<s>
In	O
object-oriented	B-Language
programming	I-Language
,	O
a	O
metaclass	B-Language
is	O
a	O
class	O
whose	O
instances	O
are	O
classes	O
.	O
</s>
<s>
Just	O
as	O
an	O
ordinary	O
class	O
defines	O
the	O
behavior	O
of	O
certain	O
objects	O
,	O
a	O
metaclass	B-Language
defines	O
the	O
behavior	O
of	O
certain	O
classes	O
and	O
their	O
instances	O
.	O
</s>
<s>
Not	O
all	O
object-oriented	B-Language
programming	I-Language
languages	I-Language
support	O
metaclasses	B-Language
.	O
</s>
<s>
Among	O
those	O
that	O
do	O
,	O
the	O
extent	O
to	O
which	O
metaclasses	B-Language
can	O
override	O
any	O
given	O
aspect	O
of	O
class	O
behavior	O
varies	O
.	O
</s>
<s>
Metaclasses	B-Language
can	O
be	O
implemented	O
by	O
having	O
classes	O
be	O
first-class	O
citizens	O
,	O
in	O
which	O
case	O
a	O
metaclass	B-Language
is	O
simply	O
an	O
object	O
that	O
constructs	O
classes	O
.	O
</s>
<s>
Each	O
language	O
has	O
its	O
own	O
metaobject	O
protocol	O
,	O
a	O
set	O
of	O
rules	O
that	O
govern	O
how	O
objects	O
,	O
classes	O
,	O
and	O
metaclasses	B-Language
interact	O
.	O
</s>
<s>
In	O
Python	B-Language
,	O
the	O
builtin	O
class	O
type	O
is	O
a	O
metaclass	B-Language
.	O
</s>
<s>
Consider	O
this	O
simple	O
Python	B-Language
class	O
:	O
</s>
<s>
In	O
languages	O
without	O
metaclasses	B-Language
,	O
these	O
details	O
are	O
defined	O
by	O
the	O
language	O
specification	O
and	O
ca	O
n't	O
be	O
overridden	O
.	O
</s>
<s>
In	O
Python	B-Language
,	O
the	O
metaclass	B-Language
-	O
type	O
-	O
controls	O
these	O
details	O
of	O
Car	O
's	O
behavior	O
.	O
</s>
<s>
They	O
can	O
be	O
overridden	O
by	O
using	O
a	O
different	O
metaclass	B-Language
instead	O
of	O
type	O
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
eliminate	O
some	O
of	O
this	O
redundancy	O
using	O
a	O
metaclass	B-Language
.	O
</s>
<s>
In	O
Python	B-Language
,	O
a	O
metaclass	B-Language
is	O
most	O
easily	O
defined	O
as	O
a	O
subclass	O
of	O
type	O
.	O
</s>
<s>
This	O
metaclass	B-Language
only	O
overrides	O
object	O
creation	O
.	O
</s>
<s>
Now	O
the	O
class	O
Car	O
can	O
be	O
rewritten	O
to	O
use	O
this	O
metaclass	B-Language
.	O
</s>
<s>
In	O
Python	B-Language
3	O
this	O
is	O
done	O
by	O
providing	O
a	O
"	O
keyword	O
argument	O
"	O
metaclass	B-Language
to	O
the	O
class	O
definition	O
:	O
</s>
<s>
In	O
Smalltalk	B-Application
,	O
everything	O
is	O
an	O
object	O
.	O
</s>
<s>
Additionally	O
,	O
Smalltalk	B-Application
is	O
a	O
class	B-Application
based	I-Application
system	O
,	O
which	O
means	O
that	O
every	O
object	O
has	O
a	O
class	O
that	O
defines	O
the	O
structure	O
of	O
that	O
object	O
(	O
i.e.	O
</s>
<s>
Together	O
this	O
implies	O
that	O
a	O
class	O
in	O
Smalltalk	B-Application
is	O
an	O
object	O
and	O
that	O
,	O
therefore	O
a	O
class	O
needs	O
to	O
be	O
an	O
instance	O
of	O
a	O
class	O
(	O
called	O
metaclass	B-Language
)	O
.	O
</s>
<s>
As	O
an	O
example	O
,	O
a	O
car	O
object	B-Language
c	I-Language
is	O
an	O
instance	O
of	O
the	O
class	O
Car	O
.	O
</s>
<s>
In	O
turn	O
,	O
the	O
class	O
Car	O
is	O
again	O
an	O
object	O
and	O
as	O
such	O
an	O
instance	O
of	O
the	O
metaclass	B-Language
of	O
Car	O
called	O
Car	O
class	O
.	O
</s>
<s>
Note	O
the	O
blank	O
in	O
the	O
name	O
of	O
the	O
metaclass	B-Language
.	O
</s>
<s>
The	O
name	O
of	O
the	O
metaclass	B-Language
is	O
the	O
Smalltalk	B-Application
expression	O
that	O
,	O
when	O
evaluated	O
,	O
results	O
in	O
the	O
metaclass	B-Language
object	O
.	O
</s>
<s>
Thus	O
evaluating	O
Car	O
class	O
results	O
in	O
the	O
metaclass	B-Language
object	O
for	O
Car	O
whose	O
name	O
is	O
Car	O
class	O
(	O
one	O
can	O
confirm	O
this	O
by	O
evaluating	O
Car	O
class	O
name	O
which	O
returns	O
the	O
name	O
of	O
the	O
metaclass	B-Language
of	O
Car	O
.	O
)	O
</s>
<s>
Class	B-Application
methods	I-Application
actually	O
belong	O
to	O
the	O
metaclass	B-Language
,	O
just	O
as	O
instance	B-Language
methods	I-Language
actually	O
belong	O
to	O
the	O
class	O
.	O
</s>
<s>
Note	O
that	O
,	O
so	O
far	O
,	O
the	O
metaclass	B-Language
inheritance	B-Language
chain	O
exactly	O
follows	O
that	O
of	O
the	O
class	B-Language
inheritance	I-Language
chain	O
.	O
</s>
<s>
But	O
the	O
metaclass	B-Language
chain	O
extends	O
further	O
because	O
Object	O
class	O
is	O
the	O
subclass	O
of	O
Class	O
.	O
</s>
<s>
All	O
metaclasses	B-Language
are	O
subclasses	O
of	O
Class	O
.	O
</s>
<s>
In	O
early	O
Smalltalks	B-Application
,	O
there	O
was	O
only	O
one	O
metaclass	B-Language
called	O
Class	O
.	O
</s>
<s>
This	O
implied	O
that	O
the	O
methods	B-Language
all	O
classes	O
have	O
were	O
the	O
same	O
,	O
in	O
particular	O
the	O
method	O
to	O
create	O
new	O
objects	O
,	O
i.e.	O
,	O
new	O
.	O
</s>
<s>
To	O
allow	O
classes	O
to	O
have	O
their	O
own	O
methods	B-Language
and	O
their	O
own	O
instance	O
variables	O
(	O
called	O
class	O
instance	O
variables	O
and	O
should	O
not	O
be	O
confused	O
with	O
class	B-Application
variables	I-Application
)	O
,	O
Smalltalk-80	B-Application
introduced	O
for	O
each	O
class	O
C	O
their	O
own	O
metaclass	B-Language
C	O
class	O
.	O
</s>
<s>
This	O
means	O
that	O
each	O
metaclass	B-Language
is	O
effectively	O
a	O
singleton	O
class	O
.	O
</s>
<s>
Since	O
there	O
is	O
no	O
requirement	O
that	O
metaclasses	B-Language
behave	O
differently	O
from	O
each	O
other	O
,	O
all	O
metaclasses	B-Language
are	O
instances	O
of	O
only	O
one	O
class	O
called	O
Metaclass	B-Language
.	O
</s>
<s>
The	O
metaclass	B-Language
of	O
Metaclass	B-Language
is	O
called	O
Metaclass	B-Language
class	O
which	O
again	O
is	O
an	O
instance	O
of	O
class	O
Metaclass	B-Language
.	O
</s>
<s>
In	O
Smalltalk-80	B-Application
,	O
every	O
class	O
(	O
except	O
Object	O
)	O
has	O
a	O
superclass	O
.	O
</s>
<s>
The	O
abstract	O
superclass	O
of	O
all	O
metaclasses	B-Language
is	O
Class	O
,	O
which	O
describes	O
the	O
general	O
nature	O
of	O
classes	O
.	O
</s>
<s>
The	O
superclass	O
hierarchy	O
for	O
metaclasses	B-Language
parallels	O
that	O
for	O
classes	O
,	O
except	O
for	O
class	O
Object	O
.	O
</s>
<s>
ALL	O
metaclasses	B-Language
are	O
subclasses	O
of	O
Class	O
,	O
therefore	O
:	O
</s>
<s>
Like	O
conjoined	O
twins	O
,	O
classes	O
and	O
metaclasses	B-Language
are	O
born	O
together	O
.	O
</s>
<s>
Metaclass	B-Language
has	O
an	O
instance	O
variable	O
thisClass	O
,	O
which	O
points	O
to	O
its	O
conjoined	O
class	O
.	O
</s>
<s>
Note	O
that	O
the	O
usual	O
Smalltalk	B-Application
class	B-Application
browser	I-Application
does	O
not	O
show	O
metaclasses	B-Language
as	O
separate	O
classes	O
.	O
</s>
<s>
Instead	O
the	O
class	B-Application
browser	I-Application
allows	O
to	O
edit	O
the	O
class	O
together	O
with	O
its	O
metaclass	B-Language
at	O
the	O
same	O
time	O
.	O
</s>
<s>
The	O
names	O
of	O
classes	O
in	O
the	O
metaclass	B-Language
hierarchy	O
are	O
easily	O
confused	O
with	O
the	O
concepts	O
of	O
the	O
same	O
name	O
.	O
</s>
<s>
Object	O
is	O
the	O
base	O
class	O
that	O
provides	O
common	O
methods	B-Language
for	O
all	O
objects	O
;	O
"	O
an	O
object	O
"	O
is	O
an	O
integer	O
,	O
or	O
a	O
widget	O
,	O
or	O
a	O
Car	O
,	O
etc	O
.	O
</s>
<s>
Class	O
is	O
the	O
base	O
of	O
the	O
metaclasses	B-Language
that	O
provides	O
common	O
methods	B-Language
for	O
all	O
classes	O
(	O
though	O
it	O
is	O
not	O
a	O
metaclass	B-Language
itself	O
)	O
;	O
"	O
a	O
class	O
"	O
is	O
something	O
like	O
Integer	O
,	O
or	O
Widget	O
,	O
or	O
Car	O
,	O
etc	O
.	O
</s>
<s>
Metaclass	B-Language
provides	O
common	O
methods	B-Language
for	O
all	O
metaclasses	B-Language
.	O
</s>
<s>
Their	O
inheritance	B-Language
hierarchy	I-Language
(	O
from	O
Object	O
)	O
,	O
and	O
the	O
main	O
facilities	O
they	O
provide	O
are	O
:	O
</s>
<s>
Ruby	B-Language
purifies	O
the	O
Smalltalk-80	B-Application
concept	O
of	O
metaclasses	B-Language
by	O
introducing	O
eigenclasses	O
,	O
</s>
<s>
removing	O
the	O
Metaclass	B-Language
class	O
,	O
</s>
<s>
Note	O
in	O
particular	O
the	O
correspondence	O
between	O
Smalltalk	B-Application
's	O
implicit	O
metaclasses	B-Language
and	O
Ruby	B-Language
's	O
eigenclasses	O
of	O
classes	O
.	O
</s>
<s>
The	O
Ruby	B-Language
eigenclass	O
model	O
makes	O
the	O
concept	O
of	O
implicit	O
metaclasses	B-Language
fully	O
uniform	O
:	O
every	O
object	O
x	O
has	O
its	O
own	O
meta-object	O
,	O
called	O
the	O
eigenclass	O
of	O
x	O
,	O
which	O
is	O
one	O
meta-level	O
higher	O
than	O
x	O
.	O
</s>
<s>
The	O
"	O
higher	O
order	O
"	O
eigenclasses	O
usually	O
exist	O
purely	O
conceptually	O
–	O
they	O
do	O
not	O
contain	O
any	O
methods	B-Language
or	O
store	O
any	O
(	O
other	O
)	O
data	O
in	O
most	O
Ruby	B-Language
programs	O
.	O
</s>
<s>
The	O
following	O
diagrams	O
show	O
a	O
sample	O
core	O
structure	O
of	O
Smalltalk-80	B-Application
and	O
Ruby	B-Language
in	O
comparison	O
.	O
</s>
<s>
Green	O
links	O
show	O
the	O
child	O
→	O
parent	O
relation	O
of	O
inheritance	B-Language
(	O
with	O
the	O
implicit	O
upward	O
direction	O
)	O
,	O
blue	O
links	O
show	O
the	O
complementary	O
member	O
→	O
container	O
relation	O
of	O
instantiation	O
(	O
a	O
blue	O
link	O
from	O
x	O
points	O
to	O
the	O
least	O
actual	O
container	O
of	O
x	O
that	O
is	O
the	O
start	O
point	O
for	O
the	O
method	O
lookup	O
when	O
a	O
method	O
is	O
invoked	O
on	O
x	O
)	O
.	O
</s>
<s>
implicit	O
metaclasses	B-Language
in	O
the	O
case	O
of	O
Smalltalk-80	B-Application
)	O
.	O
</s>
<s>
The	O
diagram	O
on	O
the	O
right	O
also	O
provides	O
a	O
picture	O
of	O
lazy	O
evaluation	O
of	O
eigenclasses	O
in	O
Ruby	B-Language
.	O
</s>
<s>
The	O
v	O
object	O
can	O
have	O
its	O
eigenclass	O
evaluated	O
(	O
allocated	O
)	O
as	O
a	O
consequence	O
of	O
adding	O
singleton	O
methods	B-Language
to	O
v	O
.	O
</s>
<s>
According	O
to	O
the	O
Ruby	B-Language
's	O
introspection	O
method	O
named	O
class	O
,	O
</s>
<s>
Following	O
the	O
standard	O
definition	O
of	O
metaclasses	B-Language
we	O
can	O
conclude	O
that	O
Class	O
and	O
Struct	O
are	O
the	O
only	O
metaclasses	B-Language
in	O
Ruby	B-Language
.	O
</s>
<s>
This	O
seems	O
to	O
contradict	O
the	O
correspondence	O
between	O
Ruby	B-Language
and	O
Smalltalk	B-Application
,	O
</s>
<s>
since	O
in	O
Smalltalk-80	B-Application
,	O
every	O
class	O
has	O
its	O
own	O
metaclass	B-Language
.	O
</s>
<s>
the	O
class	O
introspection	O
method	O
in	O
Ruby	B-Language
and	O
Smalltalk	B-Application
.	O
</s>
<s>
As	O
already	O
mentioned	O
above	O
,	O
for	O
a	O
class	O
x	O
,	O
the	O
Ruby	B-Language
expression	O
x.class	O
evaluates	O
constantly	O
to	O
Class	O
.	O
</s>
<s>
Metaclasses	B-Language
in	O
Objective-C	B-Language
are	O
almost	O
the	O
same	O
as	O
those	O
in	O
Smalltalk-80	B-Application
—	O
not	O
surprising	O
since	O
Objective-C	B-Language
borrows	O
a	O
lot	O
from	O
Smalltalk	B-Application
.	O
</s>
<s>
Like	O
Smalltalk	B-Application
,	O
in	O
Objective-C	B-Language
,	O
the	O
instance	O
variables	O
and	O
methods	B-Language
are	O
defined	O
by	O
an	O
object	O
's	O
class	O
.	O
</s>
<s>
A	O
class	O
is	O
an	O
object	O
,	O
hence	O
it	O
is	O
an	O
instance	O
of	O
a	O
metaclass	B-Language
.	O
</s>
<s>
Like	O
Smalltalk	B-Application
,	O
in	O
Objective-C	B-Language
,	O
class	B-Application
methods	I-Application
are	O
simply	O
methods	B-Language
called	O
on	O
the	O
class	O
object	O
,	O
hence	O
a	O
class	O
's	O
class	B-Application
methods	I-Application
must	O
be	O
defined	O
as	O
instance	B-Language
methods	I-Language
in	O
its	O
metaclass	B-Language
.	O
</s>
<s>
Because	O
different	O
classes	O
can	O
have	O
different	O
sets	O
of	O
class	B-Application
methods	I-Application
,	O
each	O
class	O
must	O
have	O
its	O
own	O
separate	O
metaclass	B-Language
.	O
</s>
<s>
Classes	O
and	O
metaclasses	B-Language
are	O
always	O
created	O
as	O
a	O
pair	O
:	O
the	O
runtime	O
has	O
functions	O
objc_allocateClassPair( )	O
and	O
objc_registerClassPair( )	O
to	O
create	O
and	O
register	O
class-metaclass	O
pairs	O
,	O
respectively	O
.	O
</s>
<s>
There	O
are	O
no	O
names	O
for	O
the	O
metaclasses	B-Language
;	O
however	O
,	O
a	O
pointer	O
to	O
any	O
class	O
object	O
can	O
be	O
referred	O
to	O
with	O
the	O
generic	O
type	O
Class	O
(	O
similar	O
to	O
the	O
type	O
id	O
being	O
used	O
for	O
a	O
pointer	O
to	O
any	O
object	O
)	O
.	O
</s>
<s>
Because	O
class	B-Application
methods	I-Application
are	O
inherited	O
through	O
inheritance	B-Language
,	O
like	O
Smalltalk	B-Application
,	O
metaclasses	B-Language
must	O
follow	O
an	O
inheritance	B-Language
scheme	O
paralleling	O
that	O
of	O
classes	O
(	O
e.g.	O
</s>
<s>
if	O
class	O
A	O
's	O
parent	O
class	O
is	O
class	O
B	O
,	O
then	O
A	O
's	O
metaclass	B-Language
's	O
parent	O
class	O
is	O
B	O
's	O
metaclass	B-Language
)	O
,	O
except	O
that	O
of	O
the	O
root	O
class	O
.	O
</s>
<s>
Unlike	O
Smalltalk	B-Application
,	O
the	O
metaclass	B-Language
of	O
the	O
root	O
class	O
inherits	O
from	O
the	O
root	O
class	O
(	O
usually	O
NSObject	O
using	O
the	O
Cocoa	B-Operating_System
framework	O
)	O
itself	O
.	O
</s>
<s>
This	O
ensures	O
that	O
all	O
class	O
objects	O
are	O
ultimately	O
instances	O
of	O
the	O
root	O
class	O
,	O
so	O
that	O
you	O
can	O
use	O
the	O
instance	B-Language
methods	I-Language
of	O
the	O
root	O
class	O
,	O
usually	O
useful	O
utility	O
methods	B-Language
for	O
objects	O
,	O
on	O
class	O
objects	O
themselves	O
.	O
</s>
<s>
Since	O
metaclass	B-Language
objects	O
do	O
not	O
behave	O
differently	O
(	O
you	O
cannot	O
add	O
class	B-Application
methods	I-Application
for	O
a	O
metaclass	B-Language
,	O
so	O
metaclass	B-Language
objects	O
all	O
have	O
the	O
same	O
methods	B-Language
)	O
,	O
they	O
are	O
all	O
instances	O
of	O
the	O
same	O
class	O
—	O
the	O
metaclass	B-Language
of	O
the	O
root	O
class	O
(	O
unlike	O
Smalltalk	B-Application
)	O
.	O
</s>
<s>
Thus	O
,	O
the	O
metaclass	B-Language
of	O
the	O
root	O
class	O
is	O
an	O
instance	O
of	O
itself	O
.	O
</s>
<s>
The	O
reason	O
for	O
this	O
is	O
that	O
all	O
metaclasses	B-Language
inherit	O
from	O
root	O
class	O
;	O
hence	O
,	O
they	O
must	O
inherit	O
the	O
class	B-Application
methods	I-Application
of	O
the	O
root	O
class	O
.	O
</s>
<s>
The	O
following	O
are	O
some	O
of	O
the	O
most	O
prominent	O
programming	O
languages	O
that	O
support	O
metaclasses	B-Language
.	O
</s>
<s>
Some	O
less	O
widespread	O
languages	O
that	O
support	O
metaclasses	B-Language
include	O
OpenJava	B-Application
,	O
OpenC++	O
,	O
OpenAda	O
,	O
CorbaScript	B-Language
,	O
ObjVLisp	B-Language
,	O
Object-Z	B-Application
,	O
MODEL-K	O
,	O
XOTcl	B-Language
,	O
and	O
MELDC	O
.	O
</s>
<s>
Logtalk	B-Language
,	O
an	O
object-oriented	B-Language
extension	O
of	O
Prolog	B-Language
,	O
also	O
supports	O
metaclasses	B-Language
.	O
</s>
<s>
Resource	O
Description	O
Framework	O
(	O
RDF	O
)	O
and	O
Unified	B-Language
Modeling	I-Language
Language	I-Language
(	O
UML	B-Language
)	O
both	O
support	O
metaclasses	B-Language
.	O
</s>
