What is the proper syntax here?

ALL,

class __declspec(dllexport) Foo
{
protected:
    struct Bar;
    Bar *pimpl;
};

struct Foo::Bar
{
    static std::mutex my_mutex;
};

Class Foo is interface class with a bunch of pure virtual functions that is located inside 1 header file. This header file is located inside a statically linked library.

Now in order to prevent a link error I'm trying to define my_mutex variable, but I can't figure out the syntax for it.

An obvious

Foo::pimpl::my_mutex;

gives an error.

Could someone please help?

TIA!