Re: class template

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

Hi Eric,

On Fri, Jun 10, 2011 at 04:48:01PM -0700, eric wrote:
> Dear g++ programers:
> 
>   I tried to copy a simple code about template and class which use arry
> of (int or double or char)
> but I can not get pass compile(g++4.5.2, I install gcc 4.5.2 but not
> gcc-g++ 4.5.2; however when I g++ -v, it still show it is 4.5.2 from
> my old 4.4.3) from a book (C++Primer3rdEd, chapter2).  Please help. Eric
> ----------------------------------
> 
> eric@eric-laptop:~/CppPrimer3$ g++ Array.cpp pg52.cpp
> /tmp/ccE9MPMg.o: In function `main':
> pg52.cpp:(.text+0x23): undefined reference to `Array<int>::Array(int)'
> pg52.cpp:(.text+0x37): undefined reference to
> `Array<double>::Array(int)'
> pg52.cpp:(.text+0x4b): undefined reference to `Array<char>::Array(int)'
> collect2: ld returned 1 exit status

This CAN'T work, because your code is illegal C++: you have to include
the function definitions of the template-functions Array<>::Array(int)
at least in one file where you use them.
 - when g++ compiles "Array.cpp", it does NOT GENERATE ANY CODE  --
   because no template is used here
 - when g++ compiles "pg52.cpp", it suddenly learns "Oh, I need the
   class Array<int>" -- but then it won't read again Array.cpp to learn
   how those funcions are defined & generate them to pass them to the
   linker
==> you get the undefined references - errors...

The easiest solution is to change your file "pg52.cpp":
replace 
#include "Array.h"
by
#include "Array.cpp"

and compile the program as
g++ pg52.cpp
(without mentioning Array.cpp on the commandline)
This way, while g++ compiles pg52.cpp, it also knows all the contents of
Array.cpp and thus will generate the necessary template functions.


Some gcc-specific information about template instantiation can be found
here:
http://gcc.gnu.org/onlinedocs/gcc/Template-Instantiation.html


HTH,

Axel