Why provide a declaration in the derived class for a virtual base class function

Code:

---

#include <iostream>
using namespace std;

class X
{
  public:
  X(){}

  virtual void f1()
  {
    cout << "In Base" << endl;
  }
 
  void f2()
  {
    cout << "F2 in Base" << endl;
  }
};

class Y : public X
{
  public:
  Y(){}
//  void f1(); If I uncomment this.. it works.. but why does it demand this declaration.

};

void Y::f1()
{
  cout << "F1 In Derived" << endl;
  // Since f1() is a virtual function in base,
  // why does it demand a declaration in derived too????
}

int main()
{
  Y ob;
  ob.f1();

  return 0;
}

---

Error Message:
"test.C", line 29: Error: f1() is not a member of Y.
1 Error(s) detected.

I found that by not making f1() virtual and commenting out the definition of Y::f1(), that program will compile in Visual Studio 6.0.

I think that when you declare a virtual function in a base class, it is necessary to declare and define it in the derived class. As for why the declaration is necessary, I think it's simply because the compiler is not a mind reader.

You get an error because, when that line is commented out, your declaration of class Y contains no mention of a f1 function yet you are trying to define one. Yes, the base class already has an f1 function but it doesn't matter, that is the base class f1, not the derived f1 you are trying to create. If you comment that line, you are saying that the derived class does not have an f1 of its own that overrides the virtual f1 defined by the base and so the compiler complains about your attempt at defining one.