Should random distributions be passed by reference or be object members in c++

Assuming we only instantiate less than 20 objects of class Blob and regarding efficiency (time execution) and memory management issues, is there a best option between:

• Setting the random generator and the generated distributions as private class members such as:

  class Blob {
  private:
  std::mt19937 engine;
  std::uniform_real_distribution<double> R_distribution;
  std::binomial_distribution<int> B_distribution;
  }
  
  

  and using them directly in Blob methods. Thus when we call a distribution, we also alter the state of the engine that is a member.

• Or setting the random generator as a private class members and passing the distributions by reference to the methods? For instance:

  class Blob {
  private:
  std::mt19937 engine; //engine
  }
  
  void Blob::run() {
  int blabla = 10;
  std::uniform_real_distribution<double> R_distribution(0, 10);
  do_something(blabla, R_distribution);
  ...
  }
  
  

While passing by reference induce lower overhead in general, does it matter in that case in particular? How does the overall question scale when calling the distributions a huge number of times (10^9 or more)?