Virtual pure virtual explained

Question

What exactly does it mean if a function is defined as virtual and is that the same as pure virtual

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From Wikipedia s Virtual function         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  This concept is an important part of the  runtime  polymorphism portion of object-oriented programming  OOP   In short  a virtual function defines a target function to be executed  but the target might not be known at compile time    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  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    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   whereas       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    When a pure virtual method exists  the class is  abstract  and can not be instantiated on its own  Instead  a derived class that implements the pure-virtual method s  must be used  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  Only a class that has no abstract methods can be instantiated   A virtual provides a way to override the functionality of the base class  and a pure-virtual requires it

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Virtual functions must have a definition in base class and also in derived class but not necessary  for example ToString   or toString   function is a Virtual so you can provide your own implementation by overriding it in user-defined class es   Virtual functions are declared and defined in normal class    Pure virtual function must be declared ending with    0  and it can only be declared in abstract class  An abstract class having a pure virtual function s  cannot have a definition s  of that pure virtual functions  so it implies that implementation must be provided in class es  that derived from that abstract class

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The virtual keyword gives C   its  ability to support polymorphism  When you have a pointer to an object of some class such as   class Animal     public      virtual int GetNumberOfLegs     0      class Duck   public Animal     public       int GetNumberOfLegs     return 2        class Horse   public Animal     public       int GetNumberOfLegs     return 4        void SomeFunction Animal   pAnimal      cout  lt  lt  pAnimal- gt GetNumberOfLegs        In this  silly  example  the GetNumberOfLegs   function returns the appropriate number based on the class of the object that it is called for   Now  consider the function  SomeFunction   It doesn t care what type of animal object is passed to it  as long as it is derived from Animal  The compiler will automagically cast any Animal-derived class to a Animal as it is a base class   If we do this   Duck d  SomeFunction  amp d     it d output  2   If we do this   Horse h  SomeFunction  amp h     it d output  4   We can t do this   Animal a  SomeFunction  amp a     because it won t compile due to the GetNumberOfLegs   virtual function being pure  which means it must be implemented by deriving classes  subclasses    Pure Virtual Functions are mostly used to define   a  abstract classes  These are base classes where you have to derive from them and then implement the pure virtual functions   b  interfaces  These are  empty  classes where all functions are pure virtual and hence you have to derive and then implement all of the functions

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Simula  C    and C   which use static method binding by default  the programmer can specify that particular methods should use dynamic binding by labeling them as virtual   Dynamic method binding is central to object-oriented programming    Object oriented programming requires three fundamental concepts  encapsulation  inheritance  and dynamic method binding       Encapsulation allows the implementation details of an   abstraction to be hidden behind a   simple interface          Inheritance allows a new abstraction to be defined as an   extension or refinement of some   existing abstraction  obtaining some   or all of its characteristics   automatically        Dynamic method binding allows the new abstraction to display its new   behavior even when used in a context   that expects the old abstraction

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In a C   class  virtual is the keyword which designates that  a method can be overridden  i e  implemented by  a subclass  For example   class Shape      public      Shape        virtual  Shape         std  string getName      not overridable             return m name             void setName  const std  string amp  name      not overridable             m name   name           protected      virtual void initShape      overridable             setName  Generic Shape             private      std  string m name       In this case a subclass can override the the initShape function to do some specialized work   class Square   public Shape     public       Square        virtual  Square       protected      virtual void initShape      override the Shape  initShape function             setName  Square              The term pure virtual refers to virtual functions that need to be implemented by a subclass and have not been implemented by the base class   You designate a method as pure virtual by using the virtual keyword and adding a  0 at the end of the method declaration     So  if you wanted to make Shape  initShape pure virtual you would do the following   class Shape             virtual void initShape     0     pure virtual method            By adding a pure virtual method to your class you make the class an abstract base class which is very handy for separating interfaces from implementation

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I d like to comment on Wikipedia s definition of virtual  as repeated by several here   At the time this answer was written   Wikipedia defined a virtual method as one that can be overridden in subclasses   Fortunately  Wikipedia has been edited since  and it now explains this correctly   That is incorrect  any method  not just virtual ones  can be overridden in subclasses  What virtual does is to give you polymorphism  that is  the ability to select at run-time the most-derived override of a method   Consider the following code    include  lt iostream gt  using namespace std   class Base   public      void NonVirtual             cout  lt  lt   Base NonVirtual called  n             virtual void Virtual             cout  lt  lt   Base Virtual called  n            class Derived   public Base   public      void NonVirtual             cout  lt  lt   Derived NonVirtual called  n             void Virtual             cout  lt  lt   Derived Virtual called  n             int main         Base  bBase   new Base        Base  bDerived   new Derived         bBase- gt NonVirtual        bBase- gt Virtual        bDerived- gt NonVirtual        bDerived- gt Virtual        What is the output of this program   Base NonVirtual called  Base Virtual called  Base NonVirtual called  Derived Virtual called    Derived overrides every method of Base  not just the virtual one  but also the non-virtual   We see that when you have a Base-pointer-to-Derived  bDerived   calling NonVirtual calls the Base 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   However  calling Virtual calls the Derived class implementation  Because of the keyword virtual  the selection of the method happens at run-time  not compile-time  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  This vtable is instantiated at run-time  hence the run-time resolution to the most-derived override   I hope this wasn t too confusing  In short  any method can be overridden  but only virtual methods give you polymorphism  that is  run-time selection of the most derived override  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

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Virtual  means that the method may be overridden in subclasses  but has an directly-callable implementation in the base class   Pure virtual  means it is a virtual method with no directly-callable implementation  Such a method must be overridden at least once in the inheritance hierarchy -- if a class has any unimplemented virtual methods  objects of that class cannot be constructed and compilation will fail    quark points out that pure-virtual methods can have an implementation  but as pure-virtual methods must be overridden  the default implementation can t be directly called  Here is an example of a pure-virtual method with a default    include  lt cstdio gt   class A   public      virtual void Hello     0      void A  Hello         printf  A  Hello n       class B   public A   public      void Hello             printf  B  Hello n            A  Hello              int main            Prints             B  Hello            A  Hello            B b      b Hello        return 0        According to comments  whether or not compilation will fail is compiler-specific  In GCC 4 3 3 at least  it won t compile   class A   public      virtual void Hello     0      int main         A a      return 0      Output     g   -c virt cpp  virt cpp  In function    int main       virt cpp 8  error  cannot declare variable    a    to be of abstract type    A    virt cpp 1  note    because the following virtual functions are pure within    A     virt cpp 3  note    virtual void A  Hello

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Virtual methods CAN be overridden by deriving classes  but need an implementation in the base class  the one that will be overridden   Pure virtual methods have no implementation the base class  They need to be defined by derived classes   So technically overridden is not the right term  because there s nothing to override    Virtual corresponds to the default java behaviour  when the derived class overrides a method of the base class   Pure Virtual methods correspond to the behaviour of abstract methods within abstract classes  And a class that only contains pure virtual methods and constants would be the cpp-pendant to an Interface

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A virtual function or virtual method is a function or method whose behavior can be overridden within an inheriting class by a function with the same signature  - wikipedia  This is not a good explanation for virtual functions  Because  even if a member is not virtual  inheriting classes can override it  You can try and see it yourself   The difference shows itself when a function take a base class as a parameter  When you give an inheriting class as the input  that function uses the base class implementation of the overriden function  However  if that function is virtual  it uses the one that is implemented in the deriving class

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How does the virtual keyword work   Assume that Man is a base class  Indian is derived from man    Class Man    public      virtual void do work            Class Indian   public Man    public      void do work             Declaring do work   as virtual simply means  which do work   to call will be determined ONLY at run-time   Suppose I do   Man  man  man   new Indian    man- gt do work       Indian s do work is only called    If virtual is not used  the same is statically determined or statically bound by the compiler  depending on what object is calling  So if an object of Man calls do work    Man s do work   is called EVEN THOUGH IT POINTS TO AN INDIAN OBJECT  I believe that the top voted answer is misleading - Any method whether or not virtual can have an overridden implementation in the derived class  With specific reference to C   the correct difference is run-time  when virtual is used  binding and compile-time  when virtual is not used but a method is overridden and a base pointer is pointed at a derived object  binding of associated functions   There seems to be another misleading comment that says        Justin   pure virtual  is just a term  not a keyword  see my answer   below  used to mean  this function cannot be implemented by the base   class     THIS IS WRONG  Purely virtual functions can also have a body AND CAN BE IMPLEMENTED  The truth is that an abstract class  pure virtual function can be called statically  Two very good authors are Bjarne Stroustrup and Stan Lippman     because they wrote the language

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A virtual function is a member function that is declared in a base class and that is redefined by derived class  Virtual function are hierarchical in order of inheritance   When a derived class does not override a virtual function  the function defined within its base class is used   A pure virtual function is one that contains no definition relative to the base class   It has no implementation in the base class  Any derived class must override this function

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Pure Virtual Function  try this code   include  lt iostream gt  using namespace std  class aClassWithPureVirtualFunction    public       virtual void sayHellow   0       class anotherClass aClassWithPureVirtualFunction    public       void sayHellow                  cout lt  lt  hellow World             int main           aClassWithPureVirtualFunction virtualObject              This not possible to create object of a class that contain pure virtual function            anotherClass object      object sayHellow        In class anotherClass remove the function sayHellow and run the code  you will get error Because when a class contain a pure virtual function  no object can be created from that class and it is inherited then its derived class must implement that function   Virtual function  try another code   include  lt iostream gt  using namespace std  class aClassWithPureVirtualFunction    public       virtual void sayHellow                 cout lt  lt  from base n              class anotherClass public aClassWithPureVirtualFunction    public       void sayHellow                  cout lt  lt  from derived  n             int main         aClassWithPureVirtualFunction  baseObject new aClassWithPureVirtualFunction      baseObject- gt sayHellow      call base one      baseObject new anotherClass      baseObject- gt sayHellow       call the derived one       Here the sayHellow function is marked as virtual in base class It say the compiler that try searching the function in derived class and implement the function If not found then execute the base one Thanks

[c++] Virtual/pure virtual explained

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