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

TECHNICAL NOTES

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

CAVEATS

173       In the current version of Perl, list constants are not inlined and some
174       symbols may be redefined without generating a warning.
175
176       It is not possible to have a subroutine or a keyword with the same name
177       as a constant in the same package. This is probably a Good Thing.
178
179       A constant with a name in the list "STDIN STDOUT STDERR ARGV ARGVOUT
180       ENV INC SIG" is not allowed anywhere but in package "main::", for
181       technical reasons.
182
183       Unlike constants in some languages, these cannot be overridden on the
184       command line or via environment variables.
185
186       You can get into trouble if you use constants in a context which
187       automatically quotes barewords (as is true for any subroutine call).
188       For example, you can't say $hash{CONSTANT} because "CONSTANT" will be
189       interpreted as a string.  Use $hash{CONSTANT()} or $hash{+CONSTANT} to
190       prevent the bareword quoting mechanism from kicking in.  Similarly,
191       since the "=>" operator quotes a bareword immediately to its left, you
192       have to say "CONSTANT() => 'value'" (or simply use a comma in place of
193       the big arrow) instead of "CONSTANT => 'value'".
194

SEE ALSO

196       Readonly - Facility for creating read-only scalars, arrays, hashes.
197
198       Const - Facility for creating read-only variables. Similar to
199       "Readonly", but uses "SvREADONLY" instead of "tie".
200
201       Attribute::Constant - Make read-only variables via attribute
202
203       Scalar::Readonly - Perl extension to the "SvREADONLY" scalar flag
204
205       Hash::Util - A selection of general-utility hash subroutines (mostly to
206       lock/unlock keys and values)
207

BUGS

209       Please report any bugs or feature requests via the perlbug(1) utility.
210

AUTHORS

212       Tom Phoenix, <rootbeer@redcat.com>, with help from many other folks.
213
214       Multiple constant declarations at once added by Casey West,
215       <casey@geeknest.com>.
216
217       Documentation mostly rewritten by Ilmari Karonen, <perl@itz.pp.sci.fi>.
218
219       This program is maintained by the Perl 5 Porters.  The CPAN
220       distribution is maintained by Sebastien Aperghis-Tramoni
221       <sebastien@aperghis.net>.
222
224       Copyright (C) 1997, 1999 Tom Phoenix
225
226       This module is free software; you can redistribute it or modify it
227       under the same terms as Perl itself.
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231perl v5.12.4                      2011-06-07                     constant(3pm)
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