how to realize a friend interface?

[Davide M3xican Coppola]

Hi, I would realize an interface (interface) for a pair of concrete
classes (concrete & concrete2), but I have a problem because I have to
access to a third class (element) and if I try to declare interface
class as friend, I obtain this (right) error:

[err]
main2.cpp: In member function `virtual void
concrete::method(element&)':
main2.cpp:8: error: `int element::a_int' is protected
main2.cpp:31: error: within this context
main2.cpp: In member function `virtual void
concrete2::method(element&)':
main2.cpp:8: error: `int element::a_int' is protected
main2.cpp:39: error: within this context
[/err]

So the question is, there is a way to realize this?
I don't would use the interface as a wrapper, nor solve the problem
with a design pattern, any ideas?

This is the example code:

#include <iostream>
 
using namespace std;
 
class element
{
protected:
int a_int;
public:
element(int a) { a_int = a; };
~element();
 
//  friend class interface; // --> I would declare just this line!
friend class concrete;
friend class concrete2;
};
 
class interface
{
public:
interface() {};
~interface() {};
virtual void method(element & e) = 0;
};
 
class concrete : public interface
{
public:
concrete() {};
~concrete() {};
void method(element & e) { cout << "[1] " << e.a_int <<
endl;};
};
 
class concrete2 : public interface
{
public:
concrete2() {};
~concrete2() {};
void method(element & e) { cout << "[2] " << e.a_int <<
endl;};
};
 
 
int main()
{
element * e = new element(10);
interface * i;
concrete * c = new concrete();
concrete2 * c2 = new concrete2();
 
i = c;
i->method(*e);
 
i = c2;
i->method(*e);
 
return 0;
}
 

Thanx in advance to all.

[Rolf Magnus]

Davide M3xican Coppola wrote:

Hi, I would realize an interface (interface) for a pair of concrete
classes (concrete & concrete2), but I have a problem because I have to
access to a third class (element) and if I try to declare interface
class as friend, I obtain this (right) error:

[err]
main2.cpp: In member function `virtual void
concrete::method(element&)':
main2.cpp:8: error: `int element::a_int' is protected
main2.cpp:31: error: within this context
main2.cpp: In member function `virtual void
concrete2::method(element&)':
main2.cpp:8: error: `int element::a_int' is protected
main2.cpp:39: error: within this context
[/err]

friendship is not inheritable.
So the question is, there is a way to realize this?
You could put a protected member function in the interface class that does
the actual access.
I don't would use the interface as a wrapper, nor solve the problem
with a design pattern, any ideas?
Define "design pattern". I guess my solution with that access function in
the interface can be considered a design pattern, and it probably already
has a name.
This is the example code:

 #include <iostream>

 using namespace std;

 class element
 {
     protected:
         int a_int;
     public:
         element(int a) { a_int = a; };
         ~element();

     //  friend class interface; // --> I would declare just this line!
         friend class concrete;
         friend class concrete2;
 };

 class interface
 {
     public:
         interface() {};

 

No need to define an empty constructor and clutter up the code with it. The

compiler generates one automatically for you.

         ~interface() {};

 

Shouldn't this be virtual?

         virtual void method(element & e) = 0;

 };

 class concrete : public interface

 {

     public:

         concrete() {};

         ~concrete() {};

         void method(element & e) { cout << "[1] " << e.a_int <<

 endl;};

 };

 class concrete2 : public interface

 {

     public:

        concrete2() {};

         ~concrete2() {};

         void method(element & e) { cout << "[2] " << e.a_int <<

 endl;};

 };

 int main()

 {

     element * e = new element(10);

     interface * i;

     concrete * c = new concrete();

     concrete2 * c2 = new concrete2();

     i = c;

     i->method(*e);

     i = c2;

     i->method(*e);

     return 0;

 }