What are virtual classes in c plus plus?

Answer 1

A local class refers to a class which has been defined within a function.

For example, consider a function A :

int A(int d)

{

class X

{

int a,b;

void func();

public :

int func2()

{

//do something

}

}

//rest of the function.

}

Class X is local to the function A, and can only be instantiated inside the function A. Use of such functions are usually indicative of poor function design.

Answer 2

Virtual classes are base classes that are shared between two or more classes which are themselves base classes in a multiple inheritance hierarchy. Consider the following:

struct A {};

struct B : A {};

struct C : A {};

struct D : B, C {};

Class A serves as a common base class for B and C which are inherited by D. Thus class D has two instances of class A. That is; D::B::A and D::C::A are explicit instances of A. This creates ambiguity when implicitly referring to A's members since the compiler cannot determine which instance to refer to. The ambiguity can be eliminated by referring to the explicit instance, however it is often possible for both B and C to share the same instance of A. This is achieved by declaring A as a virtual base class of B and C.

struct A {};

struct B : virtual A {};

struct C : virtual A {};

struct D : B, C {};

When you declare a virtual base class, it automatically becomes a direct ancestor of the most-derived class. The most-derived class is responsible for constructing the virtual class. All classes that inherit from the virtual class then inherit from this single virtual instance.

Thus if you declare an instance of B, B is the most-derived class (so it behaves just as if A were declared non-virtual). But if you declare an instance of D, D becomes the most-derived class.

Ideally, a virtual base class should have no data attributes since its purpose is to provide a common interface to both B and C. If A does have data attributes, keep in mind that modifying the internal state of class A will affect both B and C. Typically, class D will override all virtual methods of class A and coordinate any explicit calls to those overrides in either A, B or C.