Abstract Classes and Pure Virtual Functions
A pure virtual function or pure virtual method is a virtual function that is required to be implemented by a derived class, if that class is not abstract. Classes containing pure virtual methods are termed "abstract"; they cannot be instantiated directly. A subclass of an abstract class can only be instantiated directly if all inherited pure virtual methods have been implemented by that class or a parent class. Pure virtual methods typically have a declaration (signature) and no definition (implementation).
As an example, an abstract base class MathSymbol may provide a pure virtual function doOperation, and derived classes Plus and Minus implement doOperation to provide concrete implementations. Implementing doOperation would not make sense in the MathSymbol class, as MathSymbol is an abstract concept whose behaviour is defined solely for each given kind (subclass) of MathSymbol. Similarly, a given subclass of MathSymbol would not be complete without an implementation of doOperation.
Although pure virtual methods typically have no implementation in the class that declares them, pure virtual methods in C++ are permitted to contain an implementation in their declaring class, providing fallback or default behaviour that a derived class can delegate to, if appropriate.
Pure virtual functions can also be used where the method declarations are being used to define an interface - similar to what the interface keyword in Java explicitly specifies. In such a use, derived classes will supply all implementations. In such a design pattern, the abstract class which serves as an interface will contain only pure virtual functions, but no data members or ordinary methods. In C++, using such purely abstract classes as interfaces works because C++ supports multiple inheritance. However, because many OO languages do not support multiple inheritance, they often provide a separate interface mechanism. An example is the Java programming language.
Read more about this topic: Virtual Function
Famous quotes containing the words abstract, classes, pure, virtual and/or functions:
“If the abstract rights of man will bear discussion and explanation, those of women, by a parity of reasoning, will not shrink from the same test: though a different opinion prevails in this country.”
—Mary Wollstonecraft (1759–1797)
“When we of the so-called better classes are scared as men were never scared in history at material ugliness and hardship; when we put off marriage until our house can be artistic, and quake at the thought of having a child without a bank-account and doomed to manual labor, it is time for thinking men to protest against so unmanly and irreligious a state of opinion.”
—William James (1842–1910)
“There are moods in which one feels the impulse to enter a tacit protest against too gross an appetite for pure aesthetics in this starving and sinning world. One turns half away, musingly, from certain beautiful useless things.”
—Henry James (1843–1916)
“Neither dead nor alive, the hostage is suspended by an incalculable outcome. It is not his destiny that awaits for him, nor his own death, but anonymous chance, which can only seem to him something absolutely arbitrary.... He is in a state of radical emergency, of virtual extermination.”
—Jean Baudrillard (b. 1929)
“If photography is allowed to stand in for art in some of its functions it will soon supplant or corrupt it completely thanks to the natural support it will find in the stupidity of the multitude. It must return to its real task, which is to be the servant of the sciences and the arts, but the very humble servant, like printing and shorthand which have neither created nor supplanted literature.”
—Charles Baudelaire (1821–1867)