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Virtual functions allow a program to call methods that don't necessarily even exist at the moment the code is compiled. [citation needed] In C++, virtual methods are declared by prepending the virtual keyword to the function's declaration in the base class. This modifier is inherited by all implementations of that method in derived classes ...
To elaborate on the above example, consider a base class with no virtual functions. Whenever the base class calls another member function, it will always call its own base class functions. When we derive a class from this base class, we inherit all the member variables and member functions that were not overridden (no constructors or destructors).
Note that those functions not carrying the keyword virtual in their declaration (such as fnonvirtual() and d()) do not generally appear in the virtual method table. There are exceptions for special cases as posed by the default constructor. Also note the virtual destructors in the base classes, B1 and B2.
Perhaps the most well-known example is C++, an object-oriented extension of the C programming language. Due to the design requirements to add the object-oriented paradigm on to an existing procedural language, message passing in C++ has some unique capabilities and terminologies. For example, in C++ a method is known as a member function.
When overriding one method with another, the signatures of the two methods must be identical (and with same visibility). In C#, class methods, indexers, properties and events can all be overridden. Non-virtual or static methods cannot be overridden. The overridden base method must be virtual, abstract, or override.
struct Base {virtual void some_func (float);}; struct Derived: Base {virtual void some_func (int) override; // ill-formed - doesn't override a base class method}; The override special identifier means that the compiler will check the base class(es) to see if there is a virtual function with this exact signature.
In particular, through the curiously recurring template pattern, it is possible to implement a form of static polymorphism that closely mimics the syntax for overriding virtual functions. Because C++ templates are type-aware and Turing-complete, they can also be used to let the compiler resolve recursive conditionals and generate substantial ...
I wouldn't declare the destructors as virtual in the C++ example. It's just confusing. 212.126.224.100 14:07, 4 March 2011 (UTC) OK, I see that it is recommended to declare destructors as virtual in general. But I still think it may be more appropriate to leave the virtual out in the first example.