In a C++ class, virtual is the keyword which designates that, a method can be overridden (i.e. These are 'empty' classes where all functions are pure virtual and hence you have to derive and then implement all of the functions. These are base classes where you have to derive from them and then implement the pure virtual functions. Pure Virtual Functions are mostly used to define: We can't do this: Animal a īecause it won't compile due to the GetNumberOfLegs() virtual function being pure, which means it must be implemented by deriving classes (subclasses). The compiler will automagically cast any Animal-derived class to a Animal as it is a base class. It doesn't care what type of animal object is passed to it, as long as it is derived from Animal. Now, consider the function 'SomeFunction'. In this (silly) example, the GetNumberOfLegs() function returns the appropriate number based on the class of the object that it is called for. When you have a pointer to an object of some class such as: class Animal The virtual keyword gives C++ its' ability to support polymorphism. In practice, however, overriding a non-virtual method is considered bad practice and rarely used, so many people (including whoever wrote that Wikipedia article) think that only virtual methods can be overridden. In short, any method can be overridden, but only virtual methods give you polymorphism, that is, run-time selection of the most derived override. This vtable is instantiated at run-time, hence the run-time resolution to the most-derived override. What happens here at compile-time is that the compiler sees that this is a Base*, and that it's calling a virtual method, so it insert a call to the vtable instead of class Base. Because of the keyword virtual, the selection of the method happens at run-time, not compile-time. However, calling Virtual calls the Derived class implementation. This is resolved at compile-time: the compiler sees that bDerived is a Base*, that NonVirtual is not virtual, so it does the resolution on class Base. We see that when you have a Base-pointer-to-Derived (bDerived), calling NonVirtual calls the Base class implementation. What is the output of this program? Base NonVirtual called.ĭerived overrides every method of Base: not just the virtual one, but also the non-virtual. What virtual does is to give you polymorphism, that is, the ability to select at run-time the most-derived override of a method. That is incorrect: any method, not just virtual ones, can be overridden in subclasses. Wikipedia defined a virtual method as one that can be overridden in subclasses. I'd like to comment on Wikipedia's definition of virtual, as repeated by several here. Only a class that has no abstract methods can be instantiated.Ī virtual provides a way to override the functionality of the base class, and a pure-virtual requires it. A pure-virtual isn't defined in the base-class at all, so a derived class must define it, or that derived class is also abstract, and can not be instantiated. Instead, a derived class that implements the pure-virtual method(s) must be used. When a pure virtual method exists, the class is "abstract" and can not be instantiated on its own. This is in contrast to non-virtual functions, which can still be overridden in a derived class, but the "new" version will only be used by the derived class and below, but will not change the functionality of the base class at all.Ī 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. Therefore if one method of the base class calls a virtual method, the version defined in the derived class will be used instead of the version defined in the base class. Unlike a non-virtual function, when a virtual function is overridden the most-derived version is used at all levels of the class hierarchy, rather than just the level at which it was created. In short, a virtual function defines a target function to be executed, but the target might not be known at compile time. This concept is an important part of the (runtime) polymorphism portion of object-oriented programming (OOP). In object-oriented programming, in languages such as C++, and Object Pascal, a virtual function or virtual method is an inheritable and overridable function or method for which dynamic dispatch is facilitated.
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