[Insight-users] export template in VXL classes

[Luis Ibanez]

Hi Eric,

Thanks for pointing this out.


It seems that you are right.

The "export" statements are simply ignored.


For example, a file "export.cxx" with code:

------------------------------------------------------------
export template <class T> class foo;

template <class T>
class foo
{
public:

};
------------------------------------------------------------

when compiled as

             g++  -c   export.cxx

produces:

export.cxx:1: warning: keyword ‘export’ not implemented, and will be ignored


with gcc version:

    g++ (Ubuntu 4.4.1-4ubuntu9) 4.4.1


---

Please note that ITK itself (Insgith/Code)
doesn't use "export" statements.

--

Just for the record, you may find interesting
this Wiki page
http://www.itk.org/Wiki/Proposals:Explicit_Instantiation



    Regards,


          Luis


------------------------------------------------------------
On Wed, May 12, 2010 at 5:03 PM, Eric Nodwell <enodwell at ucalgary.ca> wrote:
> Hi,
>
> Does anyone know if the export template declarations in the VXL classes actually do anything?  For example, in vnl_matrix.h there is the following line:
>
>   export template <class T> class vnl_matrix;
>
> This is a bit puzzling, because as far as I know:
>
> 1. "export templates" is not supported by any commonly used compiler.  (In fact, why does this line not generate a compile error for most compilers?  Do they just ignore export template statements?)
>
> 2. vnl_matrix.txx provides a large number of common explicit instantiations in any case and it seems that in practice you have to provide your own explicit instantiations if you want some odd-ball type, with VXL providing a macro for this purpose (see http://public.kitware.com/vxl/doc/release/books/core/book_17.html#SEC194).
>
> 3. "export" is deprecated until some indeterminate future date when the C++ standards committee can figure out the best way to separate template definitions and implementations.  See http://en.wikipedia.org/wiki/C%2B%2B#Exported_templates .
>
> This is obviously not a burning issue, but I'm curious if this line has any actual function, or if it's a relic from the past, or if it's a hopeful anticipation of the future.
>
> Eric
>
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