Using static mutex in a class

I have a class that I can have many instances of. Inside it creates and initializes some members from a 3rd party library (that use some global variables) and is not thread-safe. I thought about using static boost::mutex, that would be locked in my class constructor and destructor. Thus creating and destroying instances among my threads would be safe for the 3rd party members. ``` class MyClass { static boost::mutex mx; // 3rd party library members public: MyClass(); ~MyClass(); }; MyClass::MyClass() { boost::mutex::scoped_lock scoped_lock(mx); // create and init 3rd party library stuff } MyClass::~MyClass() { boost::mutex::scoped_lock scoped_lock(mx); // destroy 3rd party library stuff } ``` I cannot link because I receive error: ``` undefined reference to `MyClass::mx` ``` Do I need some special initialization of such static member? Is there anything wrong about using static mutex? Edit: Linking problem is fixed with correct definition in cpp ``` boost::mutex MyClass::mx; ```