Ask about protected keyword

[micheal9000]

Please help me. I read many books and they talk: protected member can be accessed from member in same class, friend of own class, derivative class and friend of derivative class. I write code on codeblocks, all things are right, but friend of derivative class is wrong. Why ? Please help me. Here's my code
------------------

class Polygon{
protected:
int w, h;
public:
void setValue(int w, int h){
this->w = w;
this->h = h;
}
};
class Rectangle:public Polygon{
public:
int area(){
return w*h;
}
friend int area1();
};

int area1(){
return w*w;//cause error
}

[Albatross]

Small scope problem you have there. Just because something is a friend doesn't mean that the scope automatically changes. You still need to use :: or some equivalent.

EDIT: Also... erm... how is areal supposed to know which object it's accessing? It doesn't seem to take any arguments and it's not a member function...

-Albatross

[Disch]

right. area1 needs to get the area of some Polygon... but what polygon? You need to give it a polygon in order for it to get the area.

[micheal9000]

@Disch(3141) - Don't remind what I wrote. Just I want to know how to fix this error.
@Albatross (1310) - You said have to add scope operator ::, mean

int Polygon::area1(){
....
}

But impossible. Because it's friend function, not member of Polygon. And when I add a class, which is friend of Rectangle, nothing to change. Can you clearly explain ?

[Disch]

michael:

area1 gets the area of a polygon, right?

but what polygon? How do you expect to use this function?

For example:

Polygon a;
Polygon b;

cout << a.area();  // gets the area of 'a'
cout << b.area();  // gets the area of 'b'

cout << area1();  // gets the area of ??? what??? 

In order to get the area of a polygon, you need a polygon!

You can pass the polygon to the function:

int area1(const Polygon& poly)
{
  return poly.w * poly.w;
}

Then you can call it like so:

area1( b );  // gets the area of b 

[micheal9000]

I want to explain more: how to friend of derived class can access to protected member of base class. I found example on ibm, but they use pointer. I want to use above simple example. Certainly, I know how to repair to function area can access variable w and h. But in this examle, why area1 cannot?

[Zhuge]

It can't because it has no instance of Polygon to check.

[micheal9000]

Polygon doesn't have to get area method. Only Rectangle has.

[Zhuge]

Sorry, I misread. Replace Polygon with Rectangle.

[micheal9000]

ok. I solved it.

class Polygon{
    protected:
        int w, h;
};
class Rectangle:public Polygon{
    public:
        void showme(){
            cout<<w<<endl<<h;
        }
    friend void showpowerme(Rectangle*);
};

void showpowerme(Rectangle* p){
    cout<<p->w;
}

Everything's good.

[sohguanh]

void showpowerme(Rectangle* p){
cout<<p->w;
}

Not to interrupt the topic but according to Scott Meyers Effective C++ book, always try to use reference variables instead of pointers. Reason being the pointer could be NULL and then if you did not check and de-reference it, you can get error. Reference variables at run-time always point to some object so it is safe not to perform any NULL pointer check in your code.

void showpowerme(Rectangle* p){
  if (p!=NULL) cout<<p->w;
}

void showpowerme(Rectangle& p){
  cout<<p.w;
}