A constructor cannot be virtual because at the time when the constructor is invoked the virtual table would not be available in the memory. Hence we cannot have a virtual constructor. No, constructors cannot be virtual. We can't override a constructor.
When any method is declared as virtual it should be overridden in its derived class. Since constructor is used for initialising the variables and if declare a constructor as virtual it cant be overridden. Static member methods, member function templates and constructors cannot be virtual. Static member functions, member function templates and constructors cannot be virtual. You cannot.
Constructors are specific to the class in which they are declared. They cannot be inherited and so they cannot be virtual. There is no such thing as a virtual constructor. Constructors are local to the class in which they are declared, and therefore cannot be inherited. Derived classes will call the default constructor of the least-derived base class first, unless a specific base class construction overload is called via the derived class's initialisation list.
All base class constructors required by derived classes must be declared protected or public private constructors are inaccessible , but no constructor can be declared virtual. Destructors only need to be declared virtual when there are one or more virtual or pure-virtual methods declared in the class. Derived classes do not need to declare virtual destructors since it is implied by their base class. The only reason for having virtual destructors is to ensure the most-derived class destructor is called first whenever its base class falls from scope.
Destruction then cascades up the hierarchy to the base class, which must be destroyed last. The short answer is no, you cannot. The long answer is that constructors are not functions that can be called directly, and overriding a base class constructor would have no practical meaning since the derived class is itself responsible for calling its own base class constructors whether implied by omission or explicitly via the derived class' initialisation list.
Even so, the derived class isn't calling the base class constructor directly that's why constructors have no return value; the actual call is made behind the scenes. The base class itself may be derived in which case its base class must be constructed before it can be constructed.
This is the complete reverse of how a virtual function behaves, and is the reason that destructors can be virtual but constructors cannot. When a base class is destroyed, all its derivatives must be destroyed first, starting with the most-derived class of object. This can only be achieved through virtual destruction. Constructors are not functions as such, they merely describe the initialisation phase of a class.
Since they are only of relevance to the classes in which they are declared, there is no need nor want to inherit them. Every class must define its own constructors, which can include calls to any base class constructors from within the derived class initialisation list if no list is declared, or a base class constructor omitted, then the default constructor is called instead.
Class destructors are also not functions as such, but if the class has any virtual methods, or the class is not prevented from acting as a base class, the destructor must be virtual.
This is simply to ensure that when a base class object is destroyed, its derived class destructor is called first. If the destructor were not virtual, destroying a base class would not destroy the derived class, leaving it in an undefined state. Any method inherited through virtual inheritance is visible to the entire class, including its constructors. This includes virtual and pure-virtual methods. However, unless called explicitly, the most-derived implementation of the virtual function will be called.
Explicit calls allow a derived class's implementation to call its base class implementation. No you cant,you can when they are sick but,you cant revive them. Check out our Data Structures in C course to start learning today. How can we create the required type of an object at runtime? For example, see the following sample program.
The class User is utility class trying to make use of the hierarchy. The main function is consuming Base hierarchy functionality via User class. The User class constructor is creating Derived1 object, always. Recompiling is bad way of design, so we can opt for the following approach. Before going into details, let us answer, who will dictate to create either of Derived1 or Derived2 object?
Clearly, it is the consumer of User class. In simple words, if the library updates the Base class hierarchy with new class Derived3. How can the User class creates Derived3 object? The problem is with the creation of objects. Addition of new class to the hierarchy forcing dependents of User class to recompile. By delegating the object creation to class hierarchy or to a static function we can avoid the tight coupling between User and Base hierarchy.
It delegates that responsibility to Base , and provides an input in the form of ID. If the library adds new class Derived4 , the library modifier will extend the if-else ladder inside Create to return proper object.
It is not a duplicate. Add a comment. Active Oldest Votes. The Overflow Blog. Podcast The first ten years of our programming lives. Upcoming Events. Featured on Meta. Now live: A fully responsive profile.
Candidate changes in Moderator Election — review your ballot. Linked 5. Related Hot Network Questions. Stack Overflow works best with JavaScript enabled. Accept all cookies Customize settings.
0コメント