constant(3) constant(3)
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 => 'oracle@cs.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 scalar or list
value.
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.
NOTES
The value or values are evaluated in a list context. You may override
this with scalar as shown above.
These constants do not directly interpolate into double-quotish strings,
although you may do so indirectly. (See the perlref manpage for details
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
$homedir = (USERINFO)[7]; # Right
The use of all caps for constant names is merely a convention, although
it is recommended in order to make constants stand out and to help avoid
collisions with other barewords, keywords, and subroutine names. Constant
names must begin with a letter.
Constant symbols are package scoped (rather than block scoped, as use
strict is). That is, you can refer to a constant from package Other as
Other::CONST.
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As with all use directives, defining a constant happens at compile time.
Thus, it's probably not correct to put a constant declaration inside of a
conditional statement (like if ($foo) { use constant ... }).
Omitting the value for a symbol gives it the value of undef in a scalar
context or the empty list, (), in a list context. This isn't so nice as
it may sound, though, because in this case you must either quote the
symbol name, or use a big arrow, (=>), with nothing to point to. It is
probably best to declare these explicitly.
use constant UNICORNS => ();
use constant LOGFILE => undef;
The result from evaluating a list constant in a scalar context is not
documented, and is not guaranteed to be any particular value in the
future. In particular, you should not rely upon it being the number of
elements in the list, especially since it is not necessarily that value
in the current implementation.
Magical values, tied 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"
TECHNICAL NOTE
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 the section on Constant Functions
in the perlsub manpage for details about how and when this happens.
BUGS
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 keyword with the same name as
a constant. This is probably a Good Thing.
Unlike constants in some languages, these cannot be overridden on the
command line or via environment variables.
AUTHOR
Tom Phoenix, <rootbeer@teleport.com>, with help from many other folks.
COPYRIGHT
Copyright (C) 1997, Tom Phoenix
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This module is free software; you can redistribute it or modify it under
the same terms as Perl itself.
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