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tricky around c++ unique_ptr and callback

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Background

  • Currently our team is working with a kind of Graph Processing Engine. At first glance, we implement it using EDP(event-driven-programming) based on DAG. But we found that DAG is not powerful enough for future extension like:
      1. Cycle
      1. Dynamic Graph
  • So we decide to Implement it using MDP(message-driven-programming). each time a msg is received, it will trigger a callback function to process the msg.
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#include <iostream>
#include <memory>
#include <set>
#include <functional>

class Message;

class MessagePolaris {
 public:
  using Ptr = std::unique_ptr<MessagePolaris>;
  void SetCallBack(std::function<bool (const std::set<std::string>&, Message*)> call_back) {
    call_back_ = call_back;
  }

 private:
  std::function<bool (const std::set<std::string>&, Message*)> call_back_;
};

class TaskPlan {
 public:
  using Ptr = std::unique_ptr<TaskPlan>;
    
 private:
  bool OnMessage(const std::set<std::string>& receivers, Message* msg) {
    // deal msg
    return true;
  }
    
  private:
   MessagePolaris::Ptr msg_polaris_;
};


Problem

  • Consider following code:
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int main() {
  auto task_plan = std::make_unique<TaskPlan>();

  task_plan->msg_polaris_ = std::make_unique<MessagePolaris>();
  task_plan->msg_polaris_->SetCallBack(
      [&](const std::set<std::string>& receivers, Message* msg) {
        return task_plan->OnMessage(receivers, msg);
      });

  auto dup_task_plan = std::move(task_plan);

  // trigger callback

  return 0;
}
  • The above could would coredump. The right way is:
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int main() {
  auto task_plan = std::make_unique<TaskPlan>();

  task_plan->msg_polaris_ = std::make_unique<MessagePolaris>();

  auto task_plan_raw_ptr = task_plan.get();
  task_plan->msg_polaris_->SetCallBack(
      [&](const std::set<std::string>& receivers, Message* msg) {
        return task_plan_raw_ptr->OnMessage(receivers, msg);
      });

  auto dup_task_plan = std::move(task_plan);

  // trigger callback

  return 0;
}

Analysis