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