Re: [boost] static in_class constants

Subject: Re: [boost] static in_class constants
From: Mateusz Loskot (mateusz_at_[hidden])
Date: 2010-03-13 13:34:34

Mateusz Loskot wrote:
> Christian Henning wrote:
>>> 2) I notice you saying that you're getting linker errors - that
>>> should only happen if you're taking the address of the constants
>>> somewhere - if that's a use case that you want to support, then you
>>> need a definition for the constants somewhere - suitably guarded by
>>> the right Boost.Config macros (sorry can't remember what off the
>>> top of my head).
>> I'm getting a compiler error with gcc 3.4.5. The compiler that comes
>> with MinGW.
> 
> Is it compiler error or linker error?
> 
> I did a simple test of the construction you use with GCC 4.4.1
> and I'm getting linker error:
> 
> #include <boost/config.hpp>
> #include <cassert>
> 
> template<typename Property>
> struct property_base
> {
>     typedef Property type;
> };
> 
> struct pnm_image_type : property_base<int>
> {
>     typedef pnm_image_type self_type;
>     BOOST_STATIC_CONSTANT( type, _mono_asc  = 1 );
> };
> 
> int main()
> {
>     pnm_image_type imt;
>     int const& r = pnm_image_type::_mono_asc;
>     assert(1 == r);
>     int const* p = &pnm_image_type::_mono_asc;
>     assert(1 == *p);
> }
> 
> 
> gcc.link bin/gcc-4.4.1/debug/static_const
> bin/gcc-4.4.1/debug/static_const.o: In function `main':
> /home/mloskot/workshop/boost/constants/static_const.cpp:17: undefined
> reference to `pnm_image_type::_mono_asc'
> /home/mloskot/workshop/boost/constants/static_const.cpp:18: undefined
> reference to `pnm_image_type::_mono_asc'
> collect2: ld returned 1 exit status
> 
>     "g++"    -o "bin/gcc-4.4.1/debug/static_const" -Wl,--start-group
> "bin/gcc-4.4.1/debug/static_const.o"  -Wl,-Bstatic  -Wl,-Bdynamic
> -Wl,--end-group -g
> 
> ...failed gcc.link bin/gcc-4.4.1/debug/static_const...
> ...failed updating 1 target...
> ...updated 1 target...
> 
> 
> This can be fixed, inconveniently, by adding external definition:
> 
> struct pnm_image_type : property_base<int>
> {
>     typedef pnm_image_type self_type;
>     BOOST_STATIC_CONSTANT( type, _mono_asc  = 1 );
> };
> const int pnm_image_type::_mono_asc;

One more thing, I've just reminded than John has mentioned this
issue already.

Now, the question is, do you take address of any of these constants?

Best regards,
-- 
Mateusz Loskot, http://mateusz.loskot.net
Charter Member of OSGeo, http://osgeo.org