virtual

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cppreference.com > C/C++ Keywords > virtual

virtual

Syntax:

  virtual return-type name( parameter-list );
  virtual return-type name( parameter-list ) = 0;

The virtual keyword can be used to create virtual functions, which can be overridden by derived classes.

A virtual function indicates that a function can be overridden in a subclass, and that the overridden function will actually be used.
When a base object pointer points to a derived object that contains a virtual function, the decision about which version of that function to call is based on the type of object pointed to by the pointer, and this process happens at runtime.
A base object can point to different derived objects and have different versions of the virtual function run.

If the function is specified as a pure virtual function (denoted by the = 0), it must be overridden by a derived class.

Example code:

For example, the following code snippet shows how a child class can override a virtual method of its parent, and how a non-virtual method in the parent cannot be overridden:

class Base {
public:
 void nonVirtualFunc() {
   cout << "Base: non-virtual function" << endl;
 }
 virtual void virtualFunc() {
   cout << "Base: virtual function" << endl;
 }
};              

class Child : public Base {
public:
 void nonVirtualFunc() {
   cout << "Child: non-virtual function" << endl;
 }
 void virtualFunc() {
   cout << "Child: virtual function" << endl;
 }
};              

int main() {
 Base* basePointer = new Child();
 basePointer->nonVirtualFunc();
 basePointer->virtualFunc();
 return 0;
}

When run, the above code displays:

Base: non-virtual function
Child: virtual function