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constant

Section: Perl Programmers Reference Guide (3)
Updated: perl 5.004, patch 04
Index

NAME

constant - Perl pragma to declare constants

SYNOPSIS

    use constant BUFFER_SIZE    => 4096;    use constant ONE_YEAR       => 365.2425 * 24 * 60 * 60;    use constant PI             => 4 * atan2 1, 1;    use constant DEBUGGING      => 0;    use constant ORACLE         => 'oracleAATTcs.indiana.edu';    use constant USERNAME       => scalar getpwuid($<);    use constant USERINFO       => getpwuid($<);

    sub deg2rad { PI * $_[0] / 180 }

    print "This line does nothing"              unless DEBUGGING;

DESCRIPTION

This will declare a symbol to be a constant with the given scalaror list value.

When you declare a constant such as PI using the method shownabove, each machine your script runs upon can have as many digitsof accuracy as it can use. Also, your program will be easier toread, more likely to be maintained (and maintained correctly), andfar less likely to send a space probe to the wrong planet becausenobody noticed the one equation in which you wrote 3.14195.

NOTES

The value or values are evaluated in a list context. You may overridethis with scalar as shown above.

These constants do not directly interpolate into double-quotishstrings, although you may do so indirectly. (See the perlref manpage fordetails about how this works.)

    print "The value of PI is @{[ PI ]}.\n";

List constants are returned as lists, not as arrays.

    $homedir = USERINFO[7];             # WRONG    $homedir = (USERINFO)[7];           # Right

The use of all caps for constant names is merely a convention,although it is recommended in order to make constants stand outand to help avoid collisions with other barewords, keywords, andsubroutine names. Constant names must begin with a letter.

Constant symbols are package scoped (rather than block scoped, asuse strict is). That is, you can refer to a constant from packageOther as Other::CONST.

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

Omitting the value for a symbol gives it the value of undef ina scalar context or the empty list, (), in a list context. Thisisn't so nice as it may sound, though, because in this case youmust either quote the symbol name, or use a big arrow, (=>),with nothing to point to. It is probably best to declare theseexplicitly.

    use constant UNICORNS       => ();    use constant LOGFILE        => undef;

The result from evaluating a list constant in a scalar context isnot documented, and is not guaranteed to be any particular valuein the future. In particular, you should not rely upon it beingthe number of elements in the list, especially since it is notnecessarily that value in the current implementation.

Magical values, tied values, and references can be made intoconstants 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"

TECHNICAL NOTE

In the current implementation, scalar constants are actuallyinlinable subroutines. As of version 5.004 of Perl, the appropriatescalar constant is inserted directly in place of some subroutinecalls, thereby saving the overhead of a subroutine call. Seethe section on Constant Functions in the perlsub manpage for details about how and when thishappens.

BUGS

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

It is not possible to have a subroutine or keyword with the samename as a constant. This is probably a Good Thing.

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

AUTHOR

Tom Phoenix, <rootbeerAATTteleport.com>, with help frommany other folks.

COPYRIGHT

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

This document was created byman2html,using the manual pages.