constant(3) Perl Programmers Reference Guide constant(3)NAMEconstant - 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;
# references can be constants
use constant CHASH => { foo => 42 };
use constant CARRAY => [ 1,2,3,4 ];
use constant CPSEUDOHASH => [ { foo => 1}, 42 ];
use constant CCODE => sub { "bite $_[0]\n" };
print CHASH->{foo};
print CARRAY->[$i];
print CPSEUDOHASH->{foo};
print CCODE->("me");
print CHASH->[10]; # compile-time error
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 pro
gram 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
$homedir = (USERINFO)[7]; # Right
The use of all caps for constant names is merely a conven
tion, although it is recommended in order to make con
stants stand out and to help avoid collisions with other
barewords, keywords, and subroutine names. Constant names
must begin with a letter or underscore. Names beginning
with a double underscore are reserved. Some poor choices
for names will generate warnings, if warnings are enabled
at compile time.
Constant symbols are package scoped (rather than block
scoped, as "use strict" is). That is, you can refer to a
constant from package Other as "Other::CONST".
As with all "use" directives, defining a constant happens
at compile time. Thus, it's probably not correct to put a
constant declaration inside of a conditional statement
(like "if ($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 => ($! = 7);
print E2BIG, "\n"; # something like "Arg list too long"
print 0+E2BIG, "\n"; # "7"
Dereferencing constant references incorrectly (such as
using an array subscript on a constant hash reference, or
vice versa) will be trapped at compile time.
In the rare case in which you need to discover at run time
whether a particular constant has been declared via this
module, you may use this function to examine the hash
"%constant::declared". If the given constant name does not
include a package name, the current package is used.
sub declared ($) {
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};
}
TECHNICAL NOTE
In the current implementation, scalar constants are actu
ally 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 over
head of a subroutine call. See the Constant Functions
entry 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 generat
ing a warning.
It is not possible to have a subroutine or 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.
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 CARRAY => [ 1,2,3,4 ];
print CARRAY->[1];
CARRAY->[1] = " be changed";
print CARRAY->[1];
Unlike constants in some languages, these cannot be over
ridden 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 "$hash{CON
STANT}" because "CONSTANT" will be interpreted as a
string. Use "$hash{CONSTANT()}" or "$hash{+CONSTANT}" to
prevent the bareword quoting mechanism from kicking in.
Similarly, since the "=>" operator quotes a bareword imme
diately to its left, you have to say "CONSTANT() =>
'value'" (or simply use a comma in place of the big arrow)
instead of "CONSTANT => 'value'".
AUTHOR
Tom Phoenix, <rootbeer@redcat.com>, with help from many
other folks.
COPYRIGHT
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.
2001-02-22 perl v5.6.1 constant(3)
