From: thon AT irp DOT uni-stuttgart DOT de (andreas thon)
Subject: Re: Does DJGPP support C++ templates?
To: nfluhr1 AT gl DOT umbc DOT edu (Neil L. Fluhr ; BS CMSC)
Date: Thu, 16 Mar 1995 11:47:00 +0100 (MET)
Cc: djgpp AT sun DOT soe DOT clarkson DOT edu

> 
> I am trying to use a List template class and I keep getting:
> 
>    "undefined reference List<int>::Insert()"
>    "undefined reference List<int>::First()"
> 
>     ...and so on for every List function that is called in my client
>        program. 
> 
> I am using my own list implementation (not the one included with DJGPP). I 
> have my List template class declaration in my header file (list.h) and the
> implementation in my list.cc file. My client program is in its separate  
> file (intlist.cc).
> 
> I have tried the -v switch and the above output only occurs after the linker
> is called.
> 
> Any ideas? Please send responses to my e-mail address <nfluhr1 AT gl DOT umbc DOT edu>
> as I do not subscribe to the mailing list.
> 
> Thanks in advance!
> Neil
> 

I have just found the answer in a FAQ in the gnu.g++.help newsgroup:

I get undefined symbols when using templates
============================================

   (Thanks to Jason Merrill for this section).

   g++ does not automatically instantiate templates defined in other
files.  Because of this, code written for cfront will often produce
undefined symbol errors when compiled with g++.  You need to tell g++
which template instances you want, by explicitly instantiating them in
the file where they are defined.  For instance, given the files

   `templates.h':
     template <class T>
     class A {
     public:
       void f ();
       T t;
     };
     
     template <class T> void g (T a);

   `templates.cc':
     #include "templates.h"
     
     template <class T>
     void A::f () { }
     
     template <class T>
     void g (T a) { }

   main.cc:
     #include "templates.h"
     
     main ()
     {
       A<int> a;
       a.f ();
       g (a);
     }

   compiling everything with `g++ main.cc templates.cc' will result in
undefined symbol errors for `A<int>::f ()' and `g (A<int>)'.  To fix
these errors, add the lines

     template class A<int>;
     template void g (A<int>);

   to the bottom of `templates.cc' and recompile.



Andreas