A subclass, "derived class", heir class, or child class is a modular, derivative class that inherits one or more language entities from one or more other classes (called superclasses, base classes, or parent classes). The semantics of class inheritance vary from language to language, but commonly the subclass automatically inherits the instance variables and member functions of its superclasses. Some languages support the inheritance of other construct as well. For example, in Eiffel, contracts which define the specification of a class are also inherited by heirs. The superclass establishes a common interface and foundational functionality, which specialized subclasses can inherit, modify, and supplement. The software inherited by a subclass is considered reused in the subclass. A reference to an instance of a class may actually be referring one of its subclasses. The actual class of the object being referenced is impossible to predict at compile-time. A uniform interface is used to invoke the member functions of objects of a number of different classes. Subclass may replace superclass functions with entirely new functions that must share the same method signature.