Badge pattern

badge.cpp

template<typename T>
class Badge {
    friend T;
    Badge() {}
};

class Device;

class VFS {
public:
    static VFS& the() { static VFS instance; return instance; }
    void register_device(Device&, Badge<Device>)  {}
    void unregister_device(Device&, Badge<Device>){}
};

class Device {
public:
    Device()
    {
        VFS::the().register_device(*this, {});
    }
};