Provided by: libclass-methodmaker-perl_2.24-2build6_amd64 
NAME
Class::MethodMaker - Create generic methods for OO Perl
SYNOPSIS
use Class::MethodMaker
[ scalar => [qw/ foo bar baz /],
new => [qw/ new /] ,
];
DESCRIPTION
This module solves the problem of having to continually write accessor methods for your objects that
perform standard tasks.
The argument to 'use' is an arrayref, as pairs whose "keys" are the names of types of generic methods
generated by MethodMaker and whose "values" tell method maker what methods to make.
To override any generated methods, it is sufficient to ensure that the overriding method is defined when
Class::MethodMaker is called. Note that the "use" keyword introduces a "BEGIN" block, so you may need to
define (or at least declare) your overriding method in a "BEGIN" block.
Simple Use
A simple class made with "Class::MethodMaker" looks like this:
package MyClass;
use Class::MethodMaker
[ scalar => [qw/ name /],
new => [qw/ new /],
];
This creates a class, of which new instances may be created using "new", each with a single scalar
component called "name". Name may be queried and (re)set using the methods "name", "name_reset" and
"name_isset":
package main;
my $m = MyClass->new;
my $n;
$\ = "\n";
print $m->name_isset ? "true" : "false"; # false
$m->name("foo");
$n = $m->name;
print defined $n ? "->$n<-" : "*undef*"; # ->foo<-
print $m->name_isset ? "true" : "false"; # true
$m->name(undef);
$n = $m->name;
print defined $n ? "->$n<-" : "*undef*"; # *undef*
print $m->name_isset ? "true" : "false"; # true
$m->name_reset;
$n = $m->name;
print defined $n ? "->$n<-" : "*undef*"; # *undef*
print $m->name_isset ? "true" : "false"; # false
The available component types are scalar, array, hash. Certain non-data-type utilities are also
provided: new, for constructors, deep_copy and copy for object copies, and abstract for creating abstract
methods.
Each of the components take common options. These include -static, for per-class rather than per-
instance data, -type, to restrict the data stored to certain types (e.g., objects of a certain class),
-forward to forward (proxy) given methods onto components, -default/-default_ctor to set default values
for components, -tie_class to tie the storage of a data type to a given class, -read_cb/-store_cb to call
user-defined functions on read/store (without the overhead/complexity of ties; and allowing callbacks on
existing tie classes).
Detailed Use
"Class::MethodMaker" installs components into a class, by means of methods that interrogate and amend
those components. A component, sometimes referred in other documentation as a slot is a group of one or
more attributes (variables) that are associated with an instance of a class (sometimes called an object),
or occasionally a class itself (often referred to as a static component). A component is intended as a
cohesive unit of data that should only normally be interrogated or set through the methods provided.
Given an instance of a class where each instance represents a car, examples of components are the "make"
and "model" (each of which would be a simple scalar, a string), the engine (a simple scalar, an instance
of Engine::Combustion), and the wheels (an array of instances of Wheel). Note that the wheels form one
component, an array. Of course, the implementor might instead choose to use four components, each being
a scalar wheel.
To have the components created, the principle use of Class::MethodMaker is to specify the type (data-
structure) and name of each component to the import method of Class::MethodMaker
package MyClass;
use Class::MethodMaker
[ scalar => 'name',
new => [qw/ new /],
];
In this example, the import is called implicitly via the "use" statement. The components are installed
in the package in effect where the import is called. The argument to import is arranged as pairs, where
the first of each pair is the type of the data-structure, the second is the arguments for that data-
structure; in the most simple case, the name of a component to install using that data-structure. The
second of the pair should be an arrayref if not a simple name.
Data-structures may be repeated in the call:
use Class::MethodMaker
[ scalar => 'name1',
new => [qw/ new /],
scalar => 'name2',
];
It is an error to attempt to install a two or more components with the same name twice.
Options may be given to data structures to amend the nature and behaviour of the components created.
Some options are common across all data structure (e.g., "static") whilst some are specific to their
respective data structures. Option syntax is laid out in detail below. In simple, options are provided
by way of hashrefs from option name to option value. Options and component names are order-sensitive;
options appearing after a component do not affect that component. Options only apply to the data-
structure to which they are specified. Boolean options (e.g., static) may be abbreviated to -option to
set, !option to unset, without a hashref.
use Class::MethodMaker
[ scalar => [+{ -type => 'File::stat' }, qw/ -static name /],
new => 'new',
];
There are also non-data-structure methods that may be created by Class::MethodMaker. "new" is an example
of one such value; it instead causes a standard "new" method to be created for the calling class. The
arguments and options syntax remains the same, but many options clearly do not apply (e.g., "type" for
"new").
Interaction with Superclasses
Basically, "Class::MethodMaker" takes no notice of class hierarchies. If you choose to install a
component x in a class B that is a subclass of class A that already has a component x, then the methods
addressing x in B will simply override those in class A in the usual fashion. "Class::MethodMaker" takes
no special action for this situation. This is a feature.
Option Syntax
The arguments to Class::MethodMaker are passed in a single arrayref, as pairs, with the first of each
pair being the name of the data-structure, and the second being the arguments to that structure.
use Class::MethodMaker
[ scalar => 'name',
new => [qw/ new /],
];
The second of the pair may in the most simple case be a single scalar that is the name of a component to
use.
use Class::MethodMaker
[ scalar => 'bob', ];
For anything more complex, the second argument must itself be an arrayreference. Simple names within
this arrayreference are again taken as component names to use; in the following example, both "foo" and
"bar" scalar components are created:
use Class::MethodMaker
[ scalar => [qw/ foo bar /], ];
Options to the data-structure, to change the behaviour of the component, or methods available, etc., are
specified by the presence of a hash reference in line with the component names. Each key of the hashref
is the name of an option; the corresponding value is the option value. Option names are easily
recognized by a leading hyphen ("-") (or leading exclamation mark, "!"). The options affect only the
components named after the option itself. In the following example, "foo" is non-static (the default),
whilst bar is a static:
use Class::MethodMaker
[ scalar => ['foo', { -static => 1 }, 'bar'], ];
Naturally, options may be altered by later settings overriding earlier ones. The example below has
exactly the same effect as the one above:
use Class::MethodMaker
[ scalar => [{ -static => 1 }, 'bar', { -static => 0 }, 'foo'], ];
Options that are boolean (on/off) valued, such as "-static", may be specified external to any hashref as
"-optionname" to set them on and "!optionname" to set them off. The example below has exactly the same
effect as the one above:
use Class::MethodMaker
[ scalar => [ qw/ -static bar !static foo /], ];
Options that take a value, e.g., "-type", must be specified within a hashref:
use Class::MethodMaker
[ scalar => [ +{ type => 'File::stat' }, 'bob' ], ];
Options affect is limited by the scope of the nearest enclosing arrayref. This particularly means that
for multiple invocations of a data structure type, options on earlier invocations do not affect later
ones. In the following example, "foo" is non-static (the default), whilst bar is a static:
use Class::MethodMaker
[ scalar => [ qw/ -static bar /],
scalar => [ 'foo' ],
];
This is true even if later invocations do not use an arrayref. The example below has exactly the same
effect as the one above:
use Class::MethodMaker
[ scalar => [ qw/ -static bar /],
scalar => 'foo',
];
Arrayrefs may be employed within a set of arguments for a single data-structure to likewise limit scope.
The example below has exactly the same effect as the one above:
use Class::MethodMaker
[ scalar => [ [ qw/ -static bar / ], 'foo' ],
];
Method Renaming
Methods may be renamed, by providing options that map from one generic name to another. These are
identified by the presence of a '*' in the option name.
The example below installs component "a" as a scalar, but the method that would normally be installed as
"a_get" is instead installed as "get_a", and likewise "set_a" is installed in place of "a_set".
use Class::MethodMaker
[ scalar => [ { '*_get' => 'get_*',
'*_set' => 'set_*', },
'a' ],
];
Default & Optional Methods
Class::MethodMaker installs a number of methods by default. Some methods, considered to be useful only
to a subset of developers are installed only on request. Each method is marked in the text to state
whether it is installed by default or only upon request.
To request that a non-default method is installed, one needs to rename it (even possibly to its normal
name). So, to install the *_get method for a scalar attribute (as *_get), the syntax is:
package MyClass;
use Class::MethodMaker
[ scalar => [{'*_get' => '*_get'}, 'a'] ];
The method may be installed using a non-default name using similar syntax:
package MyClass;
use Class::MethodMaker
[ scalar => [{'*_get' => 'get_*'}, 'a'] ];
The client may choose to not install a default method by renaming it to undef:
use Class::MethodMaker
[ scalar => [{'*' => undef }, 'a'] ];
Note Class::MethodMaker will not install a method in place of an existing method, so if the intent is to
not install a default method because the client has their own version, an alternative to the above is to
define the client version before calling Class::MethodMaker.
Naming & Method-Design Conventions
The standard method names are designed with predictability and class extendibility in mind.
Naming
For any component x that Class::MethodMaker creates, the method names are always "x" or "x_*". This
enables predictability, for you do not need to remember which methods are named "x_*" and which *_x, and
also you can name methods that you create by avoiding prefixing them with "x", and so avoid any clash
with Class::MethodMaker-generated methods (even if Class::MethodMaker is upgraded with shiny new extra
methods). Class::MethodMaker users may rename methods (see "Method Renaming").
For any data-structure component (scalar, array, hash, etc.) x that Class::MethodMaker creates, the
method "x" sets the value of that component: i.e., overriding any existing value, not amending or
modifying. E.g., for array components, "x" does not push or pull values but all old values are removed,
and new ones placed in their stead:
package MyClass;
use Class::MethodMaker
[ array => 'a',
new => 'new',
];
package main;
my $m = MyClass->new;
$m->a(4,5);
print join(' ', $m->a), "\n"; # 4 5
$m->a(6,7);
print join(' ', $m->a), "\n"; # 6 7
The method returns the new value of the component:
print join(' ', $m->a(8,9)), "\n"; # 8 9
Note that calling the method with an empty list does not reset the value to empty; this is so that normal
lookups work on the method (i.e., if
$m->a
emptied the component, then
@a = $m->a
would always give an empty list: not that useful.
Set/Unset
Each data-structure component has the concept of being set/unset as a whole, independent of individual
members being set. Each component starts life unset (unless a default or default option or tie class has
been supplied), and is becomes set by any assignment. The component is then reset with the *_reset
method. Thus it is possible to distinguish between a component that has been set to an explicitly empty
value, and one that has not been set (or been reset). This distinction is analogous to the distinction
in hashes between a missing key and a key whose value is undef.
package MyClass;
use Class::MethodMaker
[ new => 'new',
scalar => 'x',
];
package main;
my $m = MyClass->new;
$\ = "\n";
print $m->x_isset ? "true" : "false"; # false; components start this way
my $x = $m->x;
print defined $n ? "->$n<-" : '*undef*'; # *undef*
print $m->x_isset ? "true" : "false"; # false; reading doesn't set
$m->x(undef);
$x = $m->x;
print $m->x_isset ? "true" : "false"; # true;
print defined $n ? "->$n<-" : '*undef*'; # ->foo<-
$m->x("foo");
$x = $m->x;
print $m->x_isset ? "true" : "false"; # true; undef is valid value
print defined $n ? "->$n<-" : '*undef*'; # *undef*
$m->x_reset;
$x = $m->x;
print defined $n ? "->$n<-" : '*undef*'; # *undef*
print $m->x_isset ? "true" : "false"; # false
It is not an error to query the value of an unset component: the value is undef. Querying (any passive
command, or pure function) an unset component does not cause it to become set; only assigning (any active
command, or procedure) changes the set status of a component.
NOTE THAT lvalues are still experimental (as of perl 5.8.0), and so their implementation may change r
disappear in the future. Note that lvalue use defeats type-checking. This may be considered a bug, and
so may be fixed if possible at some point in the future.
Other Design Considerations
Further design goals for Class::MethodMaker version 2:
Consistency of Options
The options passed to components are now handled in a single place, to try to be phrased
consistently. As many options as possible are common to all data-structures.
Flexibility
It is intended that all common class-construction options are supported across all data-types, so
that e.g., defaults, ties, typing may be used with your data-structure of choice, and combined.
Speed
The methods are intended to be as fast as possible, within other constraints outlined here.
Options to "use"/"import"
"-target_class"
By default, the target class is determined to be the last (latest) class in the call stack that is
not a Class::MethodMaker::Engine subtype. This is what is wanted 99% of the time, and typical users
need not worry. However, the target class may be set explicitly in the call to "use"/"import":
use Class::MethodMaker
[ -target_class => 'X',
scalar => [qw/ a /],
-target_class => 'Y',
scalar => [qw/ b /],
];
Note that the "-target_class" option is order sensitive: it affects only components requested after
it in the call to "use"/"import". As shown, the same call may handle specify multiple target
classes. Any components requested before the first "-target_class" are created in the default-
determined class, as outlined above.
Setting the target class in this way does not persist over multiple calls to "use"/"import". A
subsequent call to either will use the default-determined class as target (unless again overridden by
"-target_class").
Standard Options for Data-Structure Components.
The following options are observed by all data structure components (scalar, array, hash).
-static
package MyClass;
use Class::MethodMaker
[ scalar => [qw/ -static s /], ];
This option causes components to hold class-specific, rather than instance-specific values. Thus:
package main;
my $m = MyClass->new;
my $n = MyClass->new;
$m->a(4,5);
print join(' ', $m->a), "\n"; # 4 5
print join(' ', $n->a), "\n"; # 4 5
$n->a(6,7);
print join(' ', $n->a), "\n"; # 6 7
print join(' ', $m->a), "\n"; # 6 7
-type
use Class::MethodMaker
[ scalar => [{ -type => 'File::stat' }, 'st' ]];
Takes the name of a class, and checks that all values assigned to the component are of the
appropriate type (uses UNIVERSAL::isa, so subtypes are permissible).
-forward
This option takes as value an arrayref (or a simple scalar). The values specify a list of methods
that when called on an instance of the target class, are "forwarded on" to the given component. For
example,
package X;
use Class::MethodMaker
[scalar => [{ -type => 'File::stat',
-forward => [qw/ mode size /], },
'st1',
],
])},
any call of "mode" or "size" on an instance of "X" will simply call the method of the same name on
the value stored in the component "st1", with the same arguments, and returns the value(s) of this
call.
Forwarding only applies to the first named component (since the methodname is fixed, without the a
componentname part). This is because the components are installed in the order in which they are
created, and Class::MethodMaker never overwrites a pre-existing method. So, in the following
example, "mode" and "size" forward to the "st1" component, and "read" forwards to the "st2"
component.
package MyClass;
Class::MethodMaker->import([scalar =>
[{ -type => 'File::stat',
-forward => [qw/ mode
size /],
},
qw( st1 ),
{ -type => 'IO::Handle',
-forward => 'read', },
qw( st2 ),
]])},
Forwarding a method to a component of composite data type (e.g., array, hash) causes the method to be
mapped over the values of that component. The returned value is appropriate to the component type;
so a method forwarded to an array will return a list, like the array that is the component, but with
each value being the instead result of applying the forwarded method to the corresponding value of
the array.
The following code populates the @sizes array with the sizes of /etc/passwd, /etc/group, in that
order.
package main;
my $m = MyClass->new;
$m->st1("/etc/passwd", "/etc/group");
my @sizes = $m->size;
Calling the forwarding method in a scalar context will get the same results, but as an arrayref:
my $sizes = $m->size; # [ 921, 598 ] for example
Likewise, forwarding to a hash component will return a hash from original key to result of method on
the corresponding component, or an equivalent hashref in scalar context.
-default
use Class::MethodMaker
[ scalar => [{ -default => 7 }, 'df1' ]];
Takes a simple value; must be either undef or an instance of the appropriate type if "-type" has also
been specified. Whenever a component is new or reset, its value(s) default to the value given.
Hence *_isset will always return true for that component. For compound data-structures, the default
applies to the each element of the structure, not the compound itself. So, for array structures, the
default applies to each element of the array, not the array itself.
It is an error to specify the "-default" option and the "-default_ctor" option simultaneously.
-default_ctor
use Class::MethodMaker
[scalar => [{ -default_ctor => sub {
Y->new(-3);
},
'df2',
{ -type => 'Y',
-default_ctor => 'new' },
'df3',
]
];
Takes a coderef to call to generate the default value. This is called the first time a value is
required, and afterwards whenever reset is called. The subr is called with one argument, which is
the object upon which the component exists (or the name of the class upon which the component is
created, if the call is made on the class).
If the "-type" option is in effect, then the value may be a simple value, which shall be considered
the name of a method to call on the class specified by "-type".
It is an error to specify the "-default" option and the "-default_ctor" option simultaneously.
-tie_class
# @z is an audit trail
my @z;
package W;
use Tie::Scalar;
use base qw( Tie::StdScalar );
sub TIESCALAR { push @z, [ 'TIESCALAR' ]; $_[0]->SUPER::TIESCALAR }
sub FETCH { push @z, [ 'FETCH' ]; $_[0]->SUPER::FETCH }
sub STORE { push @z, [ STORE => $_[1] ]; $_[0]->SUPER::STORE($_[1]) }
sub DESTROY { push @z, [ 'DESTROY' ]; $_[0]->SUPER::DESTROY }
sub UNTIE { push @z, [ UNTIE => $_[1] ]; $_[0]->SUPER::UNTIE($_[1]) }
package X;
Class::MethodMaker->import([scalar =>
[{ -type => 'File::stat',
-tie_class => 'W',
-forward => [qw/ mode
size /],
},
qw( tie1 ),
new => 'new',
]]);
This option takes a simple value as argument, which is taken be the name of a class that is to be
tied to the storage for the component, e.g., for an array component, a class that implements the API
for tied arrays is needed (see Tie::Array for more information on this). Likewise for scalar
components, hash components, etc. Note that it is the component that is tied, not the data items.
package main;
my $x = X->new;
# @z is empty
my $stat1 = stat "/etc/passwd";
my $stat2 = stat "/etc/group";
$x->tie1($stat1);
# @z is (['TIESCALAR'], ['STORE', $stat1])
my $y = $x->tie1;
# $y is $stat1
# @z is (['TIESCALAR'], ['STORE', $stat1], ['FETCH'])
$x->tie1($stat2);
# @z is (['TIESCALAR'], ['STORE', $stat1], ['FETCH'], ['STORE', $stat2])
$x->tie1_reset;
# @z is (['TIESCALAR'], ['STORE', $stat1], ['FETCH'], ['STORE', $stat2],\
# ['DESTROY'])
-tie_args
package X;
Class::MethodMaker->import
([scalar => [{ -tie_class => 'V',
-tie_args => [enum => [qw/A B C/],
default => 'B'],
},
qw( tie2 ),
]]);
This option takes an array reference, whose members are passed as arguments to any tie invoked on the
component (by virtue "-tie_class"). If "-tie_class" is not in force, this is ignored.
As a convenience measure, a single argument may be passed directly, rather than embedding in an array
ref --- unless that arg is an array ref itself...
-read_cb
The implementation of this option is incomplete
package MyClass;
use Class::MethodMaker
[ scalar => [{ -read_cb => sub { ($_[1]||0) + 1 } }, 'rcb1' ]
new => 'new';
];
This option takes as argument a coderef, which is called whenever a value is read. It is called with
two arguments: the instance upon which the method was called, and the value stored in the component.
The return value of the given coderef is the value which is passed to the caller of the method as the
component value. Thus, the above example adds one to whatever the stored value is. Note that the
value is returned to the callee, but not stored in the object
package main;
my $m = MyClass->new;
$m->rcb1(4);
my $n = $x->rcb1; # 5
my $n = $x->rcb1; # 5
-store_cb
The implementation of this option is incomplete
package MyClass;
use Class::MethodMaker
[ scalar => [{ -store_cb => sub { $_[1] + 1 } }, 'scb1' ]
new => 'new';
];
This option takes as argument a coderef, which is called whenever a value is stored. It is called
with four arguments: the instance upon which the method was called, the value to store in the
component, the name of the component, and the previous value of the component (if any; if the given
element of the component was previously unset, only three arguments are passed).
The return value of the given coderef is the value which is actually stored in the component. Thus,
the above example stores 1 greater than the value passed in.
package main;
my $m = MyClass->new;
$m->scb1(4);
my $n = $x->scb1; # 5
Generally, store callbacks are cheaper than read callbacks, because values are read more often than
they are stored. But that is a generalization. YMMV.
EXPERIMENTAL & COMPATIBILITY notes
Some new facilities may be marked as EXPERIMENTAL in the documentation. These facilities are being
trialled, and whilst it is hoped that they will become mainstream code, no promises are made. They may
change or disappear at any time. Caveat Emptor. The maintainer would be delighted to hear any feedback
particularly regarding such facilities, be it good or bad, so long as it is constructive.
Some old facilities may be marked as COMPATIBILITY in the documentation. These facilities are being
maintained purely for compatibility with old versions of this module, but will ultimately disappear.
They are normally replaced by alternatives that are considered preferable. Please avoid using them, and
consider amending any existing code that does use them not to. If you believe that their removal will
cast an unacceptable pall over your life, please contact the maintainer.
SEE ALSO
Class::MethodMaker::Engine, Class::MethodMaker::scalar, Class::MethodMaker::array,
Class::MethodMaker::hash, Class::MethodMaker::V1Compat
perl v5.38.2 2024-03-31 Class::MethodMaker(3pm)