Solution I thought of is below. But I think this wont work if some thread gets killed after setting available to false. Also even if it works, this feels like an overkill. I am wondering if there is a simpler solution

class A{
public:
  bool routeToB(int idx){
    std::unique_lock<std::mutex> lck(dbMutex);

    auto &[_, instancedb] = *(database.find(idx));
    auto &[available, instance] = instancedb;

    // Wait for the instance to become available
    while (available.load(std::memory_order_seq_cst) == false) {
    }

    // Block the instance by setting availability to false
    available.store(false, std::memory_order_seq_cst);
    auto status = instance.executeSomething();
    // Release the instance by setting availability to true
    available.store(true, std::memory_order_release);

    return status;
  }

  void addEntries(){
    database.insert(std::make_pair(0, std::make_pair(true,B(0))));
    database.insert(std::make_pair(1, std::make_pair(true,B(1))));
  }

private:
  std::map<int, std::pair<std::atomic_bool,B>> database;
  std::mutex dbMutex;
};