proper way of handling std::thread termination in child process after fork()

Frown as much as you want, I'm going to do it anyway :)

My question is: in the following code, what is the proper way to handle the termination of the std::thread in the subprocess generated by fork()? std::thread::detach() or std::thread::join()?

#include <thread>
#include <iostream>
#include <unistd.h>

struct A { 

   void Fork()
   {   
      std::thread t(&A::Parallel, this);
      pid_t pid = fork();
      if(pid) {
         //parent
         t.join();
      } else {
         //child
         t.join(); // OR t.detach()?
      }   
   }   

   void Parallel()
   {   
      std::cout << "asd" << std::endl;
   }   
};

int main() {
   A a;
   a.Fork();
   return 0;
}

I know that only the thread that calls fork() is duplicated, which means that the std::thread is actually doing nothing in the child process right? Hence my doubt.