The following single-file testcase fails to link: #include <boost/thread/thread.hpp> #include <boost/thread/recursive_mutex.hpp> #include <iostream> template<typename Type> class singleton { public: typedef boost::recursive_mutex mutex_type; typedef Type * handle; private: typedef mutex_type::scoped_lock lock_type; static mutex_type mutex; static handle inst; public: static Type &instance(void) { lock_type lock(mutex); if (!inst) { inst = new Type; } return(*inst); }; }; class tester { public: void test(void) { std::cerr << "Test" << std::endl; }; }; template<> singleton<tester>::mutex_type singleton<tester>::mutex; template<> singleton<tester>::handle singleton<tester>::inst = 0; int main(void) { singleton<tester>::instance().test(); return(0); } thread.o: In function `singleton<tester>::instance()':/home/greened/projects/apsim/current/apsim/msim/test/thread.cc:21: undefined reference to `singleton<tester>::mutex' collect2: ld returned 1 exit status I was getting undefined references to "inst" before I added the initializer. Is this a new adherance to the standard by g++ 4.0.2? I didn't used to have to add the initializer to get it to link. This is troublesome with boost::recursive_mutex because it is noncopyable. How do I specify an initializer for it? Is it just not possible to have a mutex static member? -Dave