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constant(3)
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 will declare a symbol to be a constant with the given 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.
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};
}
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 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'".
AUTHOR
Tom Phoenix, <rootbeer@redcat.com>, with help from many other folks.
Multiple constant declarations at once added by Casey West,
<casey@geeknest.com>.
Documentation mostly rewritten by Ilmari Karonen, <perl@itz.pp.sci.fi>.
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.
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