Concrete class

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.

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 implementation‍—‌a concrete data structure and collection of subroutines‍—‌while a type is an . 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.

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