Friend function

[Jacko123]

Hello,
Consider this scenario,

class B;
class A
{
    private:
          int x, y;
    public:
          A(){}
    friend void func( A a, B b );
};

class B
{
    private:
          int i, j;
    public:
          B(){}
    friend void func( A a, B b );
};

Friend function is used to allow non-members to access private data defined inside a class. Now my question is I have two variables x and y in class A, and two variables i and j in class B. Is it possible that I can allow my users (outside class) to access only variable x and not y defined in class A ? Similarly, allow access outside class only to variable i and not to j in class B.
NOTE: The above scenario should happen only when friend is defined.

Thanks

[S_M_A]

No, friend declaration makes both private and protected members accessible.

[sunnypalsingh]

Directly it's not possible but you might put those variables which you don't want friend function to access in separate base classes holding that variable. Not sure on this. Maybe someone will correct/refine this.

[S_M_A]

At the first seconds I had a similar thought but since I guess that inherited class need access in some way to base class members the friend function will also have access. Maybe there is a scenario where the method can be used however. Can't think of any at the moment so I join you sunnypalsingh in the 'not sure crowd'...

[treuss]

Here is an example, how it could be done (even though this is a bit clumsy):

class A
{
private:
  int x, y;
public:
  A(){}
  class XAccessor {
    XAccessor( A& a ) : x( a.x ) {}
    friend void func( A& a );
  private:
    int& x;
  };
};

void func( A& a ) {
  A::XAccessor(a).x = 0;
  a.y = 0;  // not allowed
  A::XAccessor(a).y = 0; // not possible
}

void otherfunc( A& a ) {
  A::XAccessor(a).x = 1; // not allowed
}

[NMTop40]

Another way is to make y more private thus:

class B;
class A
{
private:
   int x;

   class very_private
   {
          int y;
          friend class A;
   } y_access;

   friend void func( A a, B b );
};

func can access a.x but cannot access a.y_access.y because friendship isn't transitive.

[DaemonAkaDevil]

Another way is to make y more private thus:
[code]


class very_private
{
int y;
friend class A;
} y_access;

sorry for interrupting but
y_access followed by class very_private braces then ";"

what kind of a syntax is this? means
am i wrong ? or you ?

[nitin1979]

sorry for interrupting but
y_access followed by class very_private braces then ";"

what kind of a syntax is this? means
am i wrong ? or you ?

Brother he is creating object of class very_private nothing more then that.

[nitin1979]

Another solution, not best but ya one of the possible solution

#include <iostream>
class B;
class tempA
{
    private:
        int x;
    public:
        tempA(){x=0;}
};
class A : private tempA
{
    private:
          int y;
    public:
          A(){y=0;}
    friend void func( A a, B b );
};
class tempB
{
    private:
        int i;
    public:
        tempB(){i=0;}
};
class B:private tempB
{
    private:
          int j;
    public:
          B(){j=0;}
    friend void func( A a, B b );
};

void func(A a, B b)
{
    std::cout<<"A.y = "<<a.y<<" , B.j = "<<b.j<<std::endl;
    //std::cout<<"A.x = "<<a.x<<" , B.j = "<<b.i<<std::endl;
    // can't use this statement because now x and i are private members
}
int main()
{
	A a;
	B b;
	func(a,b);
	return 0;
}

[NMTop40]

The problem with your solution is that x is private to A as well, and i is private to B, i.e. A cannot access x and B cannot access i.

[nitin1979]

The problem with your solution is that x is private to A as well, and i is private to B, i.e. A cannot access x and B cannot access i.

Yes you correctly identified problem in my code. Thanks for that.
I have to use tempA as friend class in class A. and same for tempB also

Thanks for correcting my code.

[exterminator]

Nice solutions there NMTop40 and treuss! But what kind of a requirement is this? Sounds like a flawed design.

[treuss]

Sounds like a flawed design.

Doesn't that apply to any use of friend?

[laserlight]

Doesn't that apply to any use of friend?

Not necessarily. For example, overloading operator<< and operator>> for I/O streams is valid use of friend functions. Consider the arguments given by Does "friend" violate encapsulation?

[exterminator]

Doesn't that apply to any use of friend?

How? Ask a question, why is that the friendship in the OP's case is causing you to "split" your original class into an internal class with an explicit accessor or causing you to create extra types (that might not sometimes make sense as individual entities or abstractions) and "different" friendships? Does it really make sense to use friends when the friend should have access to just part of the private data and not all of those? Or better put it that you want to ensure/enforce that access to other private data of a friend needs to be protected? Why need to make such a distinction?

To me it looks like:

• original friendship does not make sense
• the original class needs to be segregated into more granular distict types (in which case, your or NMTop40's solutions fit right)
• the point to protect rest of the data is immaterial

A class can have many members and considering operator>> overloads which can be friend, do you protect the data members that you don't want to publish via the operator to the streams?

Re-reading the original post,

Is it possible that I can allow my users (outside class) to access only variable x and not y defined in class A ? Similarly, allow access outside class only to variable i and not to j in class B.

I am guessing that it doesn't even concern a friend. The question is about users of the class and not friends. So, since the data members in A and B are private, nothing is accessible. For a few selected data members to be accessible the only thing needed is a modifier and an accessor (getter/setter). May be the orginal post is not clear on this point but my above arguments hold.