[Bug c++/19188] New: friend funtion inside a template class seems to have a problem

[Bug c++/19188] New: friend funtion inside a template class seems to have a problem

[max656 at hotmail dot com]

A template class(tstack class) has a singly-linked list "struct" declaration in
private section.
Another template class(tstackIterator) - iterator class is declared and defined
in global space.
If the iterator template class trys to get access to the linked list structure
inside the  previous class with a friend statement declared in   the previous
class, it does not work. It behaves in much the same way as without friend
declaration. The following code shows this:
//: TSTACK.H -- Stack using templates
#ifndef TSTACK_H_
#define TSTACK_H_

// Some implementations require this:
template<class T> class tstackIterator;


template<class T> 
class tstack {

 struct link {

    T* data;
    link* next;

    link(T* Data, link* Next){
      data = Data;
      next = Next;
    }
 } * head;
  int owns;



public:

  tstack(int Owns = 1) : head(0), owns(Owns) {}
  ~tstack();
  void push(T* Data) {
    head = new link(Data,head);
  }
  T* peek() const { return head->data; }
  T* pop();
  int Owns() const { return owns; }
  void Owns(int newownership) {
    owns = newownership;
  }

  friend class tstackIterator<T>;

};

template<class T>
T* tstack<T>::pop() {
  if(head == 0) return 0;
  T* result = head->data;
  link* oldHead = head;
  head = head->next;
  delete oldHead;
  return result;
}

template<class T>
tstack<T>::~tstack() {
  link* cursor = head;
  while(head) {
    cursor = cursor->next;
    // Conditional cleanup of data:
    if(owns) delete head->data;
    delete head;
    head = cursor;
  }
}

template<class T>
class tstackIterator {
////////////////////////////////////
  tstack<T>::link* p; // this is line 68

// when it is compiled, g++ 3.4.1 give some error message but it does not make
//sense in terms of languae. but the version g++ (GCC) 3.2.3 20030502 works fine 
// except that it gives a warning saying "tstack.h:68: warning: `typename
//tstack<T>::link' is implicitly a typename"
//The Error message is attached to the bottom
///////////////////////////////////
public:
  tstackIterator(const tstack<T>& tl)
    : p(tl.head) {}
  tstackIterator(const tstackIterator& tl)
    : p(tl.p) {}
  // operator++ returns boolean indicating end:
  int operator++() {
    if(p->next)
      p = p->next;
    else p = 0; // Indicates end of list
    return int(p);
  }
  int operator++(int) { return operator++(); }
  // Smart pointer:
  T* operator->() const {
    if(!p) return 0;
    return p->data;
  }
  T* current() const {
    if(!p) return 0;
    return p->data;
  }
  // int conversion for conditional test:
  operator int() const { return p ? 1 : 0; }
};
#endif // TSTACK_H_

////////////////////////////////////////////
the compiler complaints are as follows:

In file included from tstktst.cpp:2:
../14/tstack.h:68: error: expected `;' before '*' token
../14/tstack.h: In constructor `tstackIterator<T>::tstackIterator(const
tstack<T>&)':
../14/tstack.h:71: error: class `tstackIterator<T>' does not have any field
named `p'
../14/tstack.h: In copy constructor `tstackIterator<T>::tstackIterator(const
tstackIterator<T>&)':
../14/tstack.h:73: error: class `tstackIterator<T>' does not have any field
named `p'
../14/tstack.h: In member function `int tstackIterator<T>::operator++()':
../14/tstack.h:76: error: `p' undeclared (first use this function)
../14/tstack.h:76: error: (Each undeclared identifier is reported only once for
each function it appears in.)
../14/tstack.h: In member function `T* tstackIterator<T>::operator->() const':
../14/tstack.h:84: error: `p' undeclared (first use this function)
../14/tstack.h: In member function `T* tstackIterator<T>::current() const':
../14/tstack.h:88: error: `p' undeclared (first use this function)
../14/tstack.h: In member function `tstackIterator<T>::operator int() const':
../14/tstack.h:92: error: `p' undeclared (first use this function)

-- 
           Summary: friend funtion inside a template class seems to have a
                    problem
           Product: gcc
           Version: 3.4.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: max656 at hotmail dot com
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19188

[Bug c++/19188] friend funtion inside a template class seems to have a problem

[lerdsuwa at gcc dot gnu dot org]

------- Additional Comments From lerdsuwa at gcc dot gnu dot org  2004-12-29 14:20 -------
Not a bug.  You have to replace the code at line 68:

  tstack<T>::link* p;

with

  typename tstack<T>::link* p;

Check out the web page <http://gcc.gnu.org/gcc-3.4/changes.html>
and search for "You mustnow use the typename and template" ...
for more information.  There have been a lot of changes to the
C++ language since the old days and this is one of them.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19188

[Bug c++/19188] friend funtion inside a template class seems to have a problem

[max656 at hotmail dot com]

------- Additional Comments From max656 at hotmail dot com  2004-12-30 07:41 -------
Thanks for the reply.
In this one line of  code

tstack<T>::link* p;

"tstack<T>::link " is a typename. so I can not give another name.

Actually, I got other ways to get around this problem.
But I am interested in this style as well.
The code I am experimenting came from Bruce Eckel's old book.
The 2nd edtion uses a different way from this.

Anyway the definition of "tstack<T>::link" is not created inside the class. it
is used only to  declare a private pointer variable named "head".Actually it is
NOT defined. I think that's why g++ (GCC) 3.2.3 20030502 give the warning of
"implicite typename". But a new version, g++ 3.4.1, checks it out and send out a
error message of "../14/tstack.h:68: error: expected `;' before '*' token". I
don't have any idea of what this means. It does not make any sense at all.

Would you take a look at it and reply to me ?
I really appreciate it in advance.
Tyson

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19188

[Bug c++/19188] friend funtion inside a template class seems to have a problem

[lerdsuwa at gcc dot gnu dot org]

------- Additional Comments From lerdsuwa at gcc dot gnu dot org  2004-12-31 13:23 -------
The 'typename' keyword is required because later C++ introduces a lot more
features.  Those that interfere with your code are partial specialization 
and specialization.  For example, you can now have specialization

  template <> class tstack<bool> {
    int link;
    ...
  };

Then when you declare

  tstack<bool> t;

it will use the above declaration, where 'link' is now a member data
instead of a nested class.  So the code

  tstack<T>::link* p;

which could be interpreted as declaring a variable which is a pointer to type
'tstack<T>::link' if 'T' is 'int'.  But when 'T' is 'bool' it could means
multiplying a member data with another variable named 'p'.

To resolve the ambiguity, later C++ requires the 'typename' keyword to
treat as pointer declaration, otherwise it will be treated as muliplication.
Older C++ books don't have this 'typename' keyword but newer good books 
from respected authors coming out in the last few years should.  
(Many new C++ books are still wrong).

I know the parser error message could be improved, and there are plenty of
bug reports elsewhere about it.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19188

[Bug c++/19188] friend funtion inside a template class seems to have a problem

[max656 at hotmail dot com]

------- Additional Comments From max656 at hotmail dot com  2005-01-01 01:37 -------
Subject: RE:  friend funtion inside a template class seems to have a probl

Hi lerdsuwa

You came up with an interesting topic - class template specializtion.
I didn't know aboou it. I   need further research on that.

As you mentioned, the keyword "typename' is added and compiled and then it 
works fine.
Thanks a lot for your detailed explanation.
It haunted me a lot. I was completely stuck for a while even though I found 
some other ways.

Tyson

>From: "lerdsuwa at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>
>Reply-To: gcc-bugzilla@gcc.gnu.org
>To: max656@hotmail.com
>Subject: [Bug c++/19188] friend funtion inside a template class seems to 
>have a problem
>Date: 31 Dec 2004 13:23:03 -0000
>
>------- Additional Comments From lerdsuwa at gcc dot gnu dot org  
>2004-12-31 13:23 -------
>The 'typename' keyword is required because later C++ introduces a lot more
>features.  Those that interfere with your code are partial specialization
>and specialization.  For example, you can now have specialization
>
>   template <> class tstack<bool> {
>     int link;
>     ...
>   };
>
>Then when you declare
>
>   tstack<bool> t;
>
>it will use the above declaration, where 'link' is now a member data
>instead of a nested class.  So the code
>
>   tstack<T>::link* p;
>
>which could be interpreted as declaring a variable which is a pointer to 
>type
>'tstack<T>::link' if 'T' is 'int'.  But when 'T' is 'bool' it could means
>multiplying a member data with another variable named 'p'.
>
>To resolve the ambiguity, later C++ requires the 'typename' keyword to
>treat as pointer declaration, otherwise it will be treated as 
>muliplication.
>Older C++ books don't have this 'typename' keyword but newer good books
>from respected authors coming out in the last few years should.
>(Many new C++ books are still wrong).
>
>I know the parser error message could be improved, and there are plenty of
>bug reports elsewhere about it.
>
>--
>
>
>http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19188
>
>------- You are receiving this mail because: -------
>You reported the bug, or are watching the reporter.




-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19188