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constant (3)
  • >> constant (3) ( Solaris man: Библиотечные вызовы )
  • constant (3) ( Linux man: Библиотечные вызовы )
  • 
    
    

    NAME

         constant - Perl pragma to declare constants
    
    
    

    SYNOPSIS

             use constant BUFFER_SIZE    => 4096;
             use constant ONE_YEAR       => 365.2425 * 24 * 60 * 60;
             use constant PI             => 4 * atan2 1, 1;
             use constant DEBUGGING      => 0;
             use constant ORACLE         => '[email protected]';
             use constant USERNAME       => scalar getpwuid($<);
             use constant USERINFO       => getpwuid($<);
    
             sub deg2rad { PI * $_[0] / 180 }
    
             print "This line does nothing"              unless DEBUGGING;
    
             # references can be constants
             use constant CHASH          => { foo => 42 };
             use constant CARRAY         => [ 1,2,3,4 ];
             use constant CPSEUDOHASH    => [ { foo => 1}, 42 ];
             use constant CCODE          => sub { "bite $_[0]\n" };
    
             print CHASH->{foo};
             print CARRAY->[$i];
             print CPSEUDOHASH->{foo};
             print CCODE->("me");
             print CHASH->[10];                  # compile-time error
    
    
    
    

    DESCRIPTION

         This will declare a symbol to be a constant with the given
         scalar or list 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'.
    
    
    

    NOTES

         The value or values are evaluated in a list context. You may
         override this with `scalar' as shown above.
    
         These constants do not directly interpolate into double-
         quotish strings, although you may do so indirectly. (See the
         perlref manpage for details about how this works.)
    
             print "The value of PI is @{[ PI ]}.\n";
    
         List constants are returned as lists, not as arrays.
             $homedir = USERINFO[7];             # WRONG
             $homedir = (USERINFO)[7];           # Right
    
         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.
    
         Constant symbols are package scoped (rather than block
         scoped, as `use strict' is). That is, you can refer to a
         constant from package Other as `Other::CONST'.
    
         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 ... }').
    
         Omitting the value for a symbol gives it the value of
         `undef' in a scalar context or the empty list, `()', in a
         list context. This isn't so nice as it may sound, though,
         because in this case you must either quote the symbol name,
         or use a big arrow, (`=>'), with nothing to point to. It is
         probably best to declare these explicitly.
    
             use constant UNICORNS       => ();
             use constant LOGFILE        => undef;
    
         The result from evaluating a list constant in a scalar
         context is not documented, and is not guaranteed to be any
         particular value in the future. In particular, you should
         not rely upon it being the number of elements in the list,
         especially since it is not necessarily that value in the
         current implementation.
    
         Magical values, tied 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"
    
         Dereferencing constant references incorrectly (such as using
         an array subscript on a constant hash reference, or vice
         versa) will be trapped at compile time.
    
         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};
             }
    
    
    
    

    TECHNICAL NOTE

         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 the Constant Functions entry in the
         perlsub manpage for details about how and when this happens.
    
    
    

    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 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.
    
         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 CARRAY         => [ 1,2,3,4 ];
             print CARRAY->[1];
             CARRAY->[1] = " be changed";
             print CARRAY->[1];
    
         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.
    
    
    

    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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