calling member function without object error: C++ / (G++) oop

Question

Hi,

i was curious to know why the following throws an error in g++ (cannot call member function without object). I suppose a workaround would be to have the B class variable as static variable in A - but i was curious to find out why, when there is an instance of A's child class C created, this still throws an error - many thanks!

#include <iostream>
#include <cstring>

using namespace std;


class B {
public:
  double var;

public:
  friend class A; 
  B() : var(1) { };
  void set(double new_rate);
};


class A {
protected:
   B main_B;

public:
  virtual void set_rate(double new_rate) { cout << "test"; 
   //B.set(new_rate); 
  }

};



class C : public A {

};

/*
void B::set(double new_rate) {
  var = new_rate;
  cout << "worked " <<current_rate <<endl;
}

*/



int main() {

  C test_C;
  A::set_rate ( 2.00 );
  return 0;
}

Answer 1

Firstly,

C test_c();

does not create an instance of C, it declares a function that returns a C. You mean:

C test_c;

Secondly, non-static member functions can only be called on a specific instance of a class. So with the corrected code, you could say:

test_c.set_rate( 2.0);

Answer 2

You can use an explicit <class>:: to call a non-static member function, thereby disabling any virtual function mechanism, but for a non-static member you still need to specify a class instance on which to call the function.

e.g.

int main()
{
    C test_C;
    test_C.A::set_rate(2.00);
    return 0;
}