[Bug c++/20011] New: Templates and cyclic dependencies

[Bug c++/20011] New: Templates and cyclic dependencies

[jmoro at latentzero dot com]

Hello, the new support of two-stage name-lookup for templates in gcc 3.4.0 for
C++ brings a problem that
I don't know how to resolve.
I would like to know what is your recommended way of dealing with the following.


Here is some code that used to compile and link on gcc 3.3 (sorry to copy some
code but that's key to understanding the problem)

///////
// A.h
///////
class B;

class A
{
public: 
 
  A(){}
  ~A(){}

  template <class T> int f( T t )
  {
    B b;
    return ( b.getInt() + t );
  }

  int getInt() const
  {
    return 1;
  }
};


//////
//B.h
//////
class A;

class B
{
public:

  B(){}
  ~B(){}

  template <class T> int g( T t )
  {
    A a;
    return ( a.getInt() + t );
  }

  int getInt() const
  {
    return 2;
  }
};


///////////
//Main.cpp
///////////
#include "A.h"
#include "B.h"

int main( void )
{
  A a;
  int i = 1;
  a.f( i );

  B b;
  b.g( i );
}


As you can see in A.h, the template method f() uses B and in B.h, the template
method g() uses A.
As these methods were only compiled at instantiation time (with gcc 3.3) the
fact that A uses B and B uses A was not a problem but if I compile the same code
with gcc 3.4

I have:

> gcc -v
Reading specs from /var/gnu/3.4.0/lib/gcc/sparc-sun-solaris2.8/3.4.0/specs
Configured with: ../gcc-3.4.0/configure --prefix=/var/gnu/3.4.0 --disable-nls
--enable-languages=c,c++
Thread model: posix
gcc version 3.4.0


>  gcc -c -save-temps Main.cpp
In file included from Main.cpp:1:
A.h: In member function `int A::f(T)':
A.h:13: error: invalid use of undefined type `struct B'
A.h:1: error: forward declaration of `struct B'

This is normal as with 3.4, B needs to be known at definition time in A.h as it
is not dependent on a template parameter.
But obvioulsy, if I change my code so that A.h includes B.h, and B.h includes
B.h, I have a cyclic dependency and it won't compile.

The next step in order to remove such a dependency is usually to get the method
code out of the header file and put it in the cpp file. But of course, this is a
template method and the only method I know to do this with templates is to use
the keyword export... that you don't support.

This last point is fair enough as lots of compilers don't support it but I would
like to know what is you recommended solution to fix this problem?

-- 
           Summary: Templates and cyclic dependencies
           Product: gcc
           Version: 3.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P1
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jmoro at latentzero dot com
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20011

[Bug c++/20011] Templates and cyclic dependencies

[jmoro at latentzero dot com]

------- Additional Comments From jmoro at latentzero dot com  2005-02-16 18:06 -------
Created an attachment (id=8204)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=8204&action=view)
source code and *.ii files


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20011

[Bug c++/20011] Templates and cyclic dependencies

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-02-16 18:52 -------
The easy way to fix this is to make the template function not be in the class definition.

Anyways if you read a good C++ book, this would be explained there.

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20011

[Bug c++/20011] Templates and cyclic dependencies

[jmoro at latentzero dot com]

------- Additional Comments From jmoro at latentzero dot com  2005-02-17 10:47 -------
I am sorry but that is not good enough. If I read correcly, you recommend that
to solve the problem with my template function member of a class is ... not to
use template functions as members of the class.

My example was indeed simple and you could get the template out of the class but
if I alter slightly B.h, you can see why I want g() to be a member of the class.

I can obviouly come up with solutions where I don't use member template
functions or even where I don't use templates at all but what I want to know is
how you are supposed to deal with this problem at hand.

What is the recommended way, with gcc 3.4, to deal with template methods and
circular dependencies?

Again, when I read a good C++ book, the solution is to use the export keyword
but GCC does not support it.

///////
// B.h
///////
class A;

class B
{
public:

  B(){}
  ~B(){}

  template <class T> int g( T t )
  {
    A * a = GetA();
    return ( a->getInt() + t );
  }

  int getInt() const
  {
    return 2;
  }
  
private:

  A * GetA();
  
};

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20011

[Bug c++/20011] Templates and cyclic dependencies

[giovannibajo at libero dot it]

------- Additional Comments From giovannibajo at libero dot it  2005-02-17 11:58 -------
Maybe it is not very clear to you, but this is the GCC Bugzilla. This is where 
people report *bugs* of the GCC compiler. It is not a help desk for C++ 
programmers. If you have a BUG to report about GCC, we're eager to know. If you 
do not know how to write your program so that it follows all the ISO C++ rules, 
we really cannot help. Try asking in a newsgroup like comp.lang.c++.moderated.

Anyway, the solution for your problem is very easy, you just need to define 
your member functions outside the class definition.

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20011

[Bug c++/20011] Templates and cyclic dependencies

[jmoro at latentzero dot com]

------- Additional Comments From jmoro at latentzero dot com  2005-02-17 12:11 -------
Fair enough, you can indeed resolve the circular dependencies by moving the
definition but you still have more dependencies between files than you used to
with 3.3.

It's all good to follow the standard but implementing the two phase lookup
without implementing the export keyword makes it difficult to minimize the
dependencies.

Anyway, thanks for your "help"

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20011