Metaclass
![Diagram of the inheritance and instance relationships between classes and metaclasses in Smalltalk](/Images/godic/202501/27/Smalltalk_metaclass3009.png")
![](/Images/godic/202501/27/Smalltalk-metaclass-sample.svg3010.png")
![](/Images/godic/202501/27/Ruby-metaclass-sample.svg3010.png")
![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 have a similar hierarchy.](/Images/godic/202501/27/Objective-C_metaclass3010.png")
In object-oriented programming, a metaclass is a class whose instances are classes. Just as an ordinary class defines the behavior of certain objects, a metaclass defines the behavior of certain classes and their instances. Not all object-oriented programming languages support metaclasses. Among those that do, the extent to which metaclasses can override any given aspect of class behavior varies. Metaclasses can be implemented by having classes be first-class citizen, in which case a metaclass is simply an object that constructs classes. Each language has its own metaobject protocol, a set of rules that govern how objects, classes, and metaclasses interact.