Error: ‘int XYZ::data’ is private within this context

Question

I have one question about the following code snippet in c++ :

 #include <iostream>

 using namespace std;

 class ABC;

 class XYZ
 {
 int data;
 public:

         void setvalue(int value)
         {
         data=value;
         } 
         friend void add(ABC,XYZ);
 };

 class ABC
 {
 int data;
 public:

         void setvalue(int value)
         {
         data=value;
         }
         friend void add(ABC,XYZ);
 };

 void add(XYZ obj1, ABC obj2)
 {
      cout << "Sum of data values of XYZ and ABC objects using friend function = " << obj1.data + obj2.data;
      cout << "\n";
 }


 int main()
 {

 XYZ X;
 ABC A;

 X.setvalue(5);
 A.setvalue(50);
 add(X,A);

 return 0;
 }

In particular, when I compile, g++ complains with the following log:

g++ -Wall -Wextra -Werror friend_function_add_data.cpp -o friend_function_add_data.o
friend_function_add_data.cpp: In function ‘void add(XYZ, ABC)’:
friend_function_add_data.cpp:33:86: error: ‘int XYZ::data’ is private within this context
  33 | ut << "Sum of data values of XYZ and ABC objects using friend function = " << obj1.data + obj2.data;
     |                                                                                    ^~~~

friend_function_add_data.cpp:9:5: note: declared private here
   9 | int data;
     |     ^~~~
friend_function_add_data.cpp:33:98: error: ‘int ABC::data’ is private within this context
  33 |  data values of XYZ and ABC objects using friend function = " << obj1.data + obj2.data;
     |                                                                                   ^~~~

friend_function_add_data.cpp:21:5: note: declared private here
  21 | int data;
     |

Since it was complaining about data declared as private, I changed the declaration to public by placing data right after the public statement for each class ABC and XYZ .

In this way g++ compiles without problems.

What I wonder is : is this the only correct way to handle this issue for this example? do you think it is just a mistake by the author himself?

This piece of code is taken from Balagurusamy's "OOP programming with C++" (8th edition, page 124)

Edit: thanks Anoop for double checking! I realized I inverted the input arguments for each class ABC and XYZ, w.r.t. to the outer add function taking XYZ obj1 and ABC obj2. So it is written correctly on the book

Answer 1

The problem is that while defining the function add, the order of the two parameters named obj1 and obj2 is opposite to what it was in the friend declaration.

So to solve this, make sure that the order of the parameters match in the definition and the friend declaration as shown below:

class XYZ
{
public:
     //other code here
     friend void add(ABC,XYZ);
};
class ABC
{
public:

     //other code here
     friend void add(ABC,XYZ);
};
//ORDER OF PARAMETERS CHANGED TO MATCH WITH THE FRIEND DECLARATION
void add(ABC obj1, XYZ obj2)
{
  cout << "Sum of data values of XYZ and ABC objects using friend function = " << obj1.data + obj2.data;
  cout << "\n";
}
int main()
{

    XYZ X;
    ABC A;
    
    X.setvalue(5);
    A.setvalue(50);
    add(A,X); //ORDER OF ARGUMENTS CHANGED
}

Demo