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, "...0 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] ],
".0;

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 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.

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...0; # WRONG: does not ex; pand "PI"
print "Pi equals ".PI."...0; # 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 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.
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 pro
duce 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 con text, 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 val ues 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, "0; # something like "Arg list; too long"
print 0+E2BIG, "0; # "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 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 "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 generat
ing 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 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 immediately to
its left, you have to say "CONSTANT() => 'value'" (or sim
ply 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

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

docs.sk

comprehensive documentation repository

In section