticed the one
40       equation in which you wrote 3.14195.
41
42       When a constant is used in an expression, Perl replaces it with its
43       value at compile time, and may then optimize the expression further.
44       In particular, any code in an "if (CONSTANT)" block will be optimized
45       away if the constant is false.
46

NOTES

48       As with all "use" directives, defining a constant happens at compile
49       time.  Thus, it's probably not correct to put a constant declaration
50       inside of a conditional statement (like "if ($foo) { use constant ...
51       }").
52
53       Constants defined using this module cannot be interpolated into strings
54       like variables.  However, concatenation works just fine:
55
56           print "Pi equals PI...\n";        # WRONG: does not expand "PI"
57           print "Pi equals ".PI."...\n";    # right
58
59       Even though a reference may be declared as a constant, the reference
60       may point to data which may be changed, as this code shows.
61
62           use constant ARRAY => [ 1,2,3,4 ];
63           print ARRAY->[1];
64           ARRAY->[1] = " be changed";
65           print ARRAY->[1];
66
67       Constants belong to the package they are defined in.  To refer to a
68       constant defined in another package, specify the full package name, as
69       in "Some::Package::CONSTANT".  Constants may be exported by modules,
70       and may also be called as either class or instance methods, that is, as
71       "Some::Package->CONSTANT" or as "$obj->CONSTANT" where $obj is an
72       instance of "Some::Package".  Subclasses may define their own constants
73       to override those in their base class.
74
75       As of version 1.32 of this module, constants can be defined in packages
76       other than the caller, by including the package name in the name of the
77       constant:
78
79           use constant "OtherPackage::FWIBBLE" => 7865;
80           constant->import("Other::FWOBBLE",$value); # dynamically at run time
81
82       The use of all caps for constant names is merely a convention, although
83       it is recommended in order to make constants stand out and to help
84       avoid collisions with other barewords, keywords, and subroutine names.
85       Constant names must begin with a letter or underscore.  Names beginning
86       with a double underscore are reserved.  Some poor choices for names
87       will generate warnings, if warnings are enabled at compile time.
88
89   List constants
90       Constants may be lists of more (or less) than one value.  A constant
91       with no values evaluates to "undef" in scalar context.  Note that
92       constants with more than one value do not return their last value in
93       scalar context as one might expect.  They currently return the number
94       of values, but this may change in the future.  Do not use constants
95       with multiple values in scalar context.
96
97       NOTE: This implies that the expression defining the value of a constant
98       is evaluated in list context.  This may produce surprises:
99
100           use constant TIMESTAMP => localtime;                # WRONG!
101           use constant TIMESTAMP => scalar localtime;         # right
102
103       The first line above defines "TIMESTAMP" as a 9-element list, as
104       returned by localtime() in list context.  To set it to the string
105       returned by localtime() in scalar context, an explicit "scalar" keyword
106       is required.
107
108       List constants are lists, not arrays.  To index or slice them, they
109       must be placed in parentheses.
110
111           my @workdays = WEEKDAYS[1 .. 5];            # WRONG!
112           my @workdays = (WEEKDAYS)[1 .. 5];          # right
113
114   Defining multiple constants at once
115       Instead of writing multiple "use constant" statements, you may define
116       multiple constants in a single statement by giving, instead of the
117       constant name, a reference to a hash where the keys are the names of
118       the constants to be defined.  Obviously, all constants defined using
119       this method must have a single value.
120
121           use constant {
122               FOO => "A single value",
123               BAR => "This", "won't", "work!",        # Error!
124           };
125
126       This is a fundamental limitation of the way hashes are constructed in
127       Perl.  The error messages produced when this happens will often be
128       quite cryptic -- in the worst case there may be none at all, and you'll
129       only later find that something is broken.
130
131       When defining multiple constants, you cannot use the values of other
132       constants defined in the same declaration.  This is because the calling
133       package doesn't know about any constant within that group until after
134       the "use" statement is finished.
135
136           use constant {
137               BITMASK => 0xAFBAEBA8,
138               NEGMASK => ~BITMASK,                    # Error!
139           };
140
141   Magic constants
142       Magical values and references can be made into constants at compile
143       time, allowing for way cool stuff like this.  (These error numbers
144       aren't totally portable, alas.)
145
146           use constant E2BIG => ($! = 7);
147           print   E2BIG, "\n";        # something like "Arg list too long"
148           print 0+E2BIG, "\n";        # "7"
149
150       You can't produce a tied constant by giving a tied scalar as the value.
151       References to tied variables, however, can be used as constants without
152       any problems.
153

TECHNICAL NOTES

155       In the current implementation, scalar constants are actually inlinable
156       subroutines.  As of version 5.004 of Perl, the appropriate scalar
157       constant is inserted directly in place of some subroutine calls,
158       thereby saving the overhead of a subroutine call.  See "Constant
159       Functions" in perlsub for details about how and when this happens.
160
161       In the rare case in which you need to discover at run time whether a
162       particular constant has been declared via this module, you may use this
163       function to examine the hash %constant::declared.  If the given
164       constant name does not include a package name, the current package is
165       used.
166
167           sub declared ($) {
168               use constant 1.01;              # don't omit this!
169               my $name = shift;
170               $name =~ s/^::/main::/;
171               my $pkg = caller;
172               my $full_name = $name =~ /::/ ? $name : "${pkg}::$name";
173               $constant::declared{$full_name};
174           }
175

CAVEATS

177       List constants are not inlined unless you are using Perl v5.20 or
178       higher.  In v5.20 or higher, they are still not read-only, but that may
179       change in future versions.
180
181       It is not possible to have a subroutine or a keyword with the same name
182       as a constant in the same package.  This is probably a Good Thing.
183
184       A constant with a name in the list "STDIN STDOUT STDERR ARGV ARGVOUT
185       ENV INC SIG" is not allowed anywhere but in package "main::", for
186       technical reasons.
187
188       Unlike constants in some languages, these cannot be overridden on the
189       command line or via environment variables.
190
191       You can get into trouble if you use constants in a context which
192       automatically quotes barewords (as is true for any subroutine call).
193       For example, you can't say $hash{CONSTANT} because "CONSTANT" will be
194       interpreted as a string.  Use $hash{CONSTANT()} or $hash{+CONSTANT} to
195       prevent the bareword quoting mechanism from kicking in.  Similarly,
196       since the "=>" operator quotes a bareword immediately to its left, you
197       have to say "CONSTANT() => 'value'" (or simply use a comma in place of
198       the big arrow) instead of "CONSTANT => 'value'".
199

SEE ALSO

201       Readonly - Facility for creating read-only scalars, arrays, hashes.
202
203       Attribute::Constant - Make read-only variables via attribute
204
205       Scalar::Readonly - Perl extension to the "SvREADONLY" scalar flag
206
207       Hash::Util - A selection of general-utility hash subroutines (mostly to
208       lock/unlock keys and values)
209

BUGS

211       Please report any bugs or feature requests via the perlbug(1) utility.
212

AUTHORS

214       Tom Phoenix, <rootbeer@redcat.com>, with help from many other folks.
215
216       Multiple constant declarations at once added by Casey West,
217       <casey@geeknest.com>.
218
219       Documentation mostly rewritten by Ilmari Karonen, <perl@itz.pp.sci.fi>.
220
221       This program is maintained by the Perl 5 Porters.  The CPAN
222       distribution is maintained by Sébastien Aperghis-Tramoni
223       <sebastien@aperghis.net>.
224
226       Copyright (C) 1997, 1999 Tom Phoenix
227
228       This module is free software; you can redistribute it or modify it
229       under the same terms as Perl itself.
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233perl v5.38.0                      2023-07-21                       constant(3)
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