Question about private scope

[MattEvans]

I'm carrying this around in my signature at the moment, because I found the statement somewhat amusing:

class `Matt' is implicitly friends with itself.

(Of course, the class wasn't really called Matt. I like the look of my own name. Perhaps).

Preamble aside; the more I think about that, the more I wonder. The statement came about, because I made a class, I wanted the class to blueprint objects that would create more alike (of the same class) objects, and I didn't want to have to use accessor methods to set certain properties for the created objects. So I figured, incorrectly, that I'd need to make the class its own friend. My logic being, that private members are innaccessible from outside an object, unless whatever is accessing the object's private member is a friend of the object.

Of course, the message class `Class' is implictly friends with itself, proves that logic incorrect; the private members of an object seem to be accessible between objects of the same class. This is different to how I've always perceieved 'private'. I won't say I went from Java to C++, (I first used C++ before I first used Java), but, I've definately spent more time developing in Java than C++, and in Java, if I'm not mistaken, private means object-private not class-private.

I actually want to ask if this is complier specific (I'm using g++) As I understand C++, it's the compiler that checks access priviledges between objects/members, and then the compiled program just does what it does.

So, is this true: "the private members of an object are only accessible to this object, and to any other object of the same class", is this a G++ only thing, or is there any room for debate?

Example code follows, it should show what I'm talking about..

#include <iostream>

class Test
{
  private:
    int myvalue;
  public:
    Test(void);
    void affect_alike(Test * what);
    int get_value(void);
};

Test::Test(void):myvalue(7)
{
  cerr << "Created test object" << endl;
};

void Test::affect_alike(Test * what)
{
  what->myvalue = 100;
}

int Test::get_value(void)
{
  return myvalue;
}

int main(void)
{
  Test * test1 = new Test();
  Test * test2 = new Test();
  
  cout << endl;
  
  cout << "T1: " << test1->get_value() << endl << "T2: " << test2->get_value() << endl;
  
  test1->affect_alike(test2);
  
  cout << "T1: " << test1->get_value() << endl << "T2: " << test2->get_value() << endl;
  
  cout << "Done" << endl << endl;
  
  return 0;
}

results in:

T1: 7
T2: 7
T1: 7
T2: 100
Done

[John A]

Yes, this is normal C++ class/object behaviour.

Part of this, in my opinion, may be due to the fact that in linked lists you often need to modify the object in the next node, or the one after that. This way, you can modify the values without having to traverse through each node. Java doesn't have pointers, (right?) so then you don't really have a need for one object to modify another object's private members.

That's just a guess though, I could be wrong...

[MattEvans]

Thank you for the reply.

Java doesn't really have or not have pointers; it has object references, but they act more like C++ pointers than they do C++ references.. But, they're not really like C++ pointers either. More like 'object handles'.

Object-private seems more intuitively private to me; but can't argue with ISO C++ =P I can't honestly remember if it is like I think in Java, - I can't think in two languages at once... but I seem to remember it being one of my gripes, rather than a cherished feature... I'll have a check in Java in a moment methinks.

[MattEvans]

Haha. I am infact totally wrong about that aswell. Java does the same:

class Test
{
  private int myvalue;
  public Test()
  {
    System.out.println("Created test object");
    myvalue = 7;
  }
  public void affect_like(Test what)
  {
    what.myvalue = 100;
  }
  public int get_value()
  {
    return myvalue;
  }
}

public class TestClass
{

  public static void main(String [] args)
  {
    Test t1 = new Test();
    Test t2 = new Test();
    System.out.println("T1: " + t1.get_value());
    System.out.println("T2: " + t2.get_value());
    t1.affect_like(t2);
    System.out.println("T1: " + t1.get_value());
    System.out.println("T2: " + t2.get_value());
  }
}

Output exactly the same as the C++.

I wonder where I got that idea from. It still seems more intuitive to me for objects to deny access to private properties/methods from anything but themselves. I suppose; it would be annoying in practice though.

[thekashyap]

Very interesting. Count me in the club who didn't know that private members of one object are accesible to other objects of same type !
I tested on Microsoft Visual Studio 6.0 and Sun Workshop 6.0. It works in both places.
Statndard also doesn't seem to say anything to address this directly.