I think it is not the same as defining a global static variable. Global static variable will have internal linkage and therefore be visible inside the compilation unit where defined only. Between the compilation units these variables will be different.
No the fancy trick works. I've seen it used in XSD-tree's code which is completly a header only. They put all their globals in a template class with a single parameter char. MyClass<char>::variable has to be the same in all compilation scopes. The standard says so. So the linker has to eliminate duplicate symbols.
IMO C++ like approach would be to introduce an anonymous namespace in header files and define global variables there.
Defining static vars in the template class will result in a static variable per class template instantiation. Which is not exactly the same, as intended here.
Anonymous namespaces in headers are bad; as are declaring things static in headers. It makes the compiler generate seperate objects per compilation scope, which isn't what you want. The compiler warning is right. If I include this header in 20 compilation scopes I shouldn't have 20 copies of these constants (just refs) in my binary. At least the stuff in boost/system/error_code.hpp could easily be moved to a cxx. Boost::system is not a header only library. Chris