What is the difference between an abstract function and a virtual function

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What is the difference between an abstract function and a virtual function  In which cases is it recommended to use virtual or abstract  Which one is the best approach

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The answer has been provided a number of times but the the question about when to use each is a design-time decision  I would see it as good practice to try to bundle common method definitions into distinct interfaces and pull them into classes at appropriate abstraction levels  Dumping a common set of abstract and virtual method definitions into a class renders the class unistantiable when it may be best to define a non-abstract class that implements a set of concise interfaces  As always  it depends on what best suits your applications specific needs

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An abstract function has no implemention and it can only be declared on an abstract class  This forces the derived class to provide an implementation    A virtual function provides a default implementation and it can exist on either an abstract class or a non-abstract class    So for example   public abstract class myBase         If you derive from this class you must implement this method  notice we have no method body here either     public abstract void YouMustImplement           If you derive from this class you can change the behavior but are not required to     public virtual void YouCanOverride                   public class MyBase        This will not compile because you cannot have an abstract method in a non-abstract class     public abstract void YouMustImplement

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From a C   background  C  virtual corresponds to C   virtual  while C  abstract methods corresponds to C   pure virtual function

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Abstract method  When a class contains an abstract method  that class must be declared as abstract  The abstract method has no implementation and thus  classes that derive from that abstract class  must provide an implementation for this abstract method   Virtual method  A class can have a virtual method   The virtual method has an implementation  When you inherit from a class that has a virtual method  you can override the virtual method and provide additional logic  or replace the logic with your own implementation   When to use what  In some cases  you know that certain types should have a specific method  but  you don t know what implementation this method should have  In such cases  you can create an interface which contains a method with this signature  However  if you have such a case  but you know that implementors of that interface will also have another common method  for which you can already provide the implementation   you can create an abstract class  This abstract class then contains the abstract method  which must be overriden   and another method which contains the  common  logic   A virtual method should be used if you have a class which can be used directly  but for which you want inheritors to be able to change certain behaviour  although it is not mandatory

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Abstract function method       An abstract method is a method which is declared with the keyword abstract     It does not have body     It should be implemented by the derived class     If a method is abstract then the class should abstract   virtual function method       A virtual method is the method which is declared with the keyword virtual and it can be overridden by the derived class method by using override keyword     It s up to the derived class whether to override it or not

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Abstract methods are always virtual  They cannot have an implementation   That s the main difference   Basically  you would use a virtual method if you have the  default  implementation of it and want to allow descendants to change its behaviour   With an abstract method  you force descendants to provide an implementation

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Only abstract classes can have abstract members  A non-abstract class that inherits from an abstract class must override its abstract members  An abstract member is implicitly virtual  An abstract member cannot provide any implementation  abstract is called pure virtual in some languages

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Binding is the process of mapping a name to a unit of code   Late binding means that we use the name  but defer the mapping  In other words  we create mention the name first  and let some subsequent process handle the mapping of code to that name   Now consider    Compared to humans  machines are really good at searching and sorting Compared to machines  humans are really good at invention and innovation   So  the short answer is  virtual is a late binding instruction for the machine  runtime  whereas abstract is the late binding instruction for the human  programmer   In other words  virtual means      Dear runtime  bind the appropriate code to this name by doing what you do best  searching     Whereas abstract means      Dear programmer  please bind the appropriate code to this name by doing what you do best  inventing     For the sake of completeness  overloading means      Dear compiler  bind the appropriate code to this name by doing what you do best  sorting

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I have seen in some places the abstract method is defined as below                      An Abstract Method must have to implement in the child class       I felt it is like    It is not necessary that an abstract method has to be implemented in a child class  if the child class is also abstract      1 An abstract method cant be a private method  2 An Abstract method cant be implemented in the same abstract class   I would say   if we are implementing an abstract class  you must have to override the abstract methods from the base abstract class  Because   Implementing the abstract method is with override key word  Similar to Virtual method   It is not necessary for a virtual method to be implemented in an inherited class                                     ----------CODE--------------  public abstract class BaseClass       public int MyProperty   get  set        protected abstract void MyAbstractMethod         public virtual void MyVirtualMethod                 var x   3   4           public abstract class myClassA   BaseClass       public int MyProperty   get  set          not necessary to implement an abstract method if the child class is also abstract       protected override void MyAbstractMethod                 throw new NotImplementedException            public class myClassB   BaseClass       public int MyProperty   get  set          You must have to implement the abstract method since this class is not an abstract class       protected override void MyAbstractMethod                 throw new NotImplementedException

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You must always override an abstract function   Thus    Abstract functions - when the inheritor must provide its own implementation Virtual - when it is up to the inheritor to decide

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I have seen in some places the abstract method is defined as below                      An Abstract Method must have to implement in the child class       I felt it is like    It is not necessary that an abstract method has to be implemented in a child class  if the child class is also abstract      1 An abstract method cant be a private method  2 An Abstract method cant be implemented in the same abstract class   I would say   if we are implementing an abstract class  you must have to override the abstract methods from the base abstract class  Because   Implementing the abstract method is with override key word  Similar to Virtual method   It is not necessary for a virtual method to be implemented in an inherited class                                     ----------CODE--------------  public abstract class BaseClass       public int MyProperty   get  set        protected abstract void MyAbstractMethod         public virtual void MyVirtualMethod                 var x   3   4           public abstract class myClassA   BaseClass       public int MyProperty   get  set          not necessary to implement an abstract method if the child class is also abstract       protected override void MyAbstractMethod                 throw new NotImplementedException            public class myClassB   BaseClass       public int MyProperty   get  set          You must have to implement the abstract method since this class is not an abstract class       protected override void MyAbstractMethod                 throw new NotImplementedException

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Here I am writing some sample code hoping this may be a rather tangible example to see the behaviors of the interfaces  abstract classes and ordinary classes on a very basic level  You can also find this code in github as a project if you want to use it as a demo  https   github com usavas JavaAbstractAndInterfaceDemo   public interface ExampleInterface          public void MethodBodyInInterfaceNotPossible                void MethodInInterface        public abstract class AbstractClass       public abstract void AbstractMethod               public abstract void AbstractMethodWithBodyNotPossible                               Standard Method CAN be declared in AbstractClass     public void StandardMethod            System out println  Standard Method in AbstractClass  super  runs             public class ConcreteClass     extends AbstractClass     implements ExampleInterface         Abstract Method HAS TO be IMPLEMENTED in child class  Implemented by ConcreteClass      Override     public void AbstractMethod             System out println  AbstractMethod overridden runs                 Standard Method CAN be OVERRIDDEN       Override     public void StandardMethod             super StandardMethod            System out println  StandardMethod overridden in ConcreteClass runs               public void ConcreteMethod            System out println  Concrete method runs                 A method in interface HAS TO be IMPLEMENTED in implementer class       Override     public void MethodInInterface             System out println  MethodInInterface Implemented by ConcreteClass runs               Cannot declare abstract method in a concrete class           public abstract void AbstractMethodDeclarationInConcreteClassNotPossible

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An abstract method is a method that must be implemented to make a concrete class  The declaration is in the abstract class  and any class with an abstract method must be an abstract class  and it must be implemented in a concrete class   A virtual method is a method that can be overridden in a derived class using the override  replacing the behavior in the superclass  If you don t override  you get the original behavior  If you do  you always get the new behavior  This opposed to not virtual methods  that can not be overridden but can hide the original method  This is done using the new modifier   See the following example   public class BaseClass       public void SayHello                 Console WriteLine  Hello                public virtual void SayGoodbye                 Console WriteLine  Goodbye               public void HelloGoodbye                 this SayHello            this SayGoodbye              public class DerivedClass   BaseClass       public new void SayHello                 Console WriteLine  Hi There                public override void SayGoodbye                 Console WriteLine  See you later              When I instantiate DerivedClass and call SayHello  or SayGoodbye  I get  Hi There  and  See you later   If I call HelloGoodbye  I get  Hello  and  See you later   This is because SayGoodbye is virtual  and can be replaced by derived classes  SayHello is only hidden  so when I call that from my base class I get my original method   Abstract methods are implicitly virtual  They define behavior that must be present  more like an interface does

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You must always override an abstract function   Thus    Abstract functions - when the inheritor must provide its own implementation Virtual - when it is up to the inheritor to decide

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To my understanding   Abstract Methods    Only the abstract class can hold abstract methods  Also the derived class need to implement the method and no implementation is provided in the class   Virtual Methods   A class can declare these and also provide the implementation of the same  Also the derived class need to implement of the method to override it

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There are nothing call virtual class in C    For functions   Abstract function only have signature only the drive class should override with functionality  Virtual function will hold the part of functionality the drive class may or may not override it according to the requirement   You can decide with your requirement

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An abstract function has no implemention and it can only be declared on an abstract class  This forces the derived class to provide an implementation    A virtual function provides a default implementation and it can exist on either an abstract class or a non-abstract class    So for example   public abstract class myBase         If you derive from this class you must implement this method  notice we have no method body here either     public abstract void YouMustImplement           If you derive from this class you can change the behavior but are not required to     public virtual void YouCanOverride                   public class MyBase        This will not compile because you cannot have an abstract method in a non-abstract class     public abstract void YouMustImplement

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You basically use a virtual method when you want the inheritors to extend the functionality IF they want to   You use abstract methods when you want the inheritors to implement the functionality  and in this case they have no choice

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Abstract function cannot have a body and MUST be overridden by child classes  Virtual Function will have a body and may or may not be overridden by child classes

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Abstract function method       An abstract method is a method which is declared with the keyword abstract     It does not have body     It should be implemented by the derived class     If a method is abstract then the class should abstract   virtual function method       A virtual method is the method which is declared with the keyword virtual and it can be overridden by the derived class method by using override keyword     It s up to the derived class whether to override it or not

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An abstract function cannot have functionality  You re basically saying  any child class MUST give their own version of this method  however it s too general to even try to implement in the parent class    A virtual function  is basically saying look  here s the functionality that may or may not be good enough for the child class  So if it is good enough  use this method  if not  then override me  and provide your own functionality

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You must always override an abstract function   Thus    Abstract functions - when the inheritor must provide its own implementation Virtual - when it is up to the inheritor to decide

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Here I am writing some sample code hoping this may be a rather tangible example to see the behaviors of the interfaces  abstract classes and ordinary classes on a very basic level  You can also find this code in github as a project if you want to use it as a demo  https   github com usavas JavaAbstractAndInterfaceDemo   public interface ExampleInterface          public void MethodBodyInInterfaceNotPossible                void MethodInInterface        public abstract class AbstractClass       public abstract void AbstractMethod               public abstract void AbstractMethodWithBodyNotPossible                               Standard Method CAN be declared in AbstractClass     public void StandardMethod            System out println  Standard Method in AbstractClass  super  runs             public class ConcreteClass     extends AbstractClass     implements ExampleInterface         Abstract Method HAS TO be IMPLEMENTED in child class  Implemented by ConcreteClass      Override     public void AbstractMethod             System out println  AbstractMethod overridden runs                 Standard Method CAN be OVERRIDDEN       Override     public void StandardMethod             super StandardMethod            System out println  StandardMethod overridden in ConcreteClass runs               public void ConcreteMethod            System out println  Concrete method runs                 A method in interface HAS TO be IMPLEMENTED in implementer class       Override     public void MethodInInterface             System out println  MethodInInterface Implemented by ConcreteClass runs               Cannot declare abstract method in a concrete class           public abstract void AbstractMethodDeclarationInConcreteClassNotPossible

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If a class derives from this abstract class  it is then forced to override the abstract member  This is different from the virtual modifier  which specifies that the member may optionally be overridden

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To my understanding   Abstract Methods    Only the abstract class can hold abstract methods  Also the derived class need to implement the method and no implementation is provided in the class   Virtual Methods   A class can declare these and also provide the implementation of the same  Also the derived class need to implement of the method to override it

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There are nothing call virtual class in C    For functions   Abstract function only have signature only the drive class should override with functionality  Virtual function will hold the part of functionality the drive class may or may not override it according to the requirement   You can decide with your requirement

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You basically use a virtual method when you want the inheritors to extend the functionality IF they want to   You use abstract methods when you want the inheritors to implement the functionality  and in this case they have no choice

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From general object oriented view    Regarding abstract method  When you put an abstract method in the parent class actually your are saying to the child classes  Hey note that you have a method signature like this  And if you wanna to use it you should implement your own    Regarding virtual function  When you put a virtual method in the parent class you are saying to the derived classes   Hey there is a functionality here that do something for you  If this is useful for you just use it  If not  override this and implement your code  even you can use my implementation in your code     this is some philosophy about different between this two concept in General OO

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Abstract method doesnt have an implementation It is declared in the parent class  The child class is resposible for implementing that method   Virtual method should have an implementation in the parent class and it facilitates the child class to make the choice whether to use that implementation of the parent class or to have a new implementation for itself for that method in child class

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An abstract function cannot have functionality  You re basically saying  any child class MUST give their own version of this method  however it s too general to even try to implement in the parent class    A virtual function  is basically saying look  here s the functionality that may or may not be good enough for the child class  So if it is good enough  use this method  if not  then override me  and provide your own functionality

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The answer has been provided a number of times but the the question about when to use each is a design-time decision  I would see it as good practice to try to bundle common method definitions into distinct interfaces and pull them into classes at appropriate abstraction levels  Dumping a common set of abstract and virtual method definitions into a class renders the class unistantiable when it may be best to define a non-abstract class that implements a set of concise interfaces  As always  it depends on what best suits your applications specific needs

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An abstract function is  just  a signature  without an implementation  It is used in an interface to declare how the class can be used  It must be implemented in one of the derived classes   Virtual function  method actually   is a function you declare as well  and should implemented in one of the inheritance hierarchy classes   The inherited instances of such class  inherit the implementation as well  unless you implement it  in a lower hierarchy class

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Abstract method doesnt have an implementation It is declared in the parent class  The child class is resposible for implementing that method   Virtual method should have an implementation in the parent class and it facilitates the child class to make the choice whether to use that implementation of the parent class or to have a new implementation for itself for that method in child class

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An abstract function or method is a public  operation s name  exposed by a class  its aim  along with abstract classes  is primarily provide a form of constraint in objects design against the structure that an object have to implement    In fact the classes that inherit from its abstract class have to give an implementation to this method  generally compilers raise errors when they don t   Using abstract classes and methods is important mostly to avoid that by focusing on implementation details when designing classes  the classes structure be too related to the implementations  so creating dependences and coupling between classes that collaborate among them   A virtual function or method is simply a method that models a public behaviour of a class  but that we can leave free to modify it in the inheritance chain  because we think that child classes could have need to implement some specific extensions for that behaviour   They both represent a form of polymorpfhism in object orientation paradigm    We can use abstract methods and virtual functions together to support a good inheritance model   We design a good abstract structure of main objects of our solution  then create basic implementations by locating those more prone to further specializations and we make these ones as virtuals  finally we specialize our basic implementations  eventyually  overriding  inherited virtual ones

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explanation  with analogies  hopefully it will help you   Context  I work on the 21 st floor of a building  And I m paranoid about fire  Every now and again  somewhere in the world  a fire is burning down a sky scraper  But luckily we have an instruction manual somewhere here on what to do in case of fire   FireEscape     Don t collect belongings Walk to fire escape Walk out of building   This is basically a virtual method called FireEscape    Virtual Method  This plan is pretty good for 99  of the circumstances  It s a basic plan which works  But there is a 1  chance that the fire escape is blocked or damaged in which case you are completely screwed and you ll become toast unless you take some drastic action  With virtual methods you can do just that  you can override the basic FireEscape   plan with your own version of the plan    Run to window Jump out the window Parachute safely to the bottom   In other words virtual methods provide a basic plan  which can be overriden if you need to  Subclasses can override the parent class  virtual method if the programmer deems it appropriate   Abstract methods  Not all organisations are well drilled  Some organisations don t do fire drills  They don t have an overall escape policy  Every man is for himself  Management are only interested in such a policy existing   In other words  each person is forced to develop his own FireEscape   method  One guy will walk out the fire escape  Another guy will parachute  Another guy will use rocket propulsion technology to fly away from the building  Another guy will abseil out  Management don t care how you escape  so long as you have a basic FireEscape   plan - if they don t you can be guaranteed OHS will come down on the organisation like a tonne of bricks  This is what is meant by an abstract method    What s the difference between the two again   Abstract method  sub classes are forced to implement their own FireEscape method  With a virtual method  you have a basic plan waiting for you  but can choose to implement your own if it s not good enough   Now that wasn t so hard was it

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Abstract methods are always virtual  They cannot have an implementation   That s the main difference   Basically  you would use a virtual method if you have the  default  implementation of it and want to allow descendants to change its behaviour   With an abstract method  you force descendants to provide an implementation

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Abstract Function    It can be declared only inside abstract class   It contains only method declaration not the implementation in abstract class   It must be overridden in derived class    Virtual Function    It can be declared inside abstract as well as non abstract class  It contains method implementation  It may be overridden

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I made this simpler by making some improvements on the following classes  from other answers    using System  using System Collections Generic  using System Linq  using System Text   namespace TestOO       class Program               static void Main string   args                        BaseClass  base   new BaseClass                Console WriteLine  Calling virtual method directly                 base SayHello                Console WriteLine  Calling single method directly                 base SayGoodbye                 DerivedClass  derived   new DerivedClass                Console WriteLine  Calling new method from derived class                 derived SayHello                Console WriteLine  Calling overrided method from derived class                 derived SayGoodbye                 DerivedClass2  derived2   new DerivedClass2                Console WriteLine  Calling new method from derived2 class                 derived2 SayHello                Console WriteLine  Calling overrided method from derived2 class                 derived2 SayGoodbye                Console ReadLine                          public class BaseClass               public void SayHello                         Console WriteLine  Hello n                      public virtual void SayGoodbye                         Console WriteLine  Goodbye n                       public void HelloGoodbye                         this SayHello                this SayGoodbye                          public abstract class AbstractClass               public void SayHello                         Console WriteLine  Hello n                          public virtual void SayGoodbye                             Console WriteLine  Goodbye n                        public abstract void SayGoodbye                public class DerivedClass   BaseClass               public new void SayHello                         Console WriteLine  Hi There                       public override void SayGoodbye                         Console WriteLine  See you later                         public class DerivedClass2   AbstractClass               public new void SayHello                         Console WriteLine  Hi There                         We should use the override keyword with abstract types           public new void SayGoodbye                             Console WriteLine  See you later2                        public override void SayGoodbye                         Console WriteLine  See you later

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Most of the above examples use code - and they are very very good  I need not add to what they say  but the following is a simple explanation that makes use of analogies rather than code technical terms   Simple Explanation - Explanation using analogies  Abstract Method  Think George W Bush  He says to his soldiers   Go fight in Iraq   And that s it  All he has specified is that fighting must be done  He does not specify how exactly that will happen  But I mean  you can t just go out and  fight   what does that mean exactly  do I fight with a B-52 or my derringer  Those specific details are left to someone else  This is an abstract method   Virtual Method  David Petraeus is high up in the army  He has defined what fight means    Find the enemy Neutralise him  Have a beer afterwards   The problem is that it is a very general method  It s a good method that works  but sometimes is not specific enough  Good thing for Petraeus is that his orders have leeway and scope - he has allowed others to change his definition of  fight   according to their particular requirements   Private Job Bloggs reads Petraeus  order and is given permission to implement his own version of fight  according to his particular requirements    Find enemy  Shoot him in the head  Go Home Have beer    Nouri al Maliki also receives the same orders from Petraeus  He is to fight also  But he is a politician  not an infantry man  Obviously he cannot go around shooting his politican enemies in the head  Because Petraeus has given him a virtual method  then Maliki can implement his own version of the fight method  according to his particular circumstances     Find enemy  Have him arrested with some BS trumped up charges  Go Home Have beer    IN other words  a virtual method provides boilerplate instructions - but these are general instructions  which can be made more specific by people down the army heirarchy  according to their particular circumstances   The difference between the two   George Bush does not prove any implementation details  This must be provided by someone else  This is an abstract method  Petraeus on the other hand does provide implementation details but he has given permission for his subordinates to override his orders with their own version  if they can come up with something better    hope that helps

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Abstract Function    It can be declared only inside abstract class   It contains only method declaration not the implementation in abstract class   It must be overridden in derived class    Virtual Function    It can be declared inside abstract as well as non abstract class  It contains method implementation  It may be overridden

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An abstract function cannot have functionality  You re basically saying  any child class MUST give their own version of this method  however it s too general to even try to implement in the parent class    A virtual function  is basically saying look  here s the functionality that may or may not be good enough for the child class  So if it is good enough  use this method  if not  then override me  and provide your own functionality

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An abstract function cannot have functionality  You re basically saying  any child class MUST give their own version of this method  however it s too general to even try to implement in the parent class    A virtual function  is basically saying look  here s the functionality that may or may not be good enough for the child class  So if it is good enough  use this method  if not  then override me  and provide your own functionality

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Virtual Method    Virtual means we CAN override it   Virtual Function has an implementation  When we inherit the class we can override the virtual function and provide our own logic    We can change the return type of Virtual function while implementing the function in the child class which can be said as a concept of Shadowing     Abstract Method   Abstract means we MUST override it   An abstract function has no implementation and must be in an abstract class  It can only be declared  This forces the derived class to provide the implementation of it   An abstract member is implicitly virtual  The abstract can be called as pure virtual in some of the languages   public abstract class BaseClass        protected abstract void xAbstractMethod         public virtual void xVirtualMethod                 var x   3   4

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Abstract function cannot have a body and MUST be overridden by child classes  Virtual Function will have a body and may or may not be overridden by child classes

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Only abstract classes can have abstract members  A non-abstract class that inherits from an abstract class must override its abstract members  An abstract member is implicitly virtual  An abstract member cannot provide any implementation  abstract is called pure virtual in some languages

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Binding is the process of mapping a name to a unit of code   Late binding means that we use the name  but defer the mapping  In other words  we create mention the name first  and let some subsequent process handle the mapping of code to that name   Now consider    Compared to humans  machines are really good at searching and sorting Compared to machines  humans are really good at invention and innovation   So  the short answer is  virtual is a late binding instruction for the machine  runtime  whereas abstract is the late binding instruction for the human  programmer   In other words  virtual means      Dear runtime  bind the appropriate code to this name by doing what you do best  searching     Whereas abstract means      Dear programmer  please bind the appropriate code to this name by doing what you do best  inventing     For the sake of completeness  overloading means      Dear compiler  bind the appropriate code to this name by doing what you do best  sorting

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explanation  with analogies  hopefully it will help you   Context  I work on the 21 st floor of a building  And I m paranoid about fire  Every now and again  somewhere in the world  a fire is burning down a sky scraper  But luckily we have an instruction manual somewhere here on what to do in case of fire   FireEscape     Don t collect belongings Walk to fire escape Walk out of building   This is basically a virtual method called FireEscape    Virtual Method  This plan is pretty good for 99  of the circumstances  It s a basic plan which works  But there is a 1  chance that the fire escape is blocked or damaged in which case you are completely screwed and you ll become toast unless you take some drastic action  With virtual methods you can do just that  you can override the basic FireEscape   plan with your own version of the plan    Run to window Jump out the window Parachute safely to the bottom   In other words virtual methods provide a basic plan  which can be overriden if you need to  Subclasses can override the parent class  virtual method if the programmer deems it appropriate   Abstract methods  Not all organisations are well drilled  Some organisations don t do fire drills  They don t have an overall escape policy  Every man is for himself  Management are only interested in such a policy existing   In other words  each person is forced to develop his own FireEscape   method  One guy will walk out the fire escape  Another guy will parachute  Another guy will use rocket propulsion technology to fly away from the building  Another guy will abseil out  Management don t care how you escape  so long as you have a basic FireEscape   plan - if they don t you can be guaranteed OHS will come down on the organisation like a tonne of bricks  This is what is meant by an abstract method    What s the difference between the two again   Abstract method  sub classes are forced to implement their own FireEscape method  With a virtual method  you have a basic plan waiting for you  but can choose to implement your own if it s not good enough   Now that wasn t so hard was it

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I made this simpler by making some improvements on the following classes  from other answers    using System  using System Collections Generic  using System Linq  using System Text   namespace TestOO       class Program               static void Main string   args                        BaseClass  base   new BaseClass                Console WriteLine  Calling virtual method directly                 base SayHello                Console WriteLine  Calling single method directly                 base SayGoodbye                 DerivedClass  derived   new DerivedClass                Console WriteLine  Calling new method from derived class                 derived SayHello                Console WriteLine  Calling overrided method from derived class                 derived SayGoodbye                 DerivedClass2  derived2   new DerivedClass2                Console WriteLine  Calling new method from derived2 class                 derived2 SayHello                Console WriteLine  Calling overrided method from derived2 class                 derived2 SayGoodbye                Console ReadLine                          public class BaseClass               public void SayHello                         Console WriteLine  Hello n                      public virtual void SayGoodbye                         Console WriteLine  Goodbye n                       public void HelloGoodbye                         this SayHello                this SayGoodbye                          public abstract class AbstractClass               public void SayHello                         Console WriteLine  Hello n                          public virtual void SayGoodbye                             Console WriteLine  Goodbye n                        public abstract void SayGoodbye                public class DerivedClass   BaseClass               public new void SayHello                         Console WriteLine  Hi There                       public override void SayGoodbye                         Console WriteLine  See you later                         public class DerivedClass2   AbstractClass               public new void SayHello                         Console WriteLine  Hi There                         We should use the override keyword with abstract types           public new void SayGoodbye                             Console WriteLine  See you later2                        public override void SayGoodbye                         Console WriteLine  See you later

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An abstract function has no implemention and it can only be declared on an abstract class  This forces the derived class to provide an implementation    A virtual function provides a default implementation and it can exist on either an abstract class or a non-abstract class    So for example   public abstract class myBase         If you derive from this class you must implement this method  notice we have no method body here either     public abstract void YouMustImplement           If you derive from this class you can change the behavior but are not required to     public virtual void YouCanOverride                   public class MyBase        This will not compile because you cannot have an abstract method in a non-abstract class     public abstract void YouMustImplement

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An abstract function has no implemention and it can only be declared on an abstract class  This forces the derived class to provide an implementation    A virtual function provides a default implementation and it can exist on either an abstract class or a non-abstract class    So for example   public abstract class myBase         If you derive from this class you must implement this method  notice we have no method body here either     public abstract void YouMustImplement           If you derive from this class you can change the behavior but are not required to     public virtual void YouCanOverride                   public class MyBase        This will not compile because you cannot have an abstract method in a non-abstract class     public abstract void YouMustImplement

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An abstract method is a method that must be implemented to make a concrete class  The declaration is in the abstract class  and any class with an abstract method must be an abstract class  and it must be implemented in a concrete class   A virtual method is a method that can be overridden in a derived class using the override  replacing the behavior in the superclass  If you don t override  you get the original behavior  If you do  you always get the new behavior  This opposed to not virtual methods  that can not be overridden but can hide the original method  This is done using the new modifier   See the following example   public class BaseClass       public void SayHello                 Console WriteLine  Hello                public virtual void SayGoodbye                 Console WriteLine  Goodbye               public void HelloGoodbye                 this SayHello            this SayGoodbye              public class DerivedClass   BaseClass       public new void SayHello                 Console WriteLine  Hi There                public override void SayGoodbye                 Console WriteLine  See you later              When I instantiate DerivedClass and call SayHello  or SayGoodbye  I get  Hi There  and  See you later   If I call HelloGoodbye  I get  Hello  and  See you later   This is because SayGoodbye is virtual  and can be replaced by derived classes  SayHello is only hidden  so when I call that from my base class I get my original method   Abstract methods are implicitly virtual  They define behavior that must be present  more like an interface does

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An abstract function is  just  a signature  without an implementation  It is used in an interface to declare how the class can be used  It must be implemented in one of the derived classes   Virtual function  method actually   is a function you declare as well  and should implemented in one of the inheritance hierarchy classes   The inherited instances of such class  inherit the implementation as well  unless you implement it  in a lower hierarchy class

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From general object oriented view    Regarding abstract method  When you put an abstract method in the parent class actually your are saying to the child classes  Hey note that you have a method signature like this  And if you wanna to use it you should implement your own    Regarding virtual function  When you put a virtual method in the parent class you are saying to the derived classes   Hey there is a functionality here that do something for you  If this is useful for you just use it  If not  override this and implement your code  even you can use my implementation in your code     this is some philosophy about different between this two concept in General OO

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From a C   background  C  virtual corresponds to C   virtual  while C  abstract methods corresponds to C   pure virtual function

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Virtual Method    Virtual means we CAN override it   Virtual Function has an implementation  When we inherit the class we can override the virtual function and provide our own logic    We can change the return type of Virtual function while implementing the function in the child class which can be said as a concept of Shadowing     Abstract Method   Abstract means we MUST override it   An abstract function has no implementation and must be in an abstract class  It can only be declared  This forces the derived class to provide the implementation of it   An abstract member is implicitly virtual  The abstract can be called as pure virtual in some of the languages   public abstract class BaseClass        protected abstract void xAbstractMethod         public virtual void xVirtualMethod                 var x   3   4

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Abstract method  When a class contains an abstract method  that class must be declared as abstract  The abstract method has no implementation and thus  classes that derive from that abstract class  must provide an implementation for this abstract method   Virtual method  A class can have a virtual method   The virtual method has an implementation  When you inherit from a class that has a virtual method  you can override the virtual method and provide additional logic  or replace the logic with your own implementation   When to use what  In some cases  you know that certain types should have a specific method  but  you don t know what implementation this method should have  In such cases  you can create an interface which contains a method with this signature  However  if you have such a case  but you know that implementors of that interface will also have another common method  for which you can already provide the implementation   you can create an abstract class  This abstract class then contains the abstract method  which must be overriden   and another method which contains the  common  logic   A virtual method should be used if you have a class which can be used directly  but for which you want inheritors to be able to change certain behaviour  although it is not mandatory

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You must always override an abstract function   Thus    Abstract functions - when the inheritor must provide its own implementation Virtual - when it is up to the inheritor to decide

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Most of the above examples use code - and they are very very good  I need not add to what they say  but the following is a simple explanation that makes use of analogies rather than code technical terms   Simple Explanation - Explanation using analogies  Abstract Method  Think George W Bush  He says to his soldiers   Go fight in Iraq   And that s it  All he has specified is that fighting must be done  He does not specify how exactly that will happen  But I mean  you can t just go out and  fight   what does that mean exactly  do I fight with a B-52 or my derringer  Those specific details are left to someone else  This is an abstract method   Virtual Method  David Petraeus is high up in the army  He has defined what fight means    Find the enemy Neutralise him  Have a beer afterwards   The problem is that it is a very general method  It s a good method that works  but sometimes is not specific enough  Good thing for Petraeus is that his orders have leeway and scope - he has allowed others to change his definition of  fight   according to their particular requirements   Private Job Bloggs reads Petraeus  order and is given permission to implement his own version of fight  according to his particular requirements    Find enemy  Shoot him in the head  Go Home Have beer    Nouri al Maliki also receives the same orders from Petraeus  He is to fight also  But he is a politician  not an infantry man  Obviously he cannot go around shooting his politican enemies in the head  Because Petraeus has given him a virtual method  then Maliki can implement his own version of the fight method  according to his particular circumstances     Find enemy  Have him arrested with some BS trumped up charges  Go Home Have beer    IN other words  a virtual method provides boilerplate instructions - but these are general instructions  which can be made more specific by people down the army heirarchy  according to their particular circumstances   The difference between the two   George Bush does not prove any implementation details  This must be provided by someone else  This is an abstract method  Petraeus on the other hand does provide implementation details but he has given permission for his subordinates to override his orders with their own version  if they can come up with something better    hope that helps

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An abstract function or method is a public  operation s name  exposed by a class  its aim  along with abstract classes  is primarily provide a form of constraint in objects design against the structure that an object have to implement    In fact the classes that inherit from its abstract class have to give an implementation to this method  generally compilers raise errors when they don t   Using abstract classes and methods is important mostly to avoid that by focusing on implementation details when designing classes  the classes structure be too related to the implementations  so creating dependences and coupling between classes that collaborate among them   A virtual function or method is simply a method that models a public behaviour of a class  but that we can leave free to modify it in the inheritance chain  because we think that child classes could have need to implement some specific extensions for that behaviour   They both represent a form of polymorpfhism in object orientation paradigm    We can use abstract methods and virtual functions together to support a good inheritance model   We design a good abstract structure of main objects of our solution  then create basic implementations by locating those more prone to further specializations and we make these ones as virtuals  finally we specialize our basic implementations  eventyually  overriding  inherited virtual ones

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If a class derives from this abstract class  it is then forced to override the abstract member  This is different from the virtual modifier  which specifies that the member may optionally be overridden

[oop] What is the difference between an abstract function and a virtual function?

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