confusing usage of thread in C++ [duplicate]

I am seeing a confusing bit of code in C++. What is happening with _capture_thread here. what is abc inheriting ?

class abc
{
   private:
           std::thread _capture_thread;
   public:
           abc();
           void start();

};

abc::abc() : _capture_thread()
{
   . 
   .
}

abc::start()
{
    _capture_thread = std::thread(param1, param2,...paramn);
}