Visual C++ :: Exporting Struct From DLL

I'm trying to build the gtkmm library using MSVC. One of its source files declares a struct called BuiltinStockID together with some objects of that type - e.g.

Code:
// In the header file:-
namespace Gtk {
struct BuiltinStockID {
const char* id;

[Code] ....

GTKMM_API is obviously _declspec(dllimport) (or export) as appropriate. It seems a bit unusual to use extern in addition GTKMM_API but the code won't compile if I try to exclude it. The actual instantiations are in one of the library's source files- e.g.

Code:
// In a source file
const Gtk::BuiltinStockID APPLY = { GTK_STOCK_APPLY };
const Gtk::BuiltinStockID CANCEL = { GTK_STOCK_CANCEL };
const Gtk::BuiltinStockID OK = { GTK_STOCK_OK };

GTK_STOCK_APPLY etc are simple strings such as "gtk-apply". They mostly refer to image files for the library to load (in this case, for dialog button images) although that's probably not relevant to the problem.

If I run dumpbin /EXPORTS on the built DLL it does seem to have some exported symbols with the names Gtk::Stock::APPLY, Gtk::Stock::CANCEL, Gtk::Stock::OK etc. However, if I write this in one of my own source files:-

Code:
Gtk::BuiltinStockID MyStockID;
MyStockID = Gtk::Stock::CANCEL; // <--- debugger reports <Bad Ptr>
You can see that the Gtk::Stock::CANCEL object is invalid (my debugger reports it as a bad pointer). Likewise, if I try to pass Gtk::Stock::CANCEL as a parameter to a function.