Member objects - why are only public variables accessible?

In t开发者_如何学Gohe below code why should the variable i be public from class a? Why can't it be private or protected? I guess I am missing some basics of member objects. is it ?

#include <iostream>

class a
{
public:
    int i;
};

class b
{
private:
    a a1;
public:
    void show()
    {
        a1.i=5; 
        std::cout << a1.i;
    }
};

int _tmain(int argc, _TCHAR* argv[])
{
    b b1;
    b1.show();
    return 0;
}

---

This is not a nested class. It's just a class that happens to have a member variable whose type is another class. So the normal rules apply.

---

a and b are not nested.

'nested' would mean this:

class b
{
    class a
    {
    };
};

So what you are doing is simply creating an instance of a inside b. So b has the same access to a as you would have to a if you would define it in your main function.

---

Public members can be accessed by anything. Protected members can only be accessed by derived classes and friends. Private memebers can only be accessed by friends. As b is neither a derived class nor friend of a, it can only view public members. If you wanted a.i to be protected, A must either contain friend b, or b must inherit from a.

class b;
class a {
    friend b;
    int i;
};  

or

class b : public a {

---

You are saying that just because class b includes a class a object it should be allowed to access private members of class a? There wouldn't be a lot of point in privacy if that were how things worked. I declare things private in my class so that you don't have to worry about them, and so I can change the private members of my class without breaking your code. None of that would work if what you are asking for were true.

---

Your classes are not nested. Only members of the same class or firends can access private variables. Since "b" is not a member of or a friend of "a" you can only access the variable "i" by making it public or using geter and setter methods. See: http://www.cplusplus.com/doc/tutorial/classes/ an example of a setter method is:

class CRectangle {
    int x, y;
  public:
    void set_values (int,int);
    int area () {return (x*y);}
};

void CRectangle::set_values (int a, int b) {
  x = a;
  y = b;
}

int main () {
  CRectangle rect, rectb;
  rect.set_values (3,4);
  rectb.set_values (5,6);
  cout << "rect area: " << rect.area() << endl;
  cout << "rectb area: " << rectb.area() << endl;
  return 0;
}