If the library adds new class Derived4 , the library modifier will extend the if-else ladder inside Create to return proper object. Consumers of User need not recompile their code due to extension of Base. Note that the function Create used to return different types of Base class objects at runtime.
It acts like virtual constructor, also referred as Factory Method in pattern terminology. The pattern world demonstrates different ways to implement the above concept. Also, there are some potential design issues with the above code. Our objective is to provide some insights into virtual construction, creating objects dynamically based on some input. We have excellent books devoted to the subject, the interested reader can refer them for more information. 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. If you had one it would act like placement new. Joshua Then What's the use case? Pete Kirkham Pete Kirkham Well put. I was wondering how the question even made sense until I read this explanation of how object creation works in other languages. See users. How are vtable updated? Do they really need vptr to be updated? GCC's variable-length array extension , alloca - but leads to significant inefficiencies and complexities e.
Community Bot 1 1 1 silver badge. Tony Delroy Tony Delroy The difference is that these clone and create functions don't work with containers, don't work with pass by value, etcetera. So they don't achieve what we want -- polymorphism without slicing while preserving ordinary value semantics.
DavidSchwartz: clone and create don't work directly with Standard containers, but it's trivial to write a small management type that clone s from the copy constructor etc e. Such management objects can also be passed around by value if you find that easier than using references.
It's hardly trivial. The link goes to code that's already pretty complicated, and it's not enough to even make the standard containers work. Also, since it's not part of the language, it will be very difficult to make code that uses such a thing interoperate with code that doesn't. From this link: docwiki.
Marius Marius 3, 1 1 gold badge 27 27 silver badges 34 34 bronze badges. They exist since the loading of the code and static data of the executable. Correct, they are defined static and constant, but just not allocated and set up. They are setup at program startup. Rich No. Virtual function work in constructors exactly as anywhere else. Virtual function calls are always based on the dynamic type of the object. Rich No: inside the base class ctor, a virtual call in a constructor will dynamically call the base class version, based on the dynamic type at the time: the base class.
The virtual calls on the object under construction work the same whether they are in the ctor body or in any other function call by the ctor. The dynamic type of a base class subobject changes as construction of the derived class starts. Show 4 more comments. So, if we try to declare virtual constructor compiler throw an Error: Constructors cannot be declared virtual. UpAndAdam 4, 2 2 gold badges 25 25 silver badges 41 41 bronze badges. Neha kumari Neha kumari 71 1 1 silver badge 2 2 bronze badges.
Now a logical answer to this question according to me is: The major use of virtual keyword is to enable polymorphic behaviour when we don't know what type of the object the base class pointer will point to. Kushan Mehta Kushan Mehta 1 1 silver badge 11 11 bronze badges. Shraddha Sinha Shraddha Sinha 61 1 1 silver badge 1 1 bronze badge. Quick search will yield the answer: stackoverflow. Edouard A. Navneet Agarwal Navneet Agarwal 41 1 1 bronze badge. Tunvir Rahman Tusher Tunvir Rahman Tusher 5, 2 2 gold badges 33 33 silver badges 30 30 bronze badges.
For virtual destructor try this link. Rich Rich 3, 3 3 gold badges 22 22 silver badges 31 31 bronze badges. If we use a base class pointer to point to the base class object and call a virtual function using it.
Then it would be late binding. But, will that be a run time polymorphism? Constructors are not static, period. This is obvious nonsense. Need of virtual functions — Lets take a small example of a system which tracks information about multiple sports. In this hierarchy, the class Player is not a actual class. He always plays some game. So Player is an abstract entity, which does not exist. It is an abstract class. The RugbyPlayer, CricketPlayer are concrete entities i. The classes derived from Player may have some information to print.
So we have a Print routine in our Player class. The concrete classes will specialize this and write code to Print their own data. Now suppose we have a class Squad. This class contains a list of Players in the Squad.
The list will hold pointer of Player objects. At runtime we insert the objects in the list as per the type of game. We need to Print the Players in the squad. So we go through the list and call the Print routine of every Player in the list. This is fine, but we need to Print all the information of the Player which is game specific.
Hence we make the Print routine in Player as virtual. Now when Print is called on Player the proper Print routine will get called as per the type of Player.
The virtual function mechanism enables to call the proper routine as per the type of object the pointer points to. Need of virtual destructors — At the end of the program Squad needs to be destroyed. While destroying the Squad, as good programmers, we will cleanup the contents list. We will delete the Player in the list. Break free from the situation of mixed structure , focus In the database bit This Boolean type field , Very practical , But in c When reading from , Many people like to make mistakes , This will cause an error in operation.
ToInt16 read Control resources. String resources. Drawable Resources, etc It's understandable to store all resources according to rules Random recommendation mysql Open remote connection modify my.
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