#include <mutex>

class some_big_object {
};

void
swap(some_big_object& lhs, some_big_object& rhs)
{
}

class X {
private:
  some_big_object some_detail;
  mutable std::mutex m;

public:
  X(some_big_object const& sd) : some_detail(sd) {}

  friend void swap(X& lhs, X& rhs)
  {
    if (&lhs == &rhs)
      return;
    std::unique_lock<std::mutex> lock_a(lhs.m, std::defer_lock);
    std::unique_lock<std::mutex> lock_b(rhs.m, std::defer_lock);
    std::lock(lock_a, lock_b);
    swap(lhs.some_detail, rhs.some_detail);
  }
};

int
main()
{
}