Signifying Abstract Types
Abstract classes can be created, signified, or simulated in several ways:
- By use of the explicit keyword abstract in the class definition, as in Java, D or C#.
- By including, in the class definition, one or more abstract methods (called pure virtual functions in C++), which the class is declared to accept as part of its protocol, but for which no implementation is provided.
- By inheriting from an abstract type, and not overriding all missing features necessary to complete the class definition.
- In many dynamically typed languages such as Smalltalk, any class which sends a particular method to this, but doesn't implement that method, can be considered abstract. (However, in many such languages, like Objective-C, the error is not detected until the class is used, and the message returns results in an exception error message such as "Does not recognize selector: xxx" as
-is invoked upon detection of an unimplemented method).
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