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Like
Smalltalk, in Objective-C, class methods are simply methods called on the class object, hence a class's class methods must be defined as instance methods in its metaclass. Because different classes can have different sets of class methods, each class must have its own separate metaclass. Classes
1768:
Diagram of the inheritance and instance relationships between classes and metaclasses in
Objective-C. Note that Objective-C has multiple root classes; each root class would have a separate hierarchy. This diagram only shows the hierarchy for an example root class NSObject. Each other root class would
1051:
system, which means that every object has a class that defines the structure of that object (i.e. the instance variables the object has) and the messages an object understands. Together this implies that a class in
Smalltalk is an object and that, therefore a class needs to be an instance of a class
1472:
The following diagrams show a sample core structure of
Smalltalk-80 and Ruby in comparison. In both languages, the structure consists of a built-in part which contains the circular objects (i.e. objects that appear in a cycle formed by a combination of blue or green links) and a user-part which has
1773:
Metaclasses in
Objective-C are almost the same as those in Smalltalk-80—not surprising since Objective-C borrows a lot from Smalltalk. Like Smalltalk, in Objective-C, the instance variables and methods are defined by an object's class. A class is an object, hence it is an instance of a metaclass.
1816:
Since metaclass objects do not behave differently (you cannot add class methods for a metaclass, so metaclass objects all have the same methods), they are all instances of the same class—the metaclass of the root class (unlike
Smalltalk). Thus, the metaclass of the root class is an instance of
1800:
Because class methods are inherited through inheritance, like
Smalltalk, metaclasses must follow an inheritance scheme paralleling that of classes (e.g. if class A's parent class is class B, then A's metaclass's parent class is B's metaclass), except that of the root class.
1812:
framework) itself. This ensures that all class objects are ultimately instances of the root class, so that you can use the instance methods of the root class, usually useful utility methods for objects, on class objects themselves.
1588:
are the only metaclasses in Ruby. This seems to contradict the correspondence between Ruby and
Smalltalk, since in Smalltalk-80, every class has its own metaclass. The discrepancy is based on the disagreement between the
1452:
Note in particular the correspondence between
Smalltalk's implicit metaclasses and Ruby's eigenclasses of classes. The Ruby eigenclass model makes the concept of implicit metaclasses fully uniform: every object
1489:. Green links show the child→parent relation of inheritance (with the implicit upward direction), blue links show the complementary member→container relation of instantiation (a blue link from
270:, which are the rules that determine interactions among objects, classes, and metaclasses. Metaclasses are utilized to automate code generation and to enhance framework development.
262:
support the concept of metaclasses. For those that do, the extent of control metaclasses have over class behaviors varies. Metaclasses are often implemented by treating classes as
258:
are classes themselves. Unlike ordinary classes, which define the behaviors of objects, metaclasses specify the behaviors of classes and their instances. Not all object-oriented
1075:. Note the blank in the name of the metaclass. The name of the metaclass is the Smalltalk expression that, when evaluated, results in the metaclass object. Thus evaluating
1898:
2247:
1984:
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is accessed. In languages without metaclasses, these details are defined by the language specification and can't be overridden. In Python, the metaclass -
1124:. Note that, so far, the metaclass inheritance chain exactly follows that of the class inheritance chain. But the metaclass chain extends further because
1469:. The "higher order" eigenclasses usually exist purely conceptually – they do not contain any methods or store any (other) data in most Ruby programs.
1238:
does not show metaclasses as separate classes. Instead the class browser allows to edit the class together with its metaclass at the same time.
1519:
1258:
is the base of the metaclasses that provides common methods for all classes (though it is not a metaclass itself); "a class" is something like
1147:. To allow classes to have their own methods and their own instance variables (called class instance variables and should not be confused with
575:. It is possible to eliminate some of this redundancy using a custom metaclass. In Python, a metaclass is most easily defined as a subclass of
2063:
1817:
itself. The reason for this is that all metaclasses inherit from root class; hence, they must inherit the class methods of the root class.
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Since there is no requirement that metaclasses behave differently from each other, all metaclasses are instances of only one class called
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actually belong to the metaclass, just as instance methods actually belong to the class. When a message is sent to the object
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There are no names for the metaclasses; however, a pointer to any class object can be referred to with the generic type
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The names of classes in the metaclass hierarchy are easily confused with the concepts of the same name. For instance:
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This metaclass only overrides object creation. All other aspects of class and object behavior are still handled by
266:, making a metaclass an object that creates and manages these classes. Each programming language adheres to its own
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coincides on terminal objects, it differs in the restriction to classes. As already mentioned above, for a class
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is the base class that provides common methods for all objects; "an object" is an integer, or a widget, or a
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can be rewritten to use this metaclass. In Python 3 this is done by providing a "keyword argument"
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Diagram of the inheritance and instance relationships between classes and metaclasses in
Smalltalk
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1501:). Gray nodes display the eigenclasses (resp. implicit metaclasses in the case of Smalltalk-80).
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Unlike Smalltalk, the metaclass of the root class inherits from the root class (usually
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class, and (un)redefining the class-of map. The change can be schematized as follows:
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The superclass hierarchy for metaclasses parallels that for classes, except for class
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all classes have were the same, in particular the method to create new objects, i.e.,
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1929:. Several of these languages date from the early 1990s and are of academic interest.
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is disallowed. Following the standard definition of metaclasses we can conclude that
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A metaclass describes commen behaviour for classes, like a class does for objects
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object can have its eigenclass evaluated (allocated) as a consequence of adding
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can be instantiated as usual, but can contain any number of keyword arguments:
44:
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The above example contains some redundant code to do with the four attributes
552:'s behavior. They can be overridden by using a different metaclass instead of
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that is the start point for the method lookup when a method is invoked on
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class, shown above, does not include such details as the size in bytes of
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objects, their binary layout in memory, how they are allocated, that the
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and metaclasses are always created as a pair: the runtime has functions
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is created, and so on. These details come into play not only when a new
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1969:
1681: in this section. Unsourced material may be challenged and removed.
1560:, the class of every class (and of every eigenclass) is constantly the
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1889:(proposed for a possible inclusion in future version of the standard)
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are the only classes that have classes as instances. Subclassing of
1327:
Ruby purifies the Smalltalk-80 concept of metaclasses by introducing
1296:
1234:, which points to its conjoined class. Note that the usual Smalltalk
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810:"""Return a description of this car."""
450:"""Return a description of this car."""
1656:
1312:
Metaclass - initializing class variables, instance creation messages
962:
127:
94:. Statements consisting only of original research should be removed.
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2923:
2708:
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Object - default behavior common to all objects, like class access
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Four classes provide the facilities to describe new classes. Their
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2019:
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Some less widespread languages that support metaclasses include
1936:
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Class - similar, more comprehensive, facilities to superclasses
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is again an object and as such an instance of the metaclass of
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1922:
1886:
2177:"Object Membership: The Core Structure of Object Technology"
1786:
to create and register class-metaclass pairs, respectively.
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1867:
1503:
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introspection method in Ruby and Smalltalk. While the map
632:"""Create a new instance."""
2321:
1284:(from Object), and the main facilities they provide are:
1135:
In early Smalltalks, there was only one metaclass called
540:
object is created, but also each time any attribute of a
2829:
2248:"An implementation of mixins in Java using metaclasses"
2215:
Cocoa with Love: What is a meta-class in Objective-C?
2136:"The Ruby Object Model: Comparison with Smalltalk-80"
1027:
The Smalltalk-80 metaclass hierarchy as a UML diagram
635:# First, create the object in the normal default way.
1537:
The diagram on the right also provides a picture of
1556:According to the Ruby's introspection method named
286:is a metaclass. Consider this simple Python class:
152:. Unsourced material may be challenged and removed.
1159:. This means that each metaclass is effectively a
1820:
1201:, which describes the general nature of classes.
671:# Additionally, set attributes on the new object.
3174:
2148:
2845:
2290:
1825:The following are some of the most prominent
1226:, classes and metaclasses are born together.
1276:provides common methods for all metaclasses.
2059:IBM Metaclass programming in Python, parts
1337:
1132:. All metaclasses are subclasses of Class.
1091:which returns the name of the metaclass of
991:. Unsourced material may be challenged and
532:method is automatically called each time a
53:Learn how and when to remove these messages
2852:
2838:
2297:
2283:
2104:"A Primer on Python Metaclass Programming"
1151:), Smalltalk-80 introduced for each class
2044:
2020:Ira R. Forman and Scott Danforth (1999).
1797:being used for a pointer to any object).
1752:Learn how and when to remove this message
1697:Learn how and when to remove this message
1011:Learn how and when to remove this message
230:Learn how and when to remove this message
212:Learn how and when to remove this message
110:Learn how and when to remove this message
2086:Artima Forum: Metaclasses in Python 3.0
1763:
1493:points to the least actual container of
1120:and proceeds up the superclass chain to
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1022:
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2101:
1870:, via the metaclass pragma, as well as
1637:– which evaluates to the eigenclass of
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1465:, which is one meta-level higher than
1105:, the search for the method starts in
2833:
2278:
1299:methods and creating/running objects
1185:In Smalltalk-80, every class (except
1708:
1679:adding citations to reliable sources
1650:
1457:has its own meta-object, called the
1306:) - class/variable naming, comments
1208:. ALL metaclasses are subclasses of
1178:which again is an instance of class
1116:the search for the method starts in
1087:(one can confirm this by evaluating
1079:results in the metaclass object for
989:adding citations to reliable sources
956:
150:adding citations to reliable sources
121:
59:
18:
13:
1935:, an object-oriented extension of
1724:tone or style may not reflect the
952:
14:
3194:
1646:
1217:Object class superclass == Class.
273:
34:This article has multiple issues.
1949:(UML) both support metaclasses.
1734:guide to writing better articles
1713:
1655:
1527:
1518:
961:
126:
64:
23:
2240:
2219:
2208:
2049:. Birmingham: Packt Publishing.
1666:needs additional citations for
1623:is a class then the expression
1473:four explicit objects: classes
1047:. Additionally, Smalltalk is a
137:needs additional citations for
42:or discuss these issues on the
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2013:
1943:Resource Description Framework
1821:Support in languages and tools
1:
2363:Arbitrary-precision or bignum
2006:
1939:, also supports metaclasses.
1541:of eigenclasses in Ruby. The
3183:Class (computer programming)
1059:is an instance of the class
1055:As an example, a car object
548:- controls these details of
7:
2047:Metaprogramming with Python
2045:AloorRavi, Sulekha (2022).
2022:Putting Metaclasses to Work
1952:
244:object-oriented programming
90:the claims made and adding
10:
3201:
1829:that support metaclasses.
1630:corresponds to the Ruby's
1322:
1112:When a message is sent to
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2859:
2737:
2704:Strongly typed identifier
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2532:
2502:
2467:
2355:
2312:
1947:Unified Modeling Language
1769:have a similar hierarchy.
1411:
1365:
1230:has an instance variable
751:to the class definition:
520:. The source code of the
516:itself is an instance of
1995:Metaclass (Semantic Web)
1784:objc_registerClassPair()
1780:objc_allocateClassPair()
1613:evaluates constantly to
1139:. This implied that the
875:
753:
725:# Return the new object.
581:
288:
2779:Parametric polymorphism
2155:. Pragmatic Bookshelf.
2149:Paolo Perrotta (2010).
1845:and other versions of
1770:
1617:. In Smalltalk-80, if
1606:, the Ruby expression
1197:of all metaclasses is
1036:
1028:
1827:programming languages
1793:(similar to the type
1767:
1481:and terminal objects
1282:inheritance hierarchy
1063:. In turn, the class
1034:
1026:
869:The resulting object
260:programming languages
2152:Metaprogramming Ruby
2000:What Is a Metaclass?
1675:improve this article
1155:their own metaclass
1052:(called metaclass).
985:improve this section
282:, the builtin class
264:first-class citizens
146:improve this article
2868:Domains and methods
2784:Primitive data type
2689:Recursive data type
2542:Algebraic data type
2418:Quadruple precision
1390:
1344:
1291:Behavior - minimum
1195:abstract superclass
1170:. The metaclass of
1128:is the subclass of
1043:, everything is an
268:metaobject protocol
2914:Meta-communication
2747:Abstract data type
2428:Extended precision
2387:Reduced precision
2066:2008-09-03 at the
2024:. Addison-Wesley.
1975:Kind (type theory)
1771:
1564:class (denoted by
1388:
1342:
1302:ClassDescription (
1037:
1029:
75:possibly contains
3170:
3169:
3048:Meta-organization
3043:Meta-optimization
2827:
2826:
2559:Associative array
2423:Octuple precision
2162:978-1-934356-47-0
2116:on April 30, 2003
1762:
1761:
1754:
1728:used on Knowledge
1726:encyclopedic tone
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1568:in the diagram).
1547:singleton methods
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2799:Type constructor
2684:Opaque data type
2616:Record or Struct
2413:Double precision
2408:Single precision
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1849:influenced by it
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1736:for suggestions.
1732:See Knowledge's
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1990:Adapter pattern
1965:Metaprogramming
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1539:lazy evaluation
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1437:Eigenclasses of
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953:In Smalltalk-80
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934:
932:'Green'
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908:'Prius'
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3134:
3129:
3127:Meta-reference
3124:
3119:
3117:Metapsychology
3114:
3112:Metapsychiatry
3109:
3104:
3102:Metapopulation
3099:
3094:
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3090:
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3075:
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3058:Metaphilosophy
3055:
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3028:Metamotivation
3025:
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3015:
3010:
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2999:
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2991:Metapragmatics
2983:
2981:Meta-knowledge
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2757:Data structure
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2396:Half precision
2393:
2383:Floating point
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1742:September 2013
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1647:In Objective-C
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1304:abstract class
1278:
1277:
1271:
1253:
1220:
1219:
1212:, therefore:
1089:Car class name
1083:whose name is
1019:
1018:
969:
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951:
876:
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743:Now the class
582:
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274:Python example
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202:September 2013
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3142:Metasociology
3140:
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3053:Metaphenomics
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3003:Metamaterials
3001:
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2986:Meta-language
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2966:Metaheuristic
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2919:Metacomputing
2917:
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2902:Meta-learning
2900:
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2897:Metacognition
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2875:Meta-analysis
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2679:Function type
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2315:
2314:Uninterpreted
2311:
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2300:
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2293:
2288:
2286:
2281:
2280:
2277:
2260:on 2007-10-16
2256:
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2231:"Metaclasses"
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2115:
2111:
2110:
2105:
2102:David Mertz.
2098:
2092:
2091:(part 2 of 2)
2089:
2088:(part 1 of 2)
2083:
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2065:
2062:
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2048:
2041:
2033:
2031:0-201-43305-2
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2016:
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1847:Object Pascal
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1676:
1670:
1669:
1664:This section
1662:
1658:
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1604:
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1383:
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1354:
1347:
1346:
1343:Smalltalk-80
1340:
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1254:
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1243:
1242:
1239:
1237:
1236:class browser
1225:
1215:
1214:
1213:
1202:
1196:
1192:
1183:
1164:
1162:
1150:
1142:
1133:
1118:Integer class
1110:
1100:
1099:Class methods
1096:
1053:
1050:
1046:
1042:
1033:
1025:
1015:
1012:
1004:
994:
990:
986:
980:
979:
975:
970:This section
968:
964:
959:
958:
874:
816:" "
752:
741:
580:
557:
512:At run time,
287:
281:
271:
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191:
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177:
173:
170:
166:
163: –
162:
158:
157:Find sources:
151:
147:
141:
140:
135:This article
133:
129:
124:
123:
114:
111:
103:
93:
89:
85:
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73:This article
71:
62:
61:
56:
54:
47:
46:
41:
40:
35:
30:
21:
20:
3097:Metapolitics
3018:Metamodeling
2961:Metagenomics
2939:Meta-emotion
2890:Semantic Web
2884:
2771:
2584:Intersection
2262:. Retrieved
2255:the original
2242:
2221:
2210:
2198:. Retrieved
2194:
2185:
2171:
2151:
2144:
2130:
2118:. Retrieved
2114:the original
2107:
2097:
2082:
2055:
2046:
2040:
2021:
2015:
1941:
1931:
1892:
1824:
1815:
1803:
1799:
1788:
1776:
1772:
1748:
1739:
1723:
1693:
1687:October 2013
1684:
1673:Please help
1668:verification
1665:
1639:
1632:
1625:
1619:
1608:
1602:
1594:
1555:
1546:
1536:
1507:Smalltalk-80
1498:
1494:
1490:
1471:
1466:
1462:
1458:
1454:
1451:
1436:
1419:eigenclasses
1415:Eigenclasses
1414:
1403:
1357:
1329:eigenclasses
1326:
1279:
1240:
1221:
1203:
1184:
1165:
1134:
1126:Object class
1122:Object class
1111:
1097:
1054:
1038:
1007:
1001:October 2013
998:
983:Please help
971:
868:
742:
735:
558:
511:
277:
247:
241:
226:
208:
199:
189:
182:
175:
168:
156:
144:Please help
139:verification
136:
106:
97:
74:
50:
43:
37:
36:Please help
33:
3152:Metatheorem
3147:Meta-system
3137:Metascience
3083:Metaphysics
3038:Meta-object
2971:Metahistory
2951:Metafiction
2814:Type theory
2809:Type system
2659:Bottom type
2606:Option type
2547:generalized
2433:Long double
2378:Fixed point
2227:Herb Sutter
1907:CorbaScript
1858:Objective-C
1834:Common Lisp
1360:metaclasses
1049:class based
789:description
429:description
161:"Metaclass"
3157:Metatheory
3122:Metapuzzle
3068:Metaethics
2956:Metagaming
2929:Metadesign
2907:Metamemory
2719:Empty type
2714:Type class
2664:Collection
2621:Refinement
2599:metaobject
2447:signedness
2306:Data types
2264:2007-11-27
2007:References
1980:Reflection
1970:Metaobject
1945:(RDF) and
1808:using the
1459:eigenclass
1191:superclass
1174:is called
172:newspapers
84:improve it
39:improve it
3162:Metaverse
3073:Metalogic
3013:Metamedia
2976:Metahumor
2885:Metaclass
2794:Subtyping
2789:Interface
2772:metaclass
2724:Unit type
2694:Semaphore
2674:Exception
2579:Inductive
2569:Dependent
2534:Composite
2512:Character
2494:Reference
2391:Minifloat
2347:Bit array
1960:Metamodel
1882:Smalltalk
1333:Metaclass
1297:compiling
1274:Metaclass
1232:thisClass
1228:Metaclass
1180:Metaclass
1172:Metaclass
1168:Metaclass
1161:singleton
1085:Car class
1077:Car class
1073:Car class
1041:Smalltalk
972:does not
783:@property
771:metaclass
749:metaclass
423:@property
256:instances
248:metaclass
88:verifying
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