|
In object-oriented programming, a class is an extensible program-code-template for creating objects, providing initial values for state (member variables) and implementations of behavior (member functions or methods). In many languages, the class name is used as the name for the class (the template itself), the name for the default constructor of the class (a subroutine that creates objects), and as the type of objects generated by instantiating the class; these distinct concepts are easily conflated. When an object is created by a constructor of the class, the resulting object is called an instance of the class, and the member variables specific to the object are called instance variables, to contrast with the class variables shared across the class. In some languages, classes are only a compile-time feature (new classes cannot be declared at runtime), while in other languages classes are first-class citizens, and are generally themselves objects (typically of type Class or similar). In these languages, a class that creates classes is called a metaclass.== Class vs. type == In casual use, people often refer to the "class" of an object, but narrowly speaking objects have ''type'': the interface, namely the types of member variables, the signatures of member functions (methods), and properties these satisfy. At the same time, a class has an implementation (specifically the implementation of the methods), and can create objects of a given type, with a given implementation. In the terms of type theory, a class is an implementationa ''concrete'' data structure and collection of subroutineswhile a type is an interface. Different (concrete) classes can produce objects of the same (abstract) type (depending on type system); for example, the type Stack might be implemented with two classes SmallStack (fast for small stacks, but scales poorly) and ScalableStack (scales well but high overhead for small stacks). Similarly, a given class may have several different constructors.Types generally represent nouns, such as a person, place or thing, or something nominalized, and a class represents an implementation of these. For example, a Banana type might represent the properties and functionality of bananas in general, while the ABCBanana and XYZBanana classes would represent ways of producing bananas (say, banana suppliers or data structures and functions to represent and draw bananas in a video game). The ABCBanana class could then produce particular bananas: instances of the ABCBanana class would be objects of type Banana. Often only a single implementation of a type is given, in which case the class name is often identical with the type name.抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Class (computer programming)」の詳細全文を読む スポンサード リンク
|