Why do we need virtual functions in C++?

Question

I\'m learning C++ and I\'m just getting into virtual functions.

From what I\'ve read (in the book and online), virtual functions are functions in the base class that

Answer 1

The bottom line is that virtual functions make life easier. Let's use some of M Perry's ideas and describe what would happen if we didn't have virtual functions and instead only could use member-function pointers. We have, in the normal estimation without virtual functions:

 class base {
 public:
 void helloWorld() { std::cout << "Hello World!"; }
  };

 class derived: public base {
 public:
 void helloWorld() { std::cout << "Greetings World!"; }
 };

 int main () {
      base hwOne;
      derived hwTwo = new derived();
      base->helloWorld(); //prints "Hello World!"
      derived->helloWorld(); //prints "Hello World!"

Ok, so that is what we know. Now let's try to do it with member-function pointers:

 #include 
 using namespace std;

 class base {
 public:
 void helloWorld() { std::cout << "Hello World!"; }
 };

 class derived : public base {
 public:
 void displayHWDerived(void(derived::*hwbase)()) { (this->*hwbase)(); }
 void(derived::*hwBase)();
 void helloWorld() { std::cout << "Greetings World!"; }
 };

 int main()
 {
 base* b = new base(); //Create base object
 b->helloWorld(); // Hello World!
 void(derived::*hwBase)() = &derived::helloWorld; //create derived member 
 function pointer to base function
 derived* d = new derived(); //Create derived object. 
 d->displayHWDerived(hwBase); //Greetings World!

 char ch;
 cin >> ch;
 }

While we can do some things with member-function pointers, they aren't as flexible as virtual functions. It is tricky to use a member-function pointer in a class; the member-function pointer almost, at least in my practice, always must be called in the main function or from within a member function as in the above example.

On the other hand, virtual functions, while they might have some function-pointer overhead, do simplify things dramatically.

EDIT: There is another method which is similar by eddietree: c++ virtual function vs member function pointer (performance comparison) .