constant(3p) Perl Programmers Reference Guide constant(3p)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 (CON-
STANT)" 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 con-
stant declaration inside of a conditional statement (like
"if ($foo) { use constant ... }").
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Constants defined using this module cannot be interpolated
into strings like variables. However, concatenation works
just fine:
print "Pi equals PI...\n"; # WRONG: does not expand "PI"
print "Pi equals ".PI."...\n"; # right
Even though a reference may be declared as a constant, the
reference may point to data which may be changed, as this
code shows.
use constant ARRAY => [ 1,2,3,4 ];
print ARRAY->[1];
ARRAY->[1] = " be changed";
print ARRAY->[1];
Dereferencing constant references incorrectly (such as using
an array subscript on a constant hash reference, or vice
versa) will be trapped at compile time.
Constants belong to the package they are defined in. To
refer to a constant defined in another package, specify the
full package name, as in "Some::Package::CONSTANT". Con-
stants may be exported by modules, and may also be called as
either class or instance methods, that is, as
"Some::Package->CONSTANT" or as "$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 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 con-
text. 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:
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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 giv-
ing, 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 con-
structed 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 fin-
ished.
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.)
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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 sub-
routine 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 overrid-
den 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
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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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