What is virtual keyword and how to implement it in interface using Visual Studio

I would like to know what is virtual keyword in Visual Studio C++? How to implement it in interface? I recently saw some code like this:

Code:

---

class A
{
public:
virtual ~A();
}

class B : public A
{
public:
virtual ~B();
}

class C: public B
{
public:
virtual ~C();
}

---

I was just thinking why the destructor over here is declared as virtual. Was that required?

A base class method is defined as "virtual" if we want it to be overriden. However if you want to avoid this then use "sealed" keyword. You can declare a class method "virtual" but not a class member. A class member may be overriden even though the virtual keyword is used. You may however use "override" keyword to override virtual method.

Just to summarize some points to what opaper said:

• virtual is used when a method is to be defined as virtual.
• override is used when a method has to override a virtual method.
• abstract is used with virtual (or override) when a method has to be defined without implementation.
• final is used with virtual or override to force a method not to be overridden.

Visual Studio C++ virtual function is a member function of a class, whose functionality can be over-ridden in its derived classes. A virtual function is a member function of a class and is declared with virtual keyword. A virtual function has a different functionality in the derived class and it is resolved at run-time.