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;

Solution

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.

Comments

[Dmitry Yudakov]

10x, it did fix the linking problem. I tried defining it before but missed the type before MyClass::mx

[Gr-Disarray]

Why is this required? and what is the difference between declaration and definition?