Re: Undefined reference to template function ...

[Axel Freyn <axel-freyn at gmx dot de>]

Hi,

On Tue, May 31, 2011 at 05:59:40AM +0000, Her, Il wrote:
> From: Ian Lance Taylor [mailto:iant@google.com] 
>> "Her, Il" <il.her@hp.com> writes:
>> 
>>> I experienced "undefined reference to template function ." when I am
>>> compiling my program with gcc 4.1.2. 
>>>
>>> Almost everyone says that's because template functions weren't be
>>> implemented in the same unit where they are prototyped. (I used .cpp
>>> file for implementation and .h file for declaration).
>>>
>>> But, It works if I use gcc 3.4.6 compiler without any change of
>>> options.  Why is this happening?...
>>> I am working with my customer, I can't make them understand the
>>> common rule for using template because it works on gcc 3.4.6.
>>>
>>> Can you explain this situation to me? 
>>> (Something like it is changed after 4.0 or you have any options to avoid this.)
>> 
>> It's impossible to give you a precise answer without a small example.
>> 
>> You may find it helpful to read
>> http://gcc.gnu.org/onlinedocs/gcc-4.6.0/gcc/Template-Instantiation.html
> 
> [...]
> 
> ** Result of compiling
> [test@test-1] $ g++ -c -dynamic sub.cpp
> [test@test-1] $ g++ -shared -o libsub.so sub.o
> [test@test-1] $ g++ -g -W -o main main.cpp -lsub -L./
> /tmp/ccaUlqlQ.o: In function `main`:
> /home/guinsa/main.cpp:10: undefined reference to `Base<TEST>::dump(TEST&)`
> /home/guinsa/main.cpp:11: undefined reference to `Base<TEST>::fn(int, int)`
> collect2: ld returned 1 exit status

To my knowledge it's impossible. You want to compile a library which
contains the "template code", and which is fully independent of the
type TEST for which you instantiate the template Base<...>.
If that would be possible, we could rewrite the complete C++ standard
library and compile it into a shared library :-)

I think, you have two ways to solve it:
1) really include also the template implementations in "sub.cpp" into
   main.cpp

2) Instantiate explicitely the necessary templates.
   In your example, you would need a line
template class Base<TEST>;
   somewhere in a file which includes the implementations.

You could e.g. add to sub.cpp the two lines (it's not necessary to
include TEST.h)

struct TEST;
template class Base<TEST>;

or generate a special "template instantiation library"  (as proposed on
http://gcc.gnu.org/onlinedocs/gcc-4.6.0/gcc/Template-Instantiation.html).

Axel