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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Famous quotes containing the words signifying, abstract and/or types:
“Catholics think of grace as a supernatural power which God dispenses, primarily through the Church and its sacraments, to purify the souls of naturally sinful human beings, and render them capable of holiness.... Protestants think of grace as an attribute of God rather than a gift from God. It is a shorthand term signifying God’s determination to love, forgive, and save His human children, however little they deserve it.”
—Louis Cassels, U.S. religious columnist. “The Catholic-Protestant Differences,” What’s the Difference?, Doubleday (1965)
“When needs and means become abstract in quality, abstraction is also a character of the reciprocal relation of individuals to one another. This abstract character, universality, is the character of being recognized and is the moment which makes concrete, i.e. social, the isolated and abstract needs and their ways and means of satisfaction.”
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—Henry David Thoreau (1817–1862)