Scope-problems

[Michael S.]

Hi

I'm having some problems with a class for large Integers (called
Xuint).

I want (have) to implement it like this:
- using a simple linked list
- a reference-objekt for each value, which means if 2 Xuint-objekts
have the same value, only one list is in memory and the 2
Xuint-objekts are pointing to the ref-objekt which points to the list.

By now I have this:

class Xuint
{
//listnode
struct Xunode{
unsigned short nodeval;
Xunode *next;
};

//ref-objekt
class Rep{
friend class Xuint;
Xunode *xustart;
int refs;
//...
};

public:
//...
};

When I compile this with Forte Developer C++ 5.4 I get:
"Error: Xuint::Xunode is not accessible from Xuint::Rep"

If I make Xunode public it works, but I don't understand why, because
from my point of view the 'friend class Xuint;' should make the
private parts of Xuint accessible to Rep.

So here's the question: what am I doing wrong and is there a way to
keep Xunode private while using it from Rep?

regards,
Michael

[Victor Bazarov]

Re: Scope-problems

"Michael S." <eyesshuttight@ web.de> wrote...[color=blue]
> I'm having some problems with a class for large Integers (called
> Xuint).
>
> I want (have) to implement it like this:
> - using a simple linked list
> - a reference-objekt for each value, which means if 2 Xuint-objekts
> have the same value, only one list is in memory and the 2
> Xuint-objekts are pointing to the ref-objekt which points to the list.
>
> By now I have this:
>
> class Xuint
> {
> //listnode
> struct Xunode{
> unsigned short nodeval;
> Xunode *next;
> };
>
> //ref-objekt
> class Rep{
> friend class Xuint;
> Xunode *xustart;
> int refs;
> //...
> };
>
> public:
> //...
> };
>
> When I compile this with Forte Developer C++ 5.4 I get:
> "Error: Xuint::Xunode is not accessible from Xuint::Rep"
>
> If I make Xunode public it works, but I don't understand why, because
> from my point of view the 'friend class Xuint;' should make the
> private parts of Xuint accessible to Rep.
>
> So here's the question: what am I doing wrong and is there a way to
> keep Xunode private while using it from Rep?[/color]

I think you got the friendship wrong. Only a friend of class A
can access A's private members. If you write a class B and then
declare that A is a friend of B, it does NOT make B a friend of
A automatically.

In your case, in order for 'Rep' to access 'Xunode', 'Rep' has
to be declared a friend of 'Xuint'.

Victor

[Wagner Bruna]

Re: Scope-problems

Hi,

eyesshuttight@w eb.de (Michael S.) wrote in message news:<b5aee4d3. 0312291335.293c 0da4@posting.go ogle.com>...[color=blue]
> Hi
>
> I'm having some problems with a class for large Integers (called
> Xuint).
>
> I want (have) to implement it like this:
> - using a simple linked list
> - a reference-objekt for each value, which means if 2 Xuint-objekts
> have the same value, only one list is in memory and the 2
> Xuint-objekts are pointing to the ref-objekt which points to the list.
>
> By now I have this:
>
> class Xuint
> {
> //listnode
> struct Xunode{
> unsigned short nodeval;
> Xunode *next;
> };
>
> //ref-objekt
> class Rep{
> friend class Xuint;
> Xunode *xustart;
> int refs;
> //...
> };
>
> public:
> //...
> };
>
> When I compile this with Forte Developer C++ 5.4 I get:
> "Error: Xuint::Xunode is not accessible from Xuint::Rep"
>
> If I make Xunode public it works, but I don't understand why, because
> from my point of view the 'friend class Xuint;' should make the
> private parts of Xuint accessible to Rep.[/color]

Yes, it should; I tested your code on Comeau online and gcc 3.2.3, and
it works fine.
[color=blue]
> So here's the question: what am I doing wrong and is there a way to
> keep Xunode private while using it from Rep?[/color]

Hard to say, since it's compiler's fault... But you could use a
forward declaration to at least keep the implementation hidden:

class Xuint
{
public:
//listnode
struct Xunode;
private:
//ref-objekt
class Rep{
friend class Xuint;
Xunode *xustart;
int refs;
//...
};

public:
//...
};

// (maybe) Xuint.cpp

struct Xuint::Xunode
{
unsigned short nodeval;
Xunode *next;
};

Or maybe you could declare Xunode inside Rep.

--
Wagner

[Jeff Schwab]

Re: Scope-problems

Wagner Bruna wrote:[color=blue]
> Hi,
>
> eyesshuttight@w eb.de (Michael S.) wrote in message news:<b5aee4d3. 0312291335.293c 0da4@posting.go ogle.com>...
>[color=green]
>>Hi
>>
>>I'm having some problems with a class for large Integers (called
>>Xuint).
>>
>>I want (have) to implement it like this:
>>- using a simple linked list
>>- a reference-objekt for each value, which means if 2 Xuint-objekts
>>have the same value, only one list is in memory and the 2
>>Xuint-objekts are pointing to the ref-objekt which points to the list.
>>
>>By now I have this:
>>
>>class Xuint
>>{
>> //listnode
>> struct Xunode{
>> unsigned short nodeval;
>> Xunode *next;
>> };
>>
>> //ref-objekt
>> class Rep{
>> friend class Xuint;
>> Xunode *xustart;
>> int refs;
>> //...
>> };
>>
>> public:
>> //...
>>};
>>
>>When I compile this with Forte Developer C++ 5.4 I get:
>>"Error: Xuint::Xunode is not accessible from Xuint::Rep"
>>
>>If I make Xunode public it works, but I don't understand why, because
>>from my point of view the 'friend class Xuint;' should make the
>>private parts of Xuint accessible to Rep.[/color]
>
>
> Yes, it should; I tested your code on Comeau online and gcc 3.2.3, and
> it works fine.[/color]

I believe this issue is covered by DR 45:

http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_defects.html#45

Since Rep is a member of Xuint, it ought to have access to all the other
members (including Xunode). No friend declarations should be needed.
However, I believe Forte 5.4 is in compliance with the existing
standard. Do you have access to a newer version of the compiler?

-Jeff

[Michael S.]

Re: Scope-problems

"Victor Bazarov" <v.Abazarov@com Acast.net> wrote in message news:<Gl3Ib.696 801$Fm2.600116@ attbi_s04>...[color=blue]
> "Michael S." <eyesshuttight@ web.de> wrote...[color=green]
> > I'm having some problems with a class for large Integers (called
> > Xuint).
> >
> > I want (have) to implement it like this:
> > - using a simple linked list
> > - a reference-objekt for each value, which means if 2 Xuint-objekts
> > have the same value, only one list is in memory and the 2
> > Xuint-objekts are pointing to the ref-objekt which points to the list.
> >
> > By now I have this:
> >
> > class Xuint
> > {
> > //listnode
> > struct Xunode{
> > unsigned short nodeval;
> > Xunode *next;
> > };
> >
> > //ref-objekt
> > class Rep{
> > friend class Xuint;
> > Xunode *xustart;
> > int refs;
> > //...
> > };
> >
> > public:
> > //...
> > };
> >
> > When I compile this with Forte Developer C++ 5.4 I get:
> > "Error: Xuint::Xunode is not accessible from Xuint::Rep"
> >
> > If I make Xunode public it works, but I don't understand why, because
> > from my point of view the 'friend class Xuint;' should make the
> > private parts of Xuint accessible to Rep.
> >
> > So here's the question: what am I doing wrong and is there a way to
> > keep Xunode private while using it from Rep?[/color]
>
> I think you got the friendship wrong. Only a friend of class A
> can access A's private members. If you write a class B and then
> declare that A is a friend of B, it does NOT make B a friend of
> A automatically.
>
> In your case, in order for 'Rep' to access 'Xunode', 'Rep' has
> to be declared a friend of 'Xuint'.
>
> Victor[/color]

You're right I really got that wrong, but when I insert a 'friend
class Rep;' into class Xuint I still get the same error.

Michael

[Michael S.]

Re: Scope-problems

Jeff Schwab <jeffplus@comca st.net> wrote in message news:<U62dnRGmA f-Zb22iRVn-gw@comcast.com> ...[color=blue]
> Wagner Bruna wrote:[color=green]
> > Hi,
> >
> > eyesshuttight@w eb.de (Michael S.) wrote in message news:<b5aee4d3. 0312291335.293c 0da4@posting.go ogle.com>...
> >[color=darkred]
> >>Hi
> >>
> >>I'm having some problems with a class for large Integers (called
> >>Xuint).
> >>
> >>I want (have) to implement it like this:
> >>- using a simple linked list
> >>- a reference-objekt for each value, which means if 2 Xuint-objekts
> >>have the same value, only one list is in memory and the 2
> >>Xuint-objekts are pointing to the ref-objekt which points to the list.
> >>
> >>By now I have this:
> >>
> >>class Xuint
> >>{
> >> //listnode
> >> struct Xunode{
> >> unsigned short nodeval;
> >> Xunode *next;
> >> };
> >>
> >> //ref-objekt
> >> class Rep{
> >> friend class Xuint;
> >> Xunode *xustart;
> >> int refs;
> >> //...
> >> };
> >>
> >> public:
> >> //...
> >>};
> >>
> >>When I compile this with Forte Developer C++ 5.4 I get:
> >>"Error: Xuint::Xunode is not accessible from Xuint::Rep"
> >>
> >>If I make Xunode public it works, but I don't understand why, because
> >>from my point of view the 'friend class Xuint;' should make the
> >>private parts of Xuint accessible to Rep.[/color]
> >
> >
> > Yes, it should; I tested your code on Comeau online and gcc 3.2.3, and
> > it works fine.[/color]
>
> I believe this issue is covered by DR 45:
>
> http://anubis.dkuug.dk/jtc1/sc22/wg2...efects.html#45
>
> Since Rep is a member of Xuint, it ought to have access to all the other
> members (including Xunode). No friend declarations should be needed.
> However, I believe Forte 5.4 is in compliance with the existing
> standard. Do you have access to a newer version of the compiler?
>
> -Jeff[/color]
No, I have to use that specific Compiler, because our prof insists on
that one.

I thank all of you for your comments, but I think it will do little
harm if I make Xunode public, although I think from a stylistic point
of view it should be private.

regards,
Michael

[red floyd]

Re: Scope-problems

Michael S. wrote:[color=blue]
> Hi
>
> I'm having some problems with a class for large Integers (called
> Xuint).
>
> I want (have) to implement it like this:
> - using a simple linked list
> - a reference-objekt for each value, which means if 2 Xuint-objekts
> have the same value, only one list is in memory and the 2
> Xuint-objekts are pointing to the ref-objekt which points to the list.
>
> By now I have this:
>
> class Xuint
> {
> //listnode
> struct Xunode{
> unsigned short nodeval;
> Xunode *next;
> };
>
> //ref-objekt
> class Rep{
> friend class Xuint;
> Xunode *xustart;
> int refs;
> //...
> };
>
> public:
> //...
> };
>
> When I compile this with Forte Developer C++ 5.4 I get:
> "Error: Xuint::Xunode is not accessible from Xuint::Rep"
>
> If I make Xunode public it works, but I don't understand why, because
> from my point of view the 'friend class Xuint;' should make the
> private parts of Xuint accessible to Rep.[/color]

Youve got it backwards. The friend declaration makes the private parts of Xuint::Rep available to Xuint.
What you need in Xuint is also a friend declaration:

friend class Xuint::Rep;

Because Xunode is private within Xuint, and Xuint::Rep has no special priviliges regarding Xuint, it can't see Xunode.