I think that I've run across a bug, either in Units, or in MSVC. Because it only appears when precompiled headers are in use, I suspect it may be a bug in MSVC. Basically, if BOOST_UNITS_STATIC_CONSTANT is used in a header that is compiled into the precomiled header, there will be linker errors. Here is my information: Microsoft Windows XP Professional x64 Edition Microsoft Development Environment 2003 Version 7.1.6030 Microsoft Visual Studio .NET 2003 Service Pack 1 (KB918007) Below is the minimial code to reproduce the error. There are 4 files, stdafx.h, stdafx.cpp (which is used to create the pch), a.cpp, and b.cpp. The same code without precompiled headers turned on links fine. // start of stdafx.cpp #include "stdafx.h" // end of stdafx.cpp // start of stdafx.h #ifndef STDAFX_H_328A91DC_3924_466E_80EA_C9BB6CBF7638 #define STDAFX_H_328A91DC_3924_466E_80EA_C9BB6CBF7638 #include BOOST_UNITS_STATIC_CONSTANT(ONE_METER, boost::units::si::length); #endif // end of stdafx.h // start of a.cpp #include "stdafx.h" int main(void) { boost::units::si::meter; return 0; } // end of a.cpp // start of b.cpp #include "stdafx.h" int main2(void) { boost::units::si::meter; return 0; } // end of b.cpp Here is the error message: unit.obj : error LNK2005: "class unit >,struct boost::units::dimensionless_type>,struct boost::units::homogeneous_system > >,struct boost::units::list > > > > > > > >

,void>::boost::unit >,struct boost::units::dimensionless_type>,struct boost::units::homogeneous_system > >,struct boost::units::list > > > > > > > > >,void> const & const boost::units::si::`anonymous namespace'::meter" (?meter@?A0x490e06d9@si@units@boost@@3ABV?$unit@U?$list@U?$dim@Ulength_base_dimension@units@boost@@V?$static_rational@$00$00@23@@units@boost@@Udimensionless_type@23@@units@boost@@U?$homogeneous_system@U?$list@Umeter_base_unit@si@units@boost@@U?$list@U?$scaled_base_unit@Ugram_base_unit@cgs@units@boost@@U?$scale@$09V?$static_rational@$02$00@units@boost@@@34@@units@boost@@U?$list@Usecond_base_unit@si@units@boost@@U?$list@Uampere_base_unit@si@units@boost@@U?$list@Ukelvin_base_unit@si@units@boost@@U?$list@Umole_base_unit@si@units@boost@@U?$list@Ucandela_base_unit@si@units@boost@@U?$list@Uradian_base_unit@angle@units@boost@@U?$list@Usteradian_base_unit@angle@units@boost@@Udimensionless_type@34@@34@@34@@34@@34@@34@@34@@23@@34@@units@boost@@@23@X@34@B) already defined in stdafx.obj unit2.obj : error LNK2005: "class unit >,struct boost::units::dimensionless_type>,struct boost::units::homogeneous_system > >,struct boost::units::list > > > > > > > > ,void>::boost::unit >,struct boost::units::dimensionless_type>,struct boost::units::homogeneous_system > >,struct boost::units::list > > > > > > > > >,void> const & const boost::units::si::`anonymous namespace'::meter" (?meter@?A0x490e06d9@si@units@boost@@3ABV?$unit@U?$list@U?$dim@Ulength_base_dimension@units@boost@@V?$static_rational@$00$00@23@@units@boost@@Udimensionless_type@23@@units@boost@@U?$homogeneous_system@U?$list@Umeter_base_unit@si@units@boost@@U?$list@U?$scaled_base_unit@Ugram_base_unit@cgs@units@boost@@U?$scale@$09V?$static_rational@$02$00@units@boost@@@34@@units@boost@@U?$list@Usecond_base_unit@si@units@boost@@U?$list@Uampere_base_unit@si@units@boost@@U?$list@Ukelvin_base_unit@si@units@boost@@U?$list@Umole_base_unit@si@units@boost@@U?$list@Ucandela_base_unit@si@units@boost@@U?$list@Uradian_base_unit@angle@units@boost@@U?$list@Usteradian_base_unit@angle@units@boost@@Udimensionless_type@34@@34@@34@@34@@34@@34@@34@@23@@34@@units@boost@@@23@X@34@B) already defined in stdafx.obj

Any insights would be appreciated. Thanks, David