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.

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;
}		
		

Base: non-virtual function
Child: virtual function