Question

I have the following C++ code:

#include <iostream>
#include <vector>

using namespace std;

class A
{
   private:
      int i;
   public:
      void f1(const A& o);
      void f2()
      {
         cout<<i<<endl;
      }
};

void A::f1(const A& o)
{
   o.f2();
}

It just doesn’t compile. Can somebody give a explanation? Thanks!

Answer 1

Presumably, your compiler told you why it doesn’t compile. Mine said:

In member function ‘void A::f1(const A&)’:
passing ‘const A’ as ‘this’ argument of ‘void A::f2()’ discards qualifiers

This tells me that you’re trying to call a non-const member function (A::f2) on a reference to a const object (const A& o).

Either add a const qualifier to the function to allow it to be called on const objects:

void f2() const
          ^^^^^

or remove const from the reference to allow modification – but in this case, don’t do that, since f2() doesn’t need to modify the object.