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In software engineering, a class diagram [1] in the Unified Modeling Language (UML) is a type of static structure diagram that describes the structure of a system by showing the system's classes, their attributes, operations (or methods), and the relationships among objects. The class diagram is the main building block of object-oriented modeling.
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. 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.
Other implementations are possible: for example, objects in Python use associative key-value containers. [4] Some programming languages such as Eiffel support specification of invariants as part of the definition of the class, and enforce them through the type system.
It should be possible to define a new operation for (some) classes of an object structure without changing the classes. When new operations are needed frequently and the object structure consists of many unrelated classes, it's inflexible to add new subclasses each time a new operation is required because "[..] distributing all these operations across the various node classes leads to a system ...
The class defines the data format or type (including member variables and their types) and available procedures (class methods or member functions) for a given type or class of object. Objects are created by calling a special type of method in the class known as a constructor. Classes may inherit from other classes, so they are arranged in a ...
A sample UML class and sequence diagram for the Bridge design pattern. [3]In the above Unified Modeling Language class diagram, an abstraction (Abstraction) is not implemented as usual in a single inheritance hierarchy.
A sample UML class and sequence diagram for the Decorator design pattern. [7] In the above UML class diagram, the abstract Decorator class maintains a reference (component) to the decorated object (Component) and forwards all requests to it (component.operation()). This makes Decorator transparent (invisible) to clients of Component.
A class diagram exemplifying the singleton pattern.. In object-oriented programming, the singleton pattern is a software design pattern that restricts the instantiation of a class to a singular instance.