C++ friend function

[Der Andere]

I've got a problem concerning the << operator. Although I've declared it as
a friend function it cannot access the private member of the class AVLtree.
Some code:

------------------
class AVLtree
{
public:
friend ostream& operator << (ostream& os, AVLtree& t){};
.... some unimportant stuff ...
private:
Tree tree;
};

ostream& operator << (ostream& os, AVLtree& t)
{
// if (t.tree!=NULL)
os << *(t.tree);
return os;
}
----------------

I cannot figure out why the compiler tells

error C2248: 'tree' : cannot access private member declared in class
'AVLtree'

Any advice?

Thanks in advance,
Matthias

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[Kevin Goodsell]

Re: C++ friend function

Der Andere wrote:
[color=blue]
> I've got a problem concerning the << operator. Although I've declared it as
> a friend function it cannot access the private member of the class AVLtree.
> Some code:[/color]

When posting code, please try to make it compilable. This is FAQ 5.8.
[color=blue]
>
> ------------------
> class AVLtree
> {
> public:
> friend ostream& operator << (ostream& os, AVLtree& t){};[/color]

What's with the '{}'? I don't even know if that's legal, and it
certainly seems nonsensical. Are you defining the function here, or are
you defining it later?
[color=blue]
> ... some unimportant stuff ...
> private:
> Tree tree;
> };
>
> ostream& operator << (ostream& os, AVLtree& t)
> {
> // if (t.tree!=NULL)
> os << *(t.tree);
> return os;
> }[/color]

-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.

[Buster]

Re: C++ friend function

Der Andere wrote:
[color=blue]
> I've got a problem concerning the << operator. Although I've declared it as
> a friend function it cannot access the private member of the class AVLtree.
> Some code:
>
> ------------------
> class AVLtree
> {
> public:
> friend ostream& operator << (ostream& os, AVLtree& t){};[/color]

Defined here.
[color=blue]
> ... some unimportant stuff ...
> private:
> Tree tree;
> };
>
> ostream& operator << (ostream& os, AVLtree& t)
> {
> // if (t.tree!=NULL)
> os << *(t.tree);
> return os;
> }[/color]

And here.
[color=blue]
> ----------------
>
> I cannot figure out why the compiler tells
>
> error C2248: 'tree' : cannot access private member declared in class
> 'AVLtree'[/color]

Funny, mine says
"error: redefinition of `std::ostream& operator<<(std: :ostream&, AVLtree&)".
[color=blue]
> Any advice?[/color]

Post the real code.

--
Regards,
Buster.

[Der Andere]

Re: C++ friend function

Ok then, underneath you'll find the complete code, as I got it. It should
compile now.

Regards,
Matthias

----------------------

#include <iostream>
#include <cassert>
#include <iomanip>

using namespace std;

typedef int T;

T max(T a, T b)
{
return a > b ? a : b ;
}

T min(T a, T b)
{
return a < b ? a : b ;
}

// ----------

enum Child {Left=0, Right=1};

Child opposite(Child k)
{
return Child(1-k);
}

// ----------

enum Balance { LeftBal=-1, Balanced=0, RightBal=1};

const bool deeper = true;
const bool sameDepth = false;

// ----------

typedef class Node* Tree;
const Tree Empty = NULL;

class Node
{
enum { Arity=2 };
Tree children[Arity];
int b;
T e;
public:
Node(T & v) : e(v), b(0) { for (int i=0;i<Arity;i+= 1) children[i]=Empty; }
~Node();

friend int& bal(Tree p);
friend T& el (Tree p);
friend Tree & child(Tree p, Child k);
};

int& bal(Tree p) { assert(p); return p->b; }

T& el (Tree p) { assert(p); return p->e; }

Tree& child(Tree p, Child k) { assert(p); return p->children[k]; }

Tree& left(Tree p) { return child(p,Left); }

Tree& right(Tree p) { return child(p,Right); }

Node :: ~Node()
{
for (int i=0;i<Arity;i+= 1)
if (children[i])
delete children[i];
}

// ----------

void rotate (Tree & root, Child c)
{
Child other = opposite(c);
assert(child(ro ot,other)); // other Child should exist
Tree oldRoot = root;

root = child(root,othe r);
child(oldRoot,o ther) = child(root,c);
child(root,c) = oldRoot;

if (c==Left)
{
bal(oldRoot) -= 1+ max(bal(root),0 );
bal(root) -= 1- min(bal(oldRoot ),0);
}
else
{
bal(oldRoot) += 1- min(bal(root),0 );
bal(root) += 1+ max(bal(oldRoot ),0);
}
}

void rotateDubbel (Tree & root, Child c)
{
Child other = opposite(c);
rotate(child(ro ot,other),other );
rotate(root,c);
}

// ----------

bool insertRight(Tre e& tree, T v);
bool insertLeft(Tree & tree, T v);

bool insert(Tree& tree, T v) // returns true if tree becomes deeper
{
if (tree==Empty)
{
tree = new Node (v);
return deeper;
}
else if (v==el(tree))
return sameDepth;
else if (v<el(tree))
return insertLeft (tree,v);
else
return insertRight (tree,v);
}

bool insertRight(Tre e& tree, T v)
{
if (insert(right(t ree),v))
{
bal(tree) += 1;
switch(bal(tree ))
{
case 2:
if (bal(right(tree )) == LeftBal)
rotateDubbel(tr ee,Left);
else
rotate(tree,Lef t);
return sameDepth;
case 1: return deeper;
case 0: return sameDepth;
default: assert(false);
}
}
else
return sameDepth;
}

bool insertLeft(Tree & tree, T v)
{
if (insert(left(tr ee),v))
{
bal(tree) -= 1;
switch(bal(tree ))
{
case -2:
if (bal(left(tree) ) == RightBal)
rotateDubbel(tr ee,Right);
else
rotate(tree,Rig ht);
return sameDepth;
case -1: return deeper;
case 0: return sameDepth;
default: assert(false);
}
}
else
return sameDepth;
}

bool contains (Tree tree, T v)
{
if (tree == Empty)
return false;
else if (el(tree) == v)
return true;
else if (el(tree) < v)
return contains(right( tree),v);
else
return contains(left(t ree),v);
}

bool sorted (Tree tree, T* v=NULL)
{
if (tree==Empty)
return true;
else
{
bool sl = sorted(left(tre e),v);
if (v && el(tree) <= *v)
{
sl = false;
cout << "element " << *v << " staat op de verkeerde plaats\n";
}
v = &(el(tree));
return sorted(right(tr ee),v) && sl;
}
}

bool balanced (Tree tree, int& d)
{
if (tree==Empty)
{
d = 0;
return true;
}
else
{
int l=0, r=0;
bool bl = balanced(left(t ree),l);
bool br = balanced(right( tree),r);
if (bal(tree) != r-l)
cout << "balansfact or in knoop met " << el(tree) << " verkeerd\n";
if (abs(r-l)>1)
cout << "avl tree is op knoop met " << el(tree) << " uit balans\n";
d = max(l,r) + 1;
return bl && br && bal(tree) == r-l && abs(r-l)<=1;
}
}


ostream& operator << (ostream& os, Node& n)
{
static int indent = 0;
const int stap = 3;
indent += stap;
if (right(&n)!=Emp ty)
os << *right(&n);
os << setw(indent-stap) << "" << el(&n) << " bal=" << bal(&n) << endl;
if (left(&n)!=Empt y)
os << *left(&n);
indent -= stap;
return os;
}

// ----------
// ----------

class AVLtree
{
Tree tree;
public:
AVLtree(): tree(Empty) {}
~AVLtree() { if (tree) delete tree; }
friend ostream& operator << (ostream& os, AVLtree& t);
void Insert(T v) { insert(tree,v); }
bool Contains(T v) { return contains(tree,v ); }
bool Sorted() { return sorted(tree); }
bool Balanced() {int d =0; return balanced(tree,d ); }
};

ostream& operator << (ostream& os, AVLtree& t)
{
if (t.tree!=Empty)
os << *(t.tree);
return os;
}


// ----------

void main ()
{
// int val = 2;
// Node<int>* ptr = new Node<int>(val);
// cout << *ptr ;

/* Node<int>* t = Empty;
int rij[10] = {0,9,1,2,3,4,5, 6,7,8};
for (int i = 0;i<10;i+=1)
{
insert(t,rij[i]);
cout << *t << endl;
}
*/

AVLtree t;
// int rij[10] = {0,9,1,2,4,4,5, 6,7,8};
// int rij[10] = {5,3,7,2,4,6,1, 1,1,1};
int rij[10] = {4,2,6,1,3,5,7, 1,1,1};
for (int i = 0;i<10;i+=1)
{
t.Insert(rij[i]);
cout << t << endl;
if (!t.Sorted() || !t.Balanced())
cout << "Improper AVL tree\n";
}
for (int j = -1;j<11;j+=1)
cout << "contains " << j << " = " << boolalpha << t.Contains(j) << endl;


}

[Buster]

Re: C++ friend function

Der Andere wrote:[color=blue]
> Ok then, underneath you'll find the complete code, as I got it. It should
> compile now.[/color]

No, I get "error: `main' must return `int'".

Having fixed that, yes, it does compile. So, what, you fixed it?
What happened to "error C2248: 'tree' : cannot access private member
declared in class 'AVLtree'"?

--
Regards,
Buster.

[Der Andere]

Re: C++ friend function

> Der Andere wrote:[color=blue][color=green]
> > Ok then, underneath you'll find the complete code, as I got it. It[/color][/color]
should[color=blue][color=green]
> > compile now.[/color]
>
> No, I get "error: `main' must return `int'".
>
> Having fixed that, yes, it does compile. So, what, you fixed it?
> What happened to "error C2248: 'tree' : cannot access private member
> declared in class 'AVLtree'"?[/color]

Errh, no, I didn't fix it. I've still got it.
I've never had this kind of problem with my compiler.
But if it works on your computer, the fault may be in my system, not in the
code?!

Regards,
Matthias

[Buster]

Re: C++ friend function

Der Andere wrote:
[color=blue]
> Errh, no, I didn't fix it. I've still got it.
> I've never had this kind of problem with my compiler.
> But if it works on your computer, the fault may be in my system, not in the
> code?![/color]

What compiler is it, and what version? I gather that with Visual C++,
anything before 7.1 was pretty crappy. If I see any problems with the
code you posted I'll let you know.

--
Regards,
Buster.

[Der Andere]

Re: C++ friend function

> > Errh, no, I didn't fix it. I've still got it.[color=blue][color=green]
> > I've never had this kind of problem with my compiler.
> > But if it works on your computer, the fault may be in my system, not in[/color][/color]
the[color=blue][color=green]
> > code?![/color]
>
> What compiler is it, and what version? I gather that with Visual C++,
> anything before 7.1 was pretty crappy.[/color]

Oh, I've got Visual C++ 6.0. So maybe Microsoft is the problem, not me ;-)
[color=blue]
>If I see any problems with the
> code you posted I'll let you know.[/color]

Thanks!

Regards,
Matthias

[Leor Zolman]

Re: C++ friend function

On Tue, 13 Apr 2004 03:23:17 +0200, "Der Andere" <matieuloeschmi ch@gmx.net>
wrote:
[color=blue][color=green][color=darkred]
>> > Errh, no, I didn't fix it. I've still got it.
>> > I've never had this kind of problem with my compiler.
>> > But if it works on your computer, the fault may be in my system, not in[/color][/color]
>the[color=green][color=darkred]
>> > code?![/color]
>>
>> What compiler is it, and what version? I gather that with Visual C++,
>> anything before 7.1 was pretty crappy.[/color]
>
>Oh, I've got Visual C++ 6.0. So maybe Microsoft is the problem, not me ;-)[/color]

Anytime the words "friend function", "strange error" and "MSVC 6" are
uttered together, there's a good chance you're dealing with the infernal
"friend" bug in VC6. If you haven't yet, go download the latest service
pack (it is up to Service Pack 6 now) at:

http://msdn.microsoft.com/vstudio/downloads/updates/sp/vs6/sp6/default.aspx

-leor


--
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On-Site Training in C/C++, Java, Perl and Unix
C++ users: download BD Software's free STL Error Message Decryptor at:

https://www.bdsoft.com/tools/stlfilt.html

[Buster]

Re: C++ friend function

Der Andere wrote:
[color=blue][color=green]
>>What compiler is it, and what version? I gather that with Visual C++,
>>anything before 7.1 was pretty crappy.[/color]
>
> Oh, I've got Visual C++ 6.0. So maybe Microsoft is the problem, not me ;-)[/color]

Maybe so. I'm no expert.

Some things to try, without altering the correctness of your code:

(i) Add a forward declaration of the << overload before the definition
of the class of which it is a friend.

(ii) Define the friend function inside AVL tree class, so that it looks
a bit like an inline member function definition.
[color=blue][color=green]
>>If I see any problems with the
>>code you posted I'll let you know.[/color]
>
> Thanks![/color]

--
Regards,
Buster.

[Buster]

Re: C++ friend function

Buster wrote:
[color=blue]
> Der Andere wrote:
>[color=green]
>> Oh, I've got Visual C++ 6.0. So maybe Microsoft is the problem, not me
>> ;-)[/color]
>
> Maybe so. I'm no expert.[/color]

Looks like we're in the right place. Follow Leor's advice, not mine!

--
Regards,
Buster.

[Der Andere]

Re: C++ friend function

> >> > Errh, no, I didn't fix it. I've still got it.[color=blue][color=green][color=darkred]
> >> > I've never had this kind of problem with my compiler.
> >> > But if it works on your computer, the fault may be in my system, not[/color][/color][/color]
in[color=blue][color=green]
> >the[color=darkred]
> >> > code?!
> >>
> >> What compiler is it, and what version? I gather that with Visual C++,
> >> anything before 7.1 was pretty crappy.[/color]
> >
> >Oh, I've got Visual C++ 6.0. So maybe Microsoft is the problem, not me[/color][/color]
;-)[color=blue]
>
> Anytime the words "friend function", "strange error" and "MSVC 6" are
> uttered together, there's a good chance you're dealing with the infernal
> "friend" bug in VC6. If you haven't yet, go download the latest service
> pack (it is up to Service Pack 6 now) at:
>
>[/color]

http://msdn.microsoft.com/vstudio/downloads/updates/sp/vs6/sp6/default.aspx

Indeed. That was the problem. I've just installed the service pack and the
compiler issues no errors :-))

Regards,
Matthias