I don't know what's wrong with my very simple code

[Sam]

Hi I'm learning to code with C++ and wrote some very simple code. I
think it's consistent with every rule but always got compiling errors
that I don't understand.

The code include 5 files as following, delimited by //////:

////////////////pose.h
#ifndef pose_h
#define pose_h
#include "point.h"
class pose
{
int i;
public:
friend void point::init(pos e P);
};
#endif

////////////////pose.cpp
#include "pose.h"

////////////////point.h
#ifndef point_h
#define point_h
#include "pose.h"
class point
{
public:
void init(pose P);
};
#endif

////////////////point.cpp
#include "point.h"
void point::init(pos e P)
{
P.i=1;
}

////////////////main.cpp
#include "pose.h"
#include "point.h"
void main()
{
}
Basically the "point" class has a function to modify the "pose"
object's private data "i". VC++ always gives the error messages like:
'point' : is not a class or namespace name
'i' : cannot access private member declared in class 'pose'
syntax error : identifier 'pose'
nonexistent function 'point::init' specified as friend

I cannot understand it. Could anybody tell me what he thinks of it?
Thanks a lot.

[osmium]

Sam writes:

Hi I'm learning to code with C++ and wrote some very simple code. I
think it's consistent with every rule but always got compiling errors
that I don't understand.

The code include 5 files as following, delimited by //////:

////////////////pose.h
#ifndef pose_h
#define pose_h
#include "point.h"
class pose
{
int i;
i is, by default private. Which is what the compiler told you. Good
practice says it *should be* private. Use a friend class/function or use an
accessor to fiddle with the value of i. For a real program you should
probably rethink your overall design.

You are on the verge of developing some bad habits.
public:
friend void point::init(pos e P);
};
#endif

////////////////pose.cpp
#include "pose.h"

////////////////point.h
#ifndef point_h
#define point_h
#include "pose.h"
class point
{
public:
void init(pose P);
};
#endif

////////////////point.cpp
#include "point.h"
void point::init(pos e P)
P.i=1;
}

////////////////main.cpp
#include "pose.h"
#include "point.h"
void main()
{
}
Basically the "point" class has a function to modify the "pose"
object's private data "i". VC++ always gives the error messages like:
'point' : is not a class or namespace name
'i' : cannot access private member declared in class 'pose'
syntax error : identifier 'pose'
nonexistent function 'point::init' specified as friend

[Jakob B. Olsen]

A friend function can not be a member of a class. So point::init can not be
a friend to pose.

Jakob

"Sam" <hc***@nd.edu > wrote in message
news:f0******** *************** **@posting.goog le.com...

Hi I'm learning to code with C++ and wrote some very simple code. I
think it's consistent with every rule but always got compiling errors
that I don't understand.

The code include 5 files as following, delimited by //////:

////////////////pose.h
#ifndef pose_h
#define pose_h
#include "point.h"
class pose
{
int i;
public:
friend void point::init(pos e P);
};
#endif

////////////////pose.cpp
#include "pose.h"

////////////////point.h
#ifndef point_h
#define point_h
#include "pose.h"
class point
{
public:
void init(pose P);
};
#endif

////////////////point.cpp
#include "point.h"
void point::init(pos e P)
{
P.i=1;
}

////////////////main.cpp
#include "pose.h"
#include "point.h"
void main()
{
}
Basically the "point" class has a function to modify the "pose"
object's private data "i". VC++ always gives the error messages like:
'point' : is not a class or namespace name
'i' : cannot access private member declared in class 'pose'
syntax error : identifier 'pose'
nonexistent function 'point::init' specified as friend

I cannot understand it. Could anybody tell me what he thinks of it?
Thanks a lot.

[Adam Fineman]

osmium wrote:

Sam writes:

Hi I'm learning to code with C++ and wrote some very simple code. I
think it's consistent with every rule but always got compiling errors
that I don't understand.

The code include 5 files as following, delimited by //////:

////////////////pose.h
#ifndef pose_h
#define pose_h
#include "point.h"
class pose
{
int i;

i is, by default private. Which is what the compiler told you. Good
practice says it *should be* private. Use a friend class/function or use an
accessor to fiddle with the value of i. For a real program you should
probably rethink your overall design.

You are on the verge of developing some bad habits.

The design issue aside, the OP's question is about why the friend
declaration doesn't work as he thinks it should. He knows that i is
private, and he did have a friend function declared.

To the OP, you could get around your problem by using a combination of a
reference parameter and a forward declaration:

////////////////////////////////////////////////////////////////////////////
// A.hpp
#ifndef A_HPP_INCLUDED
#define A_HPP_INCLUDED

#include "B.hpp"

class A
{
/* ... */
friend void B:f(A& a); // A forward declaration does not suffice
// here; you must have the full definition
// of B, hence the #include above.
/* ... */
};

#endif // include guard

////////////////////////////////////////////////////////////////////////////
// B.hpp
#ifndef B_HPP_INCLUDED
#define B_HPP_INCLUDED

class A; // forward declaration

class B
{
/* ... */
public:
void f(A& a); // Note that for a reference or pointer
// to A, you only need the forward
// declaration, not the full class
// definition.

/* ... */
};
The reason what you tried earlier didn't work is that your include
directives would have resulted in a circular dependency, which your
include guard prevented. (That's what include guards are for, after all.)

If you can't use a reference parameter, then you will have to try
something else -- like a public accessor method in class A. However,
you should prefer to pass classes by reference in most situations, as
passing by value invokes the class' copy constructor. Note that in that
case, the function would only modify a copy of the parameter anyway,
which is almost certainly not what you want.

HTH,

- Adam

--
Reverse domain name to reply.

[Brian MacBride]

"Sam" <hc***@nd.edu > wrote in message
news:f0******** *************** **@posting.goog le.com...

Hi I'm learning to code with C++ and wrote some very simple code. I
think it's consistent with every rule but always got compiling errors
that I don't understand.

The code include 5 files as following, delimited by //////:

////////////////pose.h
#ifndef pose_h
#define pose_h
#include "point.h"
class pose
{
int i;
public:
friend void point::init(pos e P);
};
#endif

////////////////pose.cpp
#include "pose.h"

////////////////point.h
#ifndef point_h
#define point_h
#include "pose.h"
class point
{
public:
void init(pose P);
};
#endif

////////////////point.cpp
#include "point.h"
void point::init(pos e P)
{
P.i=1;
}

////////////////main.cpp
#include "pose.h"
#include "point.h"
void main()
{
}
Basically the "point" class has a function to modify the "pose"
object's private data "i". VC++ always gives the error messages like:
'point' : is not a class or namespace name
'i' : cannot access private member declared in class 'pose'
syntax error : identifier 'pose'
nonexistent function 'point::init' specified as friend

I cannot understand it. Could anybody tell me what he thinks of it?
Thanks a lot.

//////////////// pose.h
#ifndef pose_h
#define pose_h

#include "point.h"
class pose {
int i;
friend void point::init (pose &P);
};

#endif

//////////////// point.h
#ifndef point_h
#define point_h

class pose;

class point {
public:
void init (pose &);
};

#endif

//////////////// pose.cpp
#include "pose.h"

//////////////// point.cpp

#include "point.h"
#include "pose.h"

void point::init (pose &P) {
P.i = 1;
}

//////////////// main.cpp

#include "pose.h"
#include "point.h"
int main () {
pose p;
point pt;
pt.init (p);
}

I think you mean to pass the 'pose' by reference? As others have mentioned,
there are probably better ways of achieving your goal here.

Regards

Brian

[dwizard]

"Jakob B. Olsen" <ja***********@ hotmail.com> wrote in message
news:3f******** **************@ dread14.news.te le.dk...

A friend function can not be a member of a class. So point::init can not be a friend to pose.

This is total nonesense.

[Sam]

"Jakob B. Olsen" <ja***********@ hotmail.com> wrote in message news:<3f******* *************** @dread14.news.t ele.dk>...

A friend function can not be a member of a class. So point::init can not be
a friend to pose.

Jakob

I checked it out from the book "thinking in c++" finding that a friend
fn can be a member of a class.

Now I think the problem might be, whichever class is compiled first,
it needs the info of the the other class. When it goes to the other
class, it finds that it needs the previous class. That's the way I
understand it because I switched the order of compiling classes but
always found the compiler thinks the class (or class member fn) is not
defined.

Sam
"Sam" <hc***@nd.edu > wrote in message
news:f0******** *************** **@posting.goog le.com...

Hi I'm learning to code with C++ and wrote some very simple code. I
think it's consistent with every rule but always got compiling errors
that I don't understand.

The code include 5 files as following, delimited by //////:

////////////////pose.h
#ifndef pose_h
#define pose_h
#include "point.h"
class pose
{
int i;
public:
friend void point::init(pos e P);
};
#endif

////////////////pose.cpp
#include "pose.h"

////////////////point.h
#ifndef point_h
#define point_h
#include "pose.h"
class point
{
public:
void init(pose P);
};
#endif

////////////////point.cpp
#include "point.h"
void point::init(pos e P)
{
P.i=1;
}

////////////////main.cpp
#include "pose.h"
#include "point.h"
void main()
{
}
Basically the "point" class has a function to modify the "pose"
object's private data "i". VC++ always gives the error messages like:
'point' : is not a class or namespace name
'i' : cannot access private member declared in class 'pose'
syntax error : identifier 'pose'
nonexistent function 'point::init' specified as friend

I cannot understand it. Could anybody tell me what he thinks of it?
Thanks a lot.

[Amitashk]

In file point.cpp, the first line of code is:

#include "point.h"

Try changing it to:

#include "pose.h"

[tom_usenet]

On 6 Jan 2004 06:41:20 -0800, hc***@nd.edu (Sam) wrote:

Hi I'm learning to code with C++ and wrote some very simple code. I
think it's consistent with every rule but always got compiling errors
that I don't understand.

The code include 5 files as following, delimited by //////:

////////////////pose.h
#ifndef pose_h
#define pose_h
#include "point.h"
class pose
{
int i;
public:
friend void point::init(pos e P);
};
#endif

////////////////pose.cpp
#include "pose.h"

////////////////point.h
#ifndef point_h
#define point_h
#include "pose.h"
The above line shouldn't be there - it introduces a circular
dependency (pose.h depends on point.h which in turn depends on pose.h
and so on). Instead, just use a forward declaration:
class pose;
class point
{
public:
void init(pose P);
I think you intended to pass by reference:

void init(pose& P);
};
#endif

////////////////point.cpp
#include "point.h"
and here you now need:
#include "pose.h" //need complete definition of pose.
void point::init(pos e P)
{
P.i=1;
}

////////////////main.cpp
#include "pose.h"
#include "point.h"
void main()
{
}

Tom

C++ FAQ: http://www.parashift.com/c++-faq-lite/
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html

[dwizard]

#ifndef point_h
#define point_h

#include "pose.h"

class pose; // <--------- ADD THIS forward decleration

class point
{
public:
void init(pose P);
};

#endif