constant(3pm)                                   Perl Programmers Reference Guide                                   constant(3pm)



NAME
       constant - Perl pragma to declare constants

SYNOPSIS
           use constant PI    => 4 * atan2(1, 1);
           use constant DEBUG => 0;

           print "Pi equals ", PI, "...\n" if DEBUG;

           use constant {
               SEC   => 0,
               MIN   => 1,
               HOUR  => 2,
               MDAY  => 3,
               MON   => 4,
               YEAR  => 5,
               WDAY  => 6,
               YDAY  => 7,
               ISDST => 8,
           };

           use constant WEEKDAYS => qw(
               Sunday Monday Tuesday Wednesday Thursday Friday Saturday
           );

           print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";

DESCRIPTION
       This pragma allows you to declare constants at compile-time.

       When you declare a constant such as "PI" using the method shown above, each machine your script runs upon can have as
       many digits of accuracy as it can use. Also, your program will be easier to read, more likely to be maintained (and
       maintained correctly), and far less likely to send a space probe to the wrong planet because nobody noticed the one
       equation in which you wrote 3.14195.

       When a constant is used in an expression, Perl replaces it with its value at compile time, and may then optimize the
       expression further.  In particular, any code in an "if (CONSTANT)" block will be optimized away if the constant is false.

NOTES
       As with all "use" directives, defining a constant happens at compile time. Thus, it's probably not correct to put a
       constant declaration inside of a conditional statement (like "if ($foo) { use constant ... }").

       Constants defined using this module cannot be interpolated into strings like variables.  However, concatenation works
       just fine:

           print "Pi equals PI...\n";        # WRONG: does not expand "PI"
           print "Pi equals ".PI."...\n";    # right

       Even though a reference may be declared as a constant, the reference may point to data which may be changed, as this code
       shows.

           use constant ARRAY => [ 1,2,3,4 ];
           print ARRAY->[1];
           ARRAY->[1] = " be changed";
           print ARRAY->[1];

       Dereferencing constant references incorrectly (such as using an array subscript on a constant hash reference, or vice
       versa) will be trapped at compile time.

       Constants belong to the package they are defined in.  To refer to a constant defined in another package, specify the full
       package name, as in "Some::Package::CONSTANT".  Constants may be exported by modules, and may also be called as either
       class or instance methods, that is, as "Some::Package->CONSTANT" or as "$obj->CONSTANT" where $obj is an instance of
       "Some::Package".  Subclasses may define their own constants to override those in their base class.

       The use of all caps for constant names is merely a convention, although it is recommended in order to make constants
       stand out and to help avoid collisions with other barewords, keywords, and subroutine names. Constant names must begin
       with a letter or underscore. Names beginning with a double underscore are reserved. Some poor choices for names will
       generate warnings, if warnings are enabled at compile time.

   List constants
       Constants may be lists of more (or less) than one value.  A constant with no values evaluates to "undef" in scalar
       context.  Note that constants with more than one value do not return their last value in scalar context as one might
       expect.  They currently return the number of values, but this may change in the future.  Do not use constants with
       multiple values in scalar context.

       NOTE: This implies that the expression defining the value of a constant is evaluated in list context.  This may produce
       surprises:

           use constant TIMESTAMP => localtime;                # WRONG!
           use constant TIMESTAMP => scalar localtime;         # right

       The first line above defines "TIMESTAMP" as a 9-element list, as returned by "localtime()" in list context.  To set it to
       the string returned by "localtime()" in scalar context, an explicit "scalar" keyword is required.

       List constants are lists, not arrays.  To index or slice them, they must be placed in parentheses.

           my @workdays = WEEKDAYS[1 .. 5];            # WRONG!
           my @workdays = (WEEKDAYS)[1 .. 5];          # right

   Defining multiple constants at once
       Instead of writing multiple "use constant" statements, you may define multiple constants in a single statement by giving,
       instead of the constant name, a reference to a hash where the keys are the names of the constants to be defined.
       Obviously, all constants defined using this method must have a single value.

           use constant {
               FOO => "A single value",
               BAR => "This", "won't", "work!",        # Error!
           };

       This is a fundamental limitation of the way hashes are constructed in Perl.  The error messages produced when this
       happens will often be quite cryptic -- in the worst case there may be none at all, and you'll only later find that
       something is broken.

       When defining multiple constants, you cannot use the values of other constants defined in the same declaration.  This is
       because the calling package doesn't know about any constant within that group until after the "use" statement is
       finished.

           use constant {
               BITMASK => 0xAFBAEBA8,
               NEGMASK => ~BITMASK,                    # Error!
           };

   Magic constants
       Magical values and references can be made into constants at compile time, allowing for way cool stuff like this.  (These
       error numbers aren't totally portable, alas.)

           use constant E2BIG => ($! = 7);
           print   E2BIG, "\n";        # something like "Arg list too long"
           print 0+E2BIG, "\n";        # "7"

       You can't produce a tied constant by giving a tied scalar as the value.  References to tied variables, however, can be
       used as constants without any problems.

TECHNICAL NOTES
       In the current implementation, scalar constants are actually inlinable subroutines. As of version 5.004 of Perl, the
       appropriate scalar constant is inserted directly in place of some subroutine calls, thereby saving the overhead of a
       subroutine call. See "Constant Functions" in perlsub for details about how and when this happens.

       In the rare case in which you need to discover at run time whether a particular constant has been declared via this
       module, you may use this function to examine the hash %constant::declared. If the given constant name does not include a
       package name, the current package is used.

           sub declared ($) {
               use constant 1.01;              # don't omit this!
               my $name = shift;
               $name =~ s/^::/main::/;
               my $pkg = caller;
               my $full_name = $name =~ /::/ ? $name : "${pkg}::$name";
               $constant::declared{$full_name};
           }

CAVEATS
       In the current version of Perl, list constants are not inlined and some symbols may be redefined without generating a
       warning.

       It is not possible to have a subroutine or a keyword with the same name as a constant in the same package. This is
       probably a Good Thing.

       A constant with a name in the list "STDIN STDOUT STDERR ARGV ARGVOUT ENV INC SIG" is not allowed anywhere but in package
       "main::", for technical reasons.

       Unlike constants in some languages, these cannot be overridden on the command line or via environment variables.

       You can get into trouble if you use constants in a context which automatically quotes barewords (as is true for any
       subroutine call).  For example, you can't say $hash{CONSTANT} because "CONSTANT" will be interpreted as a string.  Use
       $hash{CONSTANT()} or $hash{+CONSTANT} to prevent the bareword quoting mechanism from kicking in.  Similarly, since the
       "=>" operator quotes a bareword immediately to its left, you have to say "CONSTANT() => 'value'" (or simply use a comma
       in place of the big arrow) instead of "CONSTANT => 'value'".

SEE ALSO
       Readonly - Facility for creating read-only scalars, arrays, hashes.

       Const - Facility for creating read-only variables. Similar to "Readonly", but uses "SvREADONLY" instead of "tie".

       Attribute::Constant - Make read-only variables via attribute

       Scalar::Readonly - Perl extension to the "SvREADONLY" scalar flag

       Hash::Util - A selection of general-utility hash subroutines (mostly to lock/unlock keys and values)

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

AUTHORS
       Tom Phoenix, <rootbeerATredcat.com>, with help from many other folks.

       Multiple constant declarations at once added by Casey West, <caseyATgeeknest.com>.

       Documentation mostly rewritten by Ilmari Karonen, <perlATitz.fi>.

       This program is maintained by the Perl 5 Porters.  The CPAN distribution is maintained by Sebastien Aperghis-Tramoni
       <sebastienATaperghis.net>.

COPYRIGHT & LICENSE
       Copyright (C) 1997, 1999 Tom Phoenix

       This module is free software; you can redistribute it or modify it under the same terms as Perl itself.



perl v5.12.4                                               2011-06-07                                              constant(3pm)