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.\" ========================================================================
.\"
.IX Title "Class::Constant 3"
.TH Class::Constant 3 "2008-01-07" "perl v5.8.8" "User Contributed Perl Documentation"
.SH "NAME"
Class::Constant \- Build constant classes
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&    use Class::Constant NORTH, EAST, SOUTH, WEST;
.Ve
.PP
.Vb 5
\&    use Class::Constant
\&        NORTH => "north",
\&        EAST  => "east",
\&        SOUTH => "south",
\&        WEST  => "west;
.Ve
.PP
.Vb 5
\&    use Class::Constant
\&        NORTH => { x =>  0, y => -1 },
\&        EAST  => { x => -1, y =>  0 },
\&        SOUTH => { x =>  0, y =>  1 },
\&        WEST  => { x =>  1, y =>  0 };
.Ve
.PP
.Vb 9
\&    use Class::Constant
\&        NORTH => "north",
\&                 { x =>  0, y => -1 },
\&        EAST  => "east",
\&                 { x => -1, y =>  0 },
\&        SOUTH => "south",
\&                 { x =>  0, y =>  1 },
\&        WEST  => "west",
\&                 { x =>  1, y =>  0 };
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Class::Constant allows you declaratively created so-called \*(L"constant
classes\*(R". These are very much like enumerated types (as close as a typeless
language like Perl can get, at least).
.PP
The classes generated by this module are modeled closely after Java's \*(L"typesafe
enumeration\*(R" pattern, but with some added spice to make them more useful to
Perl programs.
.Sh "\s-1SIMPLE\s0 \s-1USAGE\s0"
.IX Subsection "SIMPLE USAGE"
The simplese usage of Class::Constant is to use it to define a set of values
for a user-defined \*(L"type\*(R". Consider a class that defines the four main compass
points:
.PP
.Vb 1
\&    package Direction;
.Ve
.PP
.Vb 1
\&    use Class::Constant NORTH, EAST, SOUTH, WEST;
.Ve
.PP
This generates four constants which can be assigned to some variable:
.PP
.Vb 1
\&    my $facing = Direction::NORTH;
.Ve
.PP
There are two major differences between Class::Constant constants and
constants created by the constant pragma:
.IP "\(bu" 4
Class::Constant constants have no inherent value, and as such only compare
equal to themselves (but see \*(L"\s-1ORDINAL\s0 \s-1VALUES\s0\*(R", eg:
.Sp
.Vb 3
\&    if ($facing == Direction::EAST) {
\&        print "you are facing east\en";
\&    }
.Ve
.IP "\(bu" 4
Class::Constant constants are actually objects blessed into the package that
created them, so they have a \*(L"type\*(R", of sorts:
.Sp
.Vb 3
\&    if ($facing->isa("Direction")) {
\&        ...
\&    }
.Ve
.PP
Neither of these distinctions are particularly useful in this simple usage, but
are useful when using the more advanced features of this module, described
below.
.Sh "\s-1CONSTANT\s0 \s-1VALUES\s0"
.IX Subsection "CONSTANT VALUES"
Althought constants don't have a value as such, real values can be attached to
them to be used when appropriate.
.PP
\fIStringification\fR
.IX Subsection "Stringification"
.PP
Constants can be declared with a string that will be returned when the constant
is stringified (eg by \f(CW\*(C`print\*(C'\fR). For example:
.PP
.Vb 5
\&    use Class::Constant
\&        NORTH => "north",
\&        EAST  => "east",
\&        SOUTH => "south",
\&        WEST  => "west";
.Ve
.PP
This makes the following possible:
.PP
.Vb 1
\&    print "you are facing $facing\en";
.Ve
.PP
\fINamed sub-constants\fR
.IX Subsection "Named sub-constants"
.PP
You can also declare other constant values that are associated with a constant:
.PP
.Vb 5
\&    use Class::Constant
\&        NORTH => { x =>  0, y => -1 },
\&        EAST  => { x => -1, y =>  0 },
\&        SOUTH => { x =>  0, y =>  1 },
\&        WEST  => { x =>  1, y =>  0 };
.Ve
.PP
These sub-constants are accessed via \f(CW\*(C`get_*\*(C'\fR methods called on the constant
object:
.PP
.Vb 1
\&    move_player($facing->get_x, $facing->get_y);
.Ve
.PP
\fICombining the two\fR
.IX Subsection "Combining the two"
.PP
Of course both a string value and named sub-constants can be declared at the
same time:
.PP
.Vb 9
\&    use Class::Constant
\&        NORTH => "north",
\&                 { x =>  0, y => -1 },
\&        EAST  => "east",
\&                 { x => -1, y =>  0 },
\&        SOUTH => "south",
\&                 { x =>  0, y =>  1 },
\&        WEST  => "west",
\&                 { x =>  1, y =>  0 };
.Ve
.Sh "\s-1ORDINAL\s0 \s-1VALUES\s0"
.IX Subsection "ORDINAL VALUES"
Each constant has an internal value which is generated by Class::Constant as
it creates the constants. These ordinal values are unique to a package, and are
assigned sequentially to each constant create in that package. For example, in
our Direction packages, the constants would receive ordinal values as follows:
.PP
.Vb 4
\&    NORTH   0
\&    EAST    2
\&    SOUTH   1
\&    WEST    3
.Ve
.PP
The ordinal value for a constant can be retrieved by calling the \f(CW\*(C`get_ordinal\*(C'\fR
method on a constant object:
.PP
.Vb 1
\&    my $ordinal = Direction::EAST->get_ordinal;
.Ve
.PP
You can also retrieve a constant by its ordinal value using the class method
\&\f(CW\*(C`by_ordinal\*(C'\fR
.PP
.Vb 1
\&    my $west = Direction->by_ordinal(3);
.Ve
.PP
These two methods are typically used together to fetch the \*(L"next\*(R" or \*(L"previous\*(R"
constant in the sequence, eg:
.PP
.Vb 4
\&    sub turn_left {
\&        my ($facing) = @_;
\&        return Direction->by_ordinal(($facing->get_ordinal - 1) % 4);
\&    }
.Ve
.Sh "\s-1OVERLOADING\s0"
.IX Subsection "OVERLOADING"
Constant objects are blessed into the package in which they were declared. The
Class::Constant \f(CW\*(C`import\*(C'\fR method also updates the packages' \f(CW@ISA\fR to make
constant objects subclass Class::Constant::Object
.PP
Class::Constant::Object has \f(CW\*(C`as_string\*(C'\fR and \f(CW\*(C`equals\*(C'\fR methods, and also
sets up overloading for the \f(CW""\fR (stringification) and \f(CW\*(C`==\*(C'\fR and \f(CW\*(C`!=\*(C'\fR
(equality) operators to use these methods. If you override these methods in
your package, then Class::Constant::Object will arrange to call your methods
instead.
.SH "DIAGNOSTICS"
.IX Header "DIAGNOSTICS"
.ie n .IP """Can't locate constant with ordinal ""%s"" in package ""%s""""" 4
.el .IP "\f(CWCan't locate constant with ordinal ``%s'' in package ``%s''\fR" 4
.IX Item "Can't locate constant with ordinal ""%s"" in package ""%s"""
The value passed to \f(CW\*(C`by_ordinal\*(C'\fR does not corespond to any constant in the
named package. This usually means the value you've specified is greater than
the number of declared constants.
.ie n .IP """Can't locate named constant ""%s"" for ""%s""""" 4
.el .IP "\f(CWCan't locate named constant ``%s'' for ``%s''\fR" 4
.IX Item "Can't locate named constant ""%s"" for ""%s"""
A named constant associated with a declared constant was not found. It was
probably not defined; check your declarations.
.SH "AUTHOR"
.IX Header "AUTHOR"
Robert Norris (rob@cataclysm.cx)
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright (c) 2006 Robert Norris. This program is free software; you can
redistribute it and/or modify it under the same terms as Perl itself.