What is the significance of virtual polymorphism

It is possible to create a pure virtual function as follows. This has two consquences. First, it is not possible to create an instance of Shape class.

Shape class is now an abstract class. We cannot create an instance of Shape class, since it is missing definition of one of its functions. Second, it a derived from Shape class must implement draw.

Virtual functions play an important role; however, they do come with an overhead. These use more storage. The second reason is that virtual calls can lead to your code breaking if you inherit a class from a library and it changes its internal implementation of a public or private method which calls a virtual method internally without changing the base class behaviour. Show 24 more comments. Steve Steve 27k 12 12 gold badges 61 61 silver badges bronze badges.

Excellent, and gets home quickly and with the use of better examples. This is however, simplistic, and the questioner should really just read the page parashift. Other folks have already pointed to this resource in SO articles linked from this thread, but I believe this is worth re-mentioning.

Wouldn't it be nice if there was one correct word for each concept? Unfortunately, it just ain't so. Even so, I just compiled it in GCC 6. Pointer-to-derived implicitly casts to pointer-to-base more specialized implicitly casts to more general. Anything else - very prone to undefined behavior so make sure you know what you're doing. This is also referred to as "de-virtualization".

Show 12 more comments. Henk Holterman Henk Holterman k 28 28 gold badges silver badges bronze badges. Your example says that the returned string depends on whether the function is virtual, but it doesn't say which result corresponds to virtual and which corresponds to non-virtual. Additionally, it's a little confusing as you're not using the string that's being returned. With Virtual keyword: Woof.

Without Virtual keyword:? HeshamEraqi without virtual it is early binding and it will show "? Add a comment. Virtual Functions are used to support Runtime Polymorphism.

When the virtual function is called by using a Base class pointer, the compiler decides at run-time which version of the function - i. This is called Runtime Polymorphism. Yoon5oo 5 5 silver badges 11 11 bronze badges.

Very bad! The relevant implementation is the one in the most specific most derived class. Samuel Sorial 1 1 silver badge 8 8 bronze badges. Cheers and hth. Alex Martelli Alex Martelli k gold badges silver badges bronze badges. I hate to contradict you, but compile-time polymorphism is still polymorphism. Even overloading non-member functions is a form of polymorphism - ad-hoc polymorphism using the terminology in your link. The difference here is between early and late binding.

Steve, you're pedantically correct as a fellow pedant, I approve that;- -- editing the answer to add the missing adjective;-. Aryaman Gupta Aryaman Gupta 1 1 gold badge 7 7 silver badges 18 18 bronze badges.

It's worse than that. So, it's not just that some memory might leak. Rather, the program is ill-formed, so the compiler may transform it into anything: machine code that happens to work fine, or does nothing, or summons demons from your nose, or etc.

Hence with virtual function you can achieve runtime polymorphism. Without virtual you are not overloading. You are shadowing. If a base class B has one or more functions foo , and the derived class D defines a foo name, that foo hides all of those foo -s in B.

They are reached as B::foo using scope resolution. To promote B::foo functions into D for overloading, you have to use using B::foo. I've my answer in form of a conversation to be a better read: Why do we need virtual functions? Because of Polymorphism. What is Polymorphism? The fact that a base pointer can also point to derived type objects.

Well, through early binding. What is early binding? If it's not what we want to happen, why is this allowed? Because we need Polymorphism! What's the benefit of Polymorphism then? Why do we need virtual functions? Why a different implementation?

M-J M-J 8 8 silver badges 14 14 bronze badges. Quick Answer: It provides us with one of the needed "ingredients" 1 for object oriented programming. It is the fastest more efficient implementation if you need a virtual function call 2. Ziezi Ziezi 6, 3 3 gold badges 34 34 silver badges 46 46 bronze badges. So when you said : But earlier in the book, when learning about basic inheritance, I was able to override base methods in derived classes without using 'virtual'.

Echilon 9, 28 28 gold badges silver badges bronze badges. Kev Kev 1 1 silver badge 2 2 bronze badges. So it doesn't try to perform dynamic dispatch for you, unless you need it. To provide more control to the interface. NOTE: Subtype means base class, and subtyp means inherited class. When we create an object of the derived class, the compiler creates a pointer as a data member of the class containing the address of VTABLE of the derived class.

Similar concept of Late and Early Binding is used as in above example. This article is contributed by Yash Singla. If you like GeeksforGeeks and would like to contribute, you can also write an article using write. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

Skip to content. Change Language. Related Articles. When a virtual function in a base class is a created, there must be definition of a virtual function in the base class even if base class version of the function is never actually called. However pure virtual function are exception. They cannot be static member.

They can be friend function to another class. They are accessed using object pointer. A base pointer can serve as a pointer to derived object since it is type compatible where as a derived object pointer variable to base object. Its prototype in a base class and derived class must be identical for the virtual function to work properly. The class cannot have virtual constructor, but can content virtual destructor.

In fact, virtual destructor are essential to the solution of some problem. It is also possible to have virtual operator overloading. More importantly to realize the potential benefit of virtual function supporting run time polymorphism, they should be declared in the public section of class.