虚函数

• what is virtual function?

Virtual Functions

WHAT IS A VIRTUAL FUNCTION?A virtual function is a member function of a class, whose functionality can be over-ridden in its derived classes. It is one that is declared as virtual in the base class using the virtual keyword. The virtual nature is inherited in the subsequent derived classes and the virtual keyword need not be re-stated there. The whole function body can be replaced with a new set of implementation in the derived class.
What is Binding?Binding refers to the act of associating an object or a class with its member. If we can call a method fn() on an object o of a class c, we say that the object o is binded with the method fn(). This happens at compile time and is known as static or compile - time binding. The calls to the virtual member functions are resolved during run-time. This mechanism is known as dynamic binding. The most prominent reason why a virtual function will be used is to have a different functionality in the derived class. The difference between a non-virtual member function and a virtual member function is, the non-virtual member functions are resolved at compile time.
How does a Virtual Function work?Whenever a program has a virtual function declared, a v - table is constructed for the class. The v-table consists of addresses to the virtual functions for classes that contain one or more virtual functions. The object of the class containing the virtual function contains a virtual pointer that points to the base address of the virtual table in memory. Whenever there is a virtual function call, the v-table is used to resolve to the function address. An object of the class that contains one or more virtual functions contains a virtual pointer called the vptr at the very beginning of the object in the memory. Hence the size of the object in this case increases by the size of the pointer. This vptr contains the base address of the virtual table in memory. Note that virtual tables are class specific, i.e., there is only one virtual table for a class irrespective of the number of virtual functions it contains. This virtual table in turn contains the base addresses of one or more virtual functions of the class. At the time when a virtual function is called on an object, the vptr of that object provides the base address of the virtual table for that class in memory. This table is used to resolve the function call as it contains the addresses of all the virtual functions of that class. This is how dynamic binding is resolved during a virtual function call.
The following code shows how we can write a virtual function in C++ and then use the same to achieve dynamic or runtime polymorphism.

#include 
class base
{

public:

virtual void display()

{

 cout<<”\nBase”;

}

};
class derived : public base
{

 public:

 void display()

 {

    cout<<”\nDerived”;

 }

};

void main()
{

 base *ptr = new derived();

 ptr->display();

}

In the above example, the pointer is of type base but it points to the derived class object. The method display() is virtual in nature. Hence in order to resolve the virtual method call, the context of the pointer is considered, i.e., the display method of the derived class is called and not that of the base. If the method was non virtual in nature, the display() method of the base class would have been called.
What is Virtual Constructors and DestructorsA 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.
A virtual destructor is one that is declared as virtual in the base class and is used to ensure that destructors are called in the proper order. It is to be remembered that destructors are called in the reverse order of inheritance. If a base class pointer points to a derived class object and we some time later use the delete operator to delete the object, then the derived class destructor is not called. Refer to the code that follows:

#include 
class base
{

  public:

  ~base() { }

};

class derived : public base
{

  public:

~derived()  {  }

};

void main()
{

  base *ptr = new derived();
  // some code
  delete ptr;
}

In this case the type of the pointer would be considered. Hence as the pointer is of type base, the base class destructor would be called but the derived class destructor would not be called at all. The result is memory leak. In order to avoid this, we have to make the destructor virtual in the base class. This is shown in the example below:

#include 
class base
{

public:

virtual ~base(){}

};

class derived : public base
{

  public:

  ~derived()  {  }

};

void main()
{

  base *ptr = new derived();
  // some code
  delete ptr;
}

Conclusion:
Virtual methods should be used judiciously as they are slow due to the overhead involved in searching the virtual table. They also increase the size of an object of a class by the size of a pointer. The size of a pointer depends on the size of an integer. Note that in DOS based systems the size of a pointer is 2 bytes whereas in UNIX based systems it is 4 bytes.