Using static mutex in a class

Question

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` 

1. Do I need some special initialization of such static member?

2. Is there anything wrong about using static mutex?

Edit: Linking problem is fixed with correct definition in cpp

boost::mutex MyClass::mx; 

Answer 1

You have declared, but not defined your class static mutex. Just add the line

boost::mutex MyClass::mx; 

to the cpp file with the implementation of MyClass.