Editing Virtual function (section)

== 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 the derived class is not [[Abstract type|abstract]]. Classes containing pure virtual methods are termed "abstract" and they cannot be instantiated directly.  A [[Subclass (computer science)|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 ([[Type signature#Method signature|signature]]) and no definition ([[Method implementation|implementation]]).

As an example, an abstract base class <code>MathSymbol</code> may provide a pure virtual function <code>doOperation()</code>, and derived classes <code>Plus</code> and <code>Minus</code> implement <code>doOperation()</code> to provide concrete implementations. Implementing <code>doOperation()</code> would not make sense in the <code>MathSymbol</code> class, as <code>MathSymbol</code> is an abstract concept whose behaviour is defined solely for each given kind (subclass) of <code>MathSymbol</code>. Similarly, a given subclass of <code>MathSymbol</code> would not be complete without an implementation of
<code>doOperation()</code>.

Although pure virtual methods typically have no implementation in the class that declares them, pure virtual methods in some languages (e.g. C++ and Python) are permitted to contain an implementation in their declaring class, providing fallback or default behaviour that a derived class can delegate to, if appropriate.<ref>[https://en.cppreference.com/w/cpp/language/destructor#Pure_virtual_destructors Pure virtual destructors - cppreference.com]</ref><ref>[https://docs.python.org/3/library/abc.html "abc — Abstract Base Classes: @abc.abstractmethod"]</ref>

Pure virtual functions can also be used where the method declarations are being used to define an [[interface (Java)|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 OOP languages do not support multiple inheritance, they often provide a separate interface mechanism. An example is the [[Java (programming language)|Java programming language]].