constant - phpMan

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 pragma allows you to declare constants at compile-time.

    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];

    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.

    As of version 1.32 of this module, constants can be defined in packages
    other than the caller, by including the package name in the name of the
    constant:

        use constant "OtherPackage::FWIBBLE" => 7865;
        constant->import("Other::FWOBBLE",$value); # dynamically at run time

    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};
        }

CAVEATS
    List constants are not inlined unless you are using Perl v5.20 or
    higher. In v5.20 or higher, they are still not read-only, but that may
    change in future versions.

    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'".

SEE ALSO
    Readonly - Facility for creating read-only scalars, arrays, hashes.

    Attribute::Constant - Make read-only variables via attribute

    Scalar::Readonly - Perl extension to the "SvREADONLY" scalar flag

    Hash::Util - A selection of general-utility hash subroutines (mostly to
    lock/unlock keys and values)

BUGS
    Please report any bugs or feature requests via the perlbug(1) utility.

AUTHORS
    Tom Phoenix, <rootbeer AT redcat.com>, with help from many other folks.

    Multiple constant declarations at once added by Casey West,
    <casey AT geeknest.com>.

    Documentation mostly rewritten by Ilmari Karonen, <perl AT itz.fi>.

    This program is maintained by the Perl 5 Porters. The CPAN distribution
    is maintained by Sébastien Aperghis-Tramoni <sebastien AT aperghis.net>.

COPYRIGHT & LICENSE
    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.