[p5sagit/p5-mst-13.2.git] / lib / Class / Struct.pm

package Class::Struct;

## See POD after __END__

require 5.002;

use strict;
use vars qw(@ISA @EXPORT);

use Carp;

require Exporter;
@ISA = qw(Exporter);
@EXPORT = qw(struct);

## Tested on 5.002 and 5.003 without class membership tests:
my $CHECK_CLASS_MEMBERSHIP = ($] >= 5.003_95);

my $print = 0;
sub printem {
    if (@_) { $print = shift }
    else    { $print++ }
}

{
    package Class::Struct::Tie_ISA;

    sub TIEARRAY {
        my $class = shift;
        return bless [], $class;
    }

    sub STORE {
        my ($self, $index, $value) = @_;
        Class::Struct::_subclass_error();
    }

    sub FETCH {
        my ($self, $index) = @_;
        $self->[$index];
    }

    sub FETCHSIZE {
        my $self = shift;
        return scalar(@$self);
    }

    sub DESTROY { }
}

sub struct {

    # Determine parameter list structure, one of:
    #   struct( class => [ element-list ])
    #   struct( class => { element-list })
    #   struct( element-list )
    # Latter form assumes current package name as struct name.

    my ($class, @decls);
    my $base_type = ref $_[1];
    if ( $base_type eq 'HASH' ) {
        $class = shift;
        @decls = %{shift()};
        _usage_error() if @_;
    }
    elsif ( $base_type eq 'ARRAY' ) {
        $class = shift;
        @decls = @{shift()};
        _usage_error() if @_;
    }
    else {
        $base_type = 'ARRAY';
        $class = (caller())[0];
        @decls = @_;
    }
    _usage_error() if @decls % 2 == 1;

    # Ensure we are not, and will not be, a subclass.

    my $isa = do {
        no strict 'refs';
        \@{$class . '::ISA'};
    };
    _subclass_error() if @$isa;
    tie @$isa, 'Class::Struct::Tie_ISA';

    # Create constructor.

    croak "function 'new' already defined in package $class"
        if do { no strict 'refs'; defined &{$class . "::new"} };

    my @methods = ();
    my %refs = ();
    my %arrays = ();
    my %hashes = ();
    my %classes = ();
    my $got_class = 0;
    my $out = '';

    $out = "{\n  package $class;\n  use Carp;\n  sub new {\n";

    my $cnt = 0;
    my $idx = 0;
    my( $cmt, $name, $type, $elem );

    if( $base_type eq 'HASH' ){
        $out .= "    my(\$r) = {};\n";
        $cmt = '';
    }
    elsif( $base_type eq 'ARRAY' ){
        $out .= "    my(\$r) = [];\n";
    }
    while( $idx < @decls ){
        $name = $decls[$idx];
        $type = $decls[$idx+1];
        push( @methods, $name );
        if( $base_type eq 'HASH' ){
            $elem = "{'$name'}";
        }
        elsif( $base_type eq 'ARRAY' ){
            $elem = "[$cnt]";
            ++$cnt;
            $cmt = " # $name";
        }
        if( $type =~ /^\*(.)/ ){
            $refs{$name}++;
            $type = $1;
        }
        if( $type eq '@' ){
            $out .= "    \$r->$elem = [];$cmt\n";
            $arrays{$name}++;
        }
        elsif( $type eq '%' ){
            $out .= "    \$r->$elem = {};$cmt\n";
            $hashes{$name}++;
        }
        elsif ( $type eq '$') {
            $out .= "    \$r->$elem = undef;$cmt\n";
        }
        elsif( $type =~ /^\w+(?:::\w+)*$/ ){
            $out .= "    \$r->$elem = '${type}'->new();$cmt\n";
            $classes{$name} = $type;
            $got_class = 1;
        }
        else{
            croak "'$type' is not a valid struct element type";
        }
        $idx += 2;
    }
    $out .= "    bless \$r;\n  }\n";

    # Create accessor methods.

    my( $pre, $pst, $sel );
    $cnt = 0;
    foreach $name (@methods){
        if ( do { no strict 'refs'; defined &{$class . "::$name"} } ) {
            carp "function '$name' already defined, overrides struct accessor method"
                if $^W;
        }
        else {
            $pre = $pst = $cmt = $sel = '';
            if( defined $refs{$name} ){
                $pre = "\\(";
                $pst = ")";
                $cmt = " # returns ref";
            }
            $out .= "  sub $name {$cmt\n    my \$r = shift;\n";
            if( $base_type eq 'ARRAY' ){
                $elem = "[$cnt]";
                ++$cnt;
            }
            elsif( $base_type eq 'HASH' ){
                $elem = "{'$name'}";
            }
            if( defined $arrays{$name} ){
                $out .= "    my \$i;\n";
                $out .= "    \@_ ? (\$i = shift) : return $pre\$r->$elem$pst;\n";
                $sel = "->[\$i]";
            }
            elsif( defined $hashes{$name} ){
                $out .= "    my \$i;\n";
                $out .= "    \@_ ? (\$i = shift) : return $pre\$r->$elem$pst;\n";
                $sel = "->{\$i}";
            }
            elsif( defined $classes{$name} ){
                if ( $CHECK_CLASS_MEMBERSHIP ) {
                    $out .= "    croak '$name argument is wrong class' if \@_ && ! UNIVERSAL::isa(\$_[0], '$classes{$name}');\n";
                }
            }
            $out .= "    croak 'Too many args to $name' if \@_ > 1;\n";
            $out .= "    \@_ ? ($pre\$r->$elem$sel = shift$pst) : $pre\$r->$elem$sel$pst;\n";
            $out .= "  }\n";
        }
    }
    $out .= "}\n1;\n";

    print $out if $print;
    my $result = eval $out;
    carp $@ if $@;
}

sub _usage_error {
    confess "struct usage error";
}

sub _subclass_error {
    croak 'struct class cannot be a subclass (@ISA not allowed)';
}

1; # for require


__END__

=head1 NAME

Class::Struct - declare struct-like datatypes as Perl classes

=head1 SYNOPSIS

    use Class::Struct;
            # declare struct, based on array:
    struct( CLASS_NAME => [ ELEMENT_NAME => ELEMENT_TYPE, ... ]);
            # declare struct, based on hash:
    struct( CLASS_NAME => { ELEMENT_NAME => ELEMENT_TYPE, ... });

    package CLASS_NAME;
    use Class::Struct;
            # declare struct, based on array, implicit class name:
    struct( ELEMENT_NAME => ELEMENT_TYPE, ... );


    package Myobj;
    use Class::Struct;
            # declare struct with four types of elements:
    struct( s => '$', a => '@', h => '%', c => 'My_Other_Class' );

    $obj = new Myobj;               # constructor

                                    # scalar type accessor:
    $element_value = $obj->s;           # element value
    $obj->s('new value');               # assign to element

                                    # array type accessor:
    $ary_ref = $obj->a;                 # reference to whole array
    $ary_element_value = $obj->a(2);    # array element value
    $obj->a(2, 'new value');            # assign to array element

                                    # hash type accessor:
    $hash_ref = $obj->h;                # reference to whole hash
    $hash_element_value = $obj->h('x'); # hash element value
    $obj->h('x', 'new value');        # assign to hash element

                                    # class type accessor:
    $element_value = $obj->c;           # object reference
    $obj->c->method(...);               # call method of object
    $obj->c(new My_Other_Class);        # assign a new object


=head1 DESCRIPTION

C<Class::Struct> exports a single function, C<struct>.
Given a list of element names and types, and optionally
a class name, C<struct> creates a Perl 5 class that implements
a "struct-like" data structure.

The new class is given a constructor method, C<new>, for creating
struct objects.

Each element in the struct data has an accessor method, which is
used to assign to the element and to fetch its value.  The
default accessor can be overridden by declaring a C<sub> of the
same name in the package.  (See Example 2.)

Each element's type can be scalar, array, hash, or class.


=head2 The C<struct()> function

The C<struct> function has three forms of parameter-list.

    struct( CLASS_NAME => [ ELEMENT_LIST ]);
    struct( CLASS_NAME => { ELEMENT_LIST });
    struct( ELEMENT_LIST );

The first and second forms explicitly identify the name of the
class being created.  The third form assumes the current package
name as the class name.

An object of a class created by the first and third forms is
based on an array, whereas an object of a class created by the
second form is based on a hash. The array-based forms will be
somewhat faster and smaller; the hash-based forms are more
flexible.

The class created by C<struct> must not be a subclass of another
class other than C<UNIVERSAL>.

A function named C<new> must not be explicitly defined in a class
created by C<struct>.

The I<ELEMENT_LIST> has the form

    NAME => TYPE, ...

Each name-type pair declares one element of the struct. Each
element name will be defined as an accessor method unless a
method by that name is explicitly defined; in the latter case, a
warning is issued if the warning flag (B<-w>) is set.


=head2 Element Types and Accessor Methods

The four element types -- scalar, array, hash, and class -- are
represented by strings -- C<'$'>, C<'@'>, C<'%'>, and a class name --
optionally preceded by a C<'*'>.

The accessor method provided by C<struct> for an element depends
on the declared type of the element.

=over

=item Scalar (C<'$'> or C<'*$'>)

The element is a scalar, and is initialized to C<undef>.

The accessor's argument, if any, is assigned to the element.

If the element type is C<'$'>, the value of the element (after
assignment) is returned. If the element type is C<'*$'>, a reference
to the element is returned.

=item Array (C<'@'> or C<'*@'>)

The element is an array, initialized to C<()>.

With no argument, the accessor returns a reference to the
element's whole array.

With one or two arguments, the first argument is an index
specifying one element of the array; the second argument, if
present, is assigned to the array element.  If the element type
is C<'@'>, the accessor returns the array element value.  If the
element type is C<'*@'>, a reference to the array element is
returned.

=item Hash (C<'%'> or C<'*%'>)

The element is a hash, initialized to C<()>.

With no argument, the accessor returns a reference to the
element's whole hash.

With one or two arguments, the first argument is a key specifying
one element of the hash; the second argument, if present, is
assigned to the hash element.  If the element type is C<'%'>, the
accessor returns the hash element value.  If the element type is
C<'*%'>, a reference to the hash element is returned.

=item Class (C<'Class_Name'> or C<'*Class_Name'>)

The element's value must be a reference blessed to the named
class or to one of its subclasses. The element is initialized to
the result of calling the C<new> constructor of the named class.

The accessor's argument, if any, is assigned to the element. The
accessor will C<croak> if this is not an appropriate object
reference.

If the element type does not start with a C<'*'>, the accessor
returns the element value (after assignment). If the element type
starts with a C<'*'>, a reference to the element itself is returned.

=back

=head1 EXAMPLES

=over

=item Example 1

Giving a struct element a class type that is also a struct is how
structs are nested.  Here, C<timeval> represents a time (seconds and
microseconds), and C<rusage> has two elements, each of which is of
type C<timeval>.

    use Class::Struct;

    struct( rusage => {
        ru_utime => timeval,  # seconds
        ru_stime => timeval,  # microseconds
    });

    struct( timeval => [
        tv_secs  => '$',
        tv_usecs => '$',
    ]);

        # create an object:
    my $t = new rusage;
    	# $t->ru_utime and $t->ru_stime are objects of type timeval.

        # set $t->ru_utime to 100.0 sec and $t->ru_stime to 5.0 sec.
    $t->ru_utime->tv_secs(100);
    $t->ru_utime->tv_usecs(0);
    $t->ru_stime->tv_secs(5);
    $t->ru_stime->tv_usecs(0);


=item Example 2

An accessor function can be redefined in order to provide
additional checking of values, etc.  Here, we want the C<count>
element always to be nonnegative, so we redefine the C<count>
accessor accordingly.

    package MyObj;
    use Class::Struct;

    		# declare the struct
    struct ( 'MyObj', { count => '$', stuff => '%' } );

    		# override the default accessor method for 'count'
    sub count {
        my $self = shift;
        if ( @_ ) {
            die 'count must be nonnegative' if $_[0] < 0;
            $self->{'count'} = shift;
            warn "Too many args to count" if @_;
        }
        return $self->{'count'};
    }

    package main;
    $x = new MyObj;
    print "\$x->count(5) = ", $x->count(5), "\n";
                            # prints '$x->count(5) = 5'

    print "\$x->count = ", $x->count, "\n";
                            # prints '$x->count = 5'

    print "\$x->count(-5) = ", $x->count(-5), "\n";
                            # dies due to negative argument!


=head1 Author and Modification History


Renamed to C<Class::Struct> and modified by Jim Miner, 1997-04-02.

    members() function removed.
    Documentation corrected and extended.
    Use of struct() in a subclass prohibited.
    User definition of accessor allowed.
    Treatment of '*' in element types corrected.
    Treatment of classes as element types corrected.
    Class name to struct() made optional.
    Diagnostic checks added.


Originally C<Class::Template> by Dean Roehrich.

    # Template.pm   --- struct/member template builder
    #   12mar95
    #   Dean Roehrich
    #
    # changes/bugs fixed since 28nov94 version:
    #  - podified
    # changes/bugs fixed since 21nov94 version:
    #  - Fixed examples.
    # changes/bugs fixed since 02sep94 version:
    #  - Moved to Class::Template.
    # changes/bugs fixed since 20feb94 version:
    #  - Updated to be a more proper module.
    #  - Added "use strict".
    #  - Bug in build_methods, was using @var when @$var needed.
    #  - Now using my() rather than local().
    #
    # Uses perl5 classes to create nested data types.
    # This is offered as one implementation of Tom Christiansen's "structs.pl"
    # idea.

=cut
Commit	Line	Data
8cc95fdb	1	package Class::Struct;
	2
	3	## See POD after __END__
	4
	5	require 5.002;
	6
	7	use strict;
	8	use vars qw(@ISA @EXPORT);
	9
	10	use Carp;
	11
	12	require Exporter;
	13	@ISA = qw(Exporter);
	14	@EXPORT = qw(struct);
	15
	16	## Tested on 5.002 and 5.003 without class membership tests:
	17	my $CHECK_CLASS_MEMBERSHIP = ($] >= 5.003_95);
	18
	19	my $print = 0;
	20	sub printem {
	21	if (@_) { $print = shift }
	22	else { $print++ }
	23	}
	24
	25	{
	26	package Class::Struct::Tie_ISA;
	27
	28	sub TIEARRAY {
	29	my $class = shift;
	30	return bless [], $class;
	31	}
	32
	33	sub STORE {
	34	my ($self, $index, $value) = @_;
	35	Class::Struct::_subclass_error();
	36	}
	37
	38	sub FETCH {
	39	my ($self, $index) = @_;
	40	$self->[$index];
	41	}
	42
f740b751	43	sub FETCHSIZE {
	44	my $self = shift;
	45	return scalar(@$self);
	46	}
	47
8cc95fdb	48	sub DESTROY { }
	49	}
	50
	51	sub struct {
	52
	53	# Determine parameter list structure, one of:
	54	# struct( class => [ element-list ])
	55	# struct( class => { element-list })
	56	# struct( element-list )
	57	# Latter form assumes current package name as struct name.
	58
	59	my ($class, @decls);
	60	my $base_type = ref $_[1];
	61	if ( $base_type eq 'HASH' ) {
	62	$class = shift;
	63	@decls = %{shift()};
	64	_usage_error() if @_;
	65	}
	66	elsif ( $base_type eq 'ARRAY' ) {
	67	$class = shift;
	68	@decls = @{shift()};
	69	_usage_error() if @_;
	70	}
	71	else {
	72	$base_type = 'ARRAY';
	73	$class = (caller())[0];
	74	@decls = @_;
	75	}
	76	_usage_error() if @decls % 2 == 1;
	77
	78	# Ensure we are not, and will not be, a subclass.
	79
	80	my $isa = do {
	81	no strict 'refs';
	82	\@{$class . '::ISA'};
	83	};
	84	_subclass_error() if @$isa;
	85	tie @$isa, 'Class::Struct::Tie_ISA';
	86
	87	# Create constructor.
	88
	89	croak "function 'new' already defined in package $class"
	90	if do { no strict 'refs'; defined &{$class . "::new"} };
	91
	92	my @methods = ();
	93	my %refs = ();
	94	my %arrays = ();
	95	my %hashes = ();
	96	my %classes = ();
	97	my $got_class = 0;
	98	my $out = '';
	99
	100	$out = "{\n package $class;\n use Carp;\n sub new {\n";
	101
	102	my $cnt = 0;
	103	my $idx = 0;
	104	my( $cmt, $name, $type, $elem );
	105
	106	if( $base_type eq 'HASH' ){
	107	$out .= " my(\$r) = {};\n";
	108	$cmt = '';
	109	}
	110	elsif( $base_type eq 'ARRAY' ){
	111	$out .= " my(\$r) = [];\n";
112	}
113	while( $idx < @decls ){
114	$name = $decls[$idx];
115	$type = $decls[$idx+1];
116	push( @methods, $name );
117	if( $base_type eq 'HASH' ){
118	$elem = "{'$name'}";
119	}
120	elsif( $base_type eq 'ARRAY' ){
121	$elem = "[$cnt]";
122	++$cnt;
123	$cmt = " # $name";
124	}
125	if( $type =~ /^\*(.)/ ){
126	$refs{$name}++;
127	$type = $1;
128	}
129	if( $type eq '@' ){
130	$out .= " \$r->$elem = [];$cmt\n";
131	$arrays{$name}++;
132	}
133	elsif( $type eq '%' ){
134	$out .= " \$r->$elem = {};$cmt\n";
135	$hashes{$name}++;
136	}
137	elsif ( $type eq '$') {
138	$out .= " \$r->$elem = undef;$cmt\n";
139	}
140	elsif( $type =~ /^\w+(?:::\w+)*$/ ){
141	$out .= " \$r->$elem = '${type}'->new();$cmt\n";
142	$classes{$name} = $type;
143	$got_class = 1;
144	}
145	else{
146	croak "'$type' is not a valid struct element type";
147	}
148	$idx += 2;
149	}
150	$out .= " bless \$r;\n }\n";
151
152	# Create accessor methods.
153
8cc95fdb	154	my( $pre, $pst, $sel );
	155	$cnt = 0;
	156	foreach $name (@methods){
	157	if ( do { no strict 'refs'; defined &{$class . "::$name"} } ) {
	158	carp "function '$name' already defined, overrides struct accessor method"
	159	if $^W;
	160	}
	161	else {
	162	$pre = $pst = $cmt = $sel = '';
	163	if( defined $refs{$name} ){
	164	$pre = "\\(";
	165	$pst = ")";
	166	$cmt = " # returns ref";
	167	}
	168	$out .= " sub $name {$cmt\n my \$r = shift;\n";
	169	if( $base_type eq 'ARRAY' ){
	170	$elem = "[$cnt]";
	171	++$cnt;
	172	}
	173	elsif( $base_type eq 'HASH' ){
	174	$elem = "{'$name'}";
	175	}
	176	if( defined $arrays{$name} ){
	177	$out .= " my \$i;\n";
	178	$out .= " \@_ ? (\$i = shift) : return $pre\$r->$elem$pst;\n";
	179	$sel = "->[\$i]";
	180	}
	181	elsif( defined $hashes{$name} ){
	182	$out .= " my \$i;\n";
	183	$out .= " \@_ ? (\$i = shift) : return $pre\$r->$elem$pst;\n";
	184	$sel = "->{\$i}";
	185	}
	186	elsif( defined $classes{$name} ){
	187	if ( $CHECK_CLASS_MEMBERSHIP ) {
20408e3c	188	$out .= " croak '$name argument is wrong class' if \@_ && ! UNIVERSAL::isa(\$_[0], '$classes{$name}');\n";
8cc95fdb	189	}
	190	}
	191	$out .= " croak 'Too many args to $name' if \@_ > 1;\n";
	192	$out .= " \@_ ? ($pre\$r->$elem$sel = shift$pst) : $pre\$r->$elem$sel$pst;\n";
	193	$out .= " }\n";
	194	}
	195	}
	196	$out .= "}\n1;\n";
	197
	198	print $out if $print;
	199	my $result = eval $out;
	200	carp $@ if $@;
	201	}
	202
	203	sub _usage_error {
	204	confess "struct usage error";
	205	}
	206
	207	sub _subclass_error {
	208	croak 'struct class cannot be a subclass (@ISA not allowed)';
	209	}
	210
	211	1; # for require
	212
	213
	214	__END__
	215
	216	=head1 NAME
	217
	218	Class::Struct - declare struct-like datatypes as Perl classes
	219
	220	=head1 SYNOPSIS
	221
	222	use Class::Struct;
	223	# declare struct, based on array:
	224	struct( CLASS_NAME => [ ELEMENT_NAME => ELEMENT_TYPE, ... ]);
	225	# declare struct, based on hash:
	226	struct( CLASS_NAME => { ELEMENT_NAME => ELEMENT_TYPE, ... });
	227
	228	package CLASS_NAME;
	229	use Class::Struct;
	230	# declare struct, based on array, implicit class name:
	231	struct( ELEMENT_NAME => ELEMENT_TYPE, ... );
	232
	233
	234	package Myobj;
	235	use Class::Struct;
	236	# declare struct with four types of elements:
	237	struct( s => '$', a => '@', h => '%', c => 'My_Other_Class' );
	238
	239	$obj = new Myobj; # constructor
	240
	241	# scalar type accessor:
	242	$element_value = $obj->s; # element value
	243	$obj->s('new value'); # assign to element
	244
	245	# array type accessor:
	246	$ary_ref = $obj->a; # reference to whole array
	247	$ary_element_value = $obj->a(2); # array element value
	248	$obj->a(2, 'new value'); # assign to array element
	249
	250	# hash type accessor:
	251	$hash_ref = $obj->h; # reference to whole hash
	252	$hash_element_value = $obj->h('x'); # hash element value
253	$obj->h('x', 'new value'); # assign to hash element
254
255	# class type accessor:
256	$element_value = $obj->c; # object reference
257	$obj->c->method(...); # call method of object
258	$obj->c(new My_Other_Class); # assign a new object
259
260
261	=head1 DESCRIPTION
262
263	C<Class::Struct> exports a single function, C<struct>.
264	Given a list of element names and types, and optionally
265	a class name, C<struct> creates a Perl 5 class that implements
266	a "struct-like" data structure.
267
268	The new class is given a constructor method, C<new>, for creating
269	struct objects.
270
271	Each element in the struct data has an accessor method, which is
272	used to assign to the element and to fetch its value. The
273	default accessor can be overridden by declaring a C<sub> of the
274	same name in the package. (See Example 2.)
275
276	Each element's type can be scalar, array, hash, or class.
277
278
279	=head2 The C<struct()> function
280
281	The C<struct> function has three forms of parameter-list.
282
283	struct( CLASS_NAME => [ ELEMENT_LIST ]);
284	struct( CLASS_NAME => { ELEMENT_LIST });
285	struct( ELEMENT_LIST );
286
287	The first and second forms explicitly identify the name of the
288	class being created. The third form assumes the current package
289	name as the class name.
290
291	An object of a class created by the first and third forms is
292	based on an array, whereas an object of a class created by the
293	second form is based on a hash. The array-based forms will be
294	somewhat faster and smaller; the hash-based forms are more
295	flexible.
296
297	The class created by C<struct> must not be a subclass of another
298	class other than C<UNIVERSAL>.
299
300	A function named C<new> must not be explicitly defined in a class
301	created by C<struct>.
302
303	The I<ELEMENT_LIST> has the form
304
305	NAME => TYPE, ...
306
307	Each name-type pair declares one element of the struct. Each
308	element name will be defined as an accessor method unless a
309	method by that name is explicitly defined; in the latter case, a
103ff8e3	310	warning is issued if the warning flag (B<-w>) is set.
8cc95fdb	311
	312
	313	=head2 Element Types and Accessor Methods
	314
	315	The four element types -- scalar, array, hash, and class -- are
	316	represented by strings -- C<'$'>, C<'@'>, C<'%'>, and a class name --
	317	optionally preceded by a C<'*'>.
	318
	319	The accessor method provided by C<struct> for an element depends
	320	on the declared type of the element.
	321
	322	=over
	323
	324	=item Scalar (C<'$'> or C<'*$'>)
	325
	326	The element is a scalar, and is initialized to C<undef>.
	327
	328	The accessor's argument, if any, is assigned to the element.
	329
	330	If the element type is C<'$'>, the value of the element (after
	331	assignment) is returned. If the element type is C<'*$'>, a reference
	332	to the element is returned.
	333
	334	=item Array (C<'@'> or C<'*@'>)
	335
	336	The element is an array, initialized to C<()>.
	337
	338	With no argument, the accessor returns a reference to the
	339	element's whole array.
	340
	341	With one or two arguments, the first argument is an index
	342	specifying one element of the array; the second argument, if
	343	present, is assigned to the array element. If the element type
	344	is C<'@'>, the accessor returns the array element value. If the
	345	element type is C<'*@'>, a reference to the array element is
	346	returned.
	347
	348	=item Hash (C<'%'> or C<'*%'>)
	349
	350	The element is a hash, initialized to C<()>.
	351
	352	With no argument, the accessor returns a reference to the
	353	element's whole hash.
	354
	355	With one or two arguments, the first argument is a key specifying
	356	one element of the hash; the second argument, if present, is
	357	assigned to the hash element. If the element type is C<'%'>, the
	358	accessor returns the hash element value. If the element type is
	359	C<'*%'>, a reference to the hash element is returned.
	360
	361	=item Class (C<'Class_Name'> or C<'*Class_Name'>)
	362
	363	The element's value must be a reference blessed to the named
	364	class or to one of its subclasses. The element is initialized to
	365	the result of calling the C<new> constructor of the named class.
	366
	367	The accessor's argument, if any, is assigned to the element. The
	368	accessor will C<croak> if this is not an appropriate object
	369	reference.
	370
	371	If the element type does not start with a C<'*'>, the accessor
	372	returns the element value (after assignment). If the element type
	373	starts with a C<'*'>, a reference to the element itself is returned.
	374
375	=back
376
377	=head1 EXAMPLES
378
379	=over
380
381	=item Example 1
382
383	Giving a struct element a class type that is also a struct is how
384	structs are nested. Here, C<timeval> represents a time (seconds and
385	microseconds), and C<rusage> has two elements, each of which is of
386	type C<timeval>.
387
388	use Class::Struct;
389
390	struct( rusage => {
391	ru_utime => timeval, # seconds
392	ru_stime => timeval, # microseconds
393	});
394
395	struct( timeval => [
396	tv_secs => '$',
397	tv_usecs => '$',
398	]);
399
400	# create an object:
401	my $t = new rusage;
402	# $t->ru_utime and $t->ru_stime are objects of type timeval.
403
404	# set $t->ru_utime to 100.0 sec and $t->ru_stime to 5.0 sec.
405	$t->ru_utime->tv_secs(100);
406	$t->ru_utime->tv_usecs(0);
407	$t->ru_stime->tv_secs(5);
408	$t->ru_stime->tv_usecs(0);
409
410
411	=item Example 2
412
413	An accessor function can be redefined in order to provide
414	additional checking of values, etc. Here, we want the C<count>
415	element always to be nonnegative, so we redefine the C<count>
416	accessor accordingly.
417
418	package MyObj;
419	use Class::Struct;
420
421	# declare the struct
422	struct ( 'MyObj', { count => '$', stuff => '%' } );
423
424	# override the default accessor method for 'count'
425	sub count {
426	my $self = shift;
427	if ( @_ ) {
428	die 'count must be nonnegative' if $_[0] < 0;
429	$self->{'count'} = shift;
430	warn "Too many args to count" if @_;
431	}
432	return $self->{'count'};
433	}
434
435	package main;
436	$x = new MyObj;
437	print "\$x->count(5) = ", $x->count(5), "\n";
438	# prints '$x->count(5) = 5'
439
440	print "\$x->count = ", $x->count, "\n";
441	# prints '$x->count = 5'
442
443	print "\$x->count(-5) = ", $x->count(-5), "\n";
444	# dies due to negative argument!
445
446
447	=head1 Author and Modification History
448
449
450	Renamed to C<Class::Struct> and modified by Jim Miner, 1997-04-02.
451
452	members() function removed.
453	Documentation corrected and extended.
454	Use of struct() in a subclass prohibited.
455	User definition of accessor allowed.
456	Treatment of '*' in element types corrected.
457	Treatment of classes as element types corrected.
458	Class name to struct() made optional.
459	Diagnostic checks added.
460
461
462	Originally C<Class::Template> by Dean Roehrich.
463
464	# Template.pm --- struct/member template builder
465	# 12mar95
466	# Dean Roehrich
467	#
468	# changes/bugs fixed since 28nov94 version:
469	# - podified
470	# changes/bugs fixed since 21nov94 version:
471	# - Fixed examples.
472	# changes/bugs fixed since 02sep94 version:
473	# - Moved to Class::Template.
474	# changes/bugs fixed since 20feb94 version:
475	# - Updated to be a more proper module.
476	# - Added "use strict".
477	# - Bug in build_methods, was using @var when @$var needed.
478	# - Now using my() rather than local().
479	#
480	# Uses perl5 classes to create nested data types.
481	# This is offered as one implementation of Tom Christiansen's "structs.pl"
482	# idea.
483
484	=cut