Thread: 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.