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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 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:



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constant(3p)     Perl Programmers Reference Guide    constant(3p)


           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 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



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constant(3p)     Perl Programmers Reference Guide    constant(3p)


       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, "\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.




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constant(3p)     Perl Programmers Reference Guide    constant(3p)


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.




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constant(3p)     Perl Programmers Reference Guide    constant(3p)


       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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