How to let std::thread delete the object automatically after excuting its member function

Question

I want want to implement a cmmand class which does some work in an another thread, and I don't want to let users to delete that object manually.My command class like this:

class Cmd {
 public:
  void excute() {
    std::cout << "thread begins" << std::endl;
    std::this_thread::sleep_for(std::chrono::seconds(2));  // do some work
    std::cout << "thread ends" << std::endl;
  }

  void run() {
    // I want std::unique_ptr to delete 'this' after work is done,but does't work
    std::thread td(&Cmd::excute, std::unique_ptr<Cmd>(this));
    td.detach();
  }

  // test if this object is still alive
  void ok() { std::cout << "OK" << std::endl; }
};

I use it like this:

int main() {
  Cmd *p = new Cmd();
  p->run();

  // waiting for cmd thread ends
  std::this_thread::sleep_for(std::chrono::seconds(3));

  p->ok();  // I thought p was deleted but not

  return 0;
}

As in the comments, the object is still alive after cmd thread finishes, I want to know how to implement such funtionality.

EDIT

users of cmd does't know when cmd will finish, so the flowing use case will leads to UB.

std::unique_ptr<Cmd> up(new Cmd);  // or just Cmd c;
up->run();
// cmd will be deleted after out of scope but cmd::excute may still need it

CLOSED

I made a mistake about test, in fact the object is deleted after the thread ends.It is more clear with following test with a additional member variable int i.

#include <functional>
#include <iostream>
#include <stack>
#include <thread>

using namespace std;

class Cmd {
 public:
  ~Cmd() { std::cout << "destructor" << std::endl; }

  void excute() {
    std::cout << i << " thread begins" << std::endl;
    std::this_thread::sleep_for(std::chrono::seconds(2));  // do some work
    std::cout << i << " thread ends" << std::endl;
  }

  void run() {
    // I want std::unique_ptr to delete 'this' after work is done,but it seems
    // not working
    std::thread td(&Cmd::excute, std::unique_ptr<Cmd>(this));
    td.detach();
  }

  // test if this object is still alive
  void ok() { std::cout << i << " OK" << std::endl; }

  int i;
};

int main() {
  Cmd *p = new Cmd();
  p->i = 10;
  p->run();

  // waiting for cmd thread ends
  std::this_thread::sleep_for(std::chrono::seconds(3));

  p->ok();  // I thought p was deleted but not

  return 0;
}

The flowwing outputs proved the object is deleted.

10 thread begins
10 thread ends
destructor
-572662307 OK

But just as some kind guys suggests, this is not a good design, avoid it as you can.

Answer 1

Instead of thread, you can use std::future to signal the state. You can then either wait for the task to finish, or ignore the future completely.

#include <future>
#include <chrono>
#include <mutex>
#include <iostream>

class Cmd {
public:
    std::future<void> run() {
        std::lock_guard<std::mutex> lock(startMutex);
        if (started) {
            throw std::logic_error("already started");
        }
        started = true;

        // Take copies here, so that it doesn't matter if Cmd is destroyed
        int i_ = i;
        return std::async(std::launch::async, [i_]() {
            std::this_thread::sleep_for(std::chrono::seconds(2));
            std::cout << i_ << std::endl;
        });
    }

    int i = 0;

private:
    std::mutex startMutex;
    bool started = false;
};

int main() {
    auto p = std::make_unique<Cmd>();
    p->i = 10;
    auto f = p->run();
    p.reset();

    // Do some other work

    // Wait for the task to finish (or use f.get() if there is no need to
    // do some other work while waiting)
    if (f.valid()) {
        std::future_status operation;
        do {
            // Do some other work

            operation = f.wait_for(std::chrono::milliseconds(1));
        } while (operation != std::future_status::ready);
    }
}

Answer 2

There are two ways to automatically delete the memory created dynamically.

1. In client code(main function), you should use some smart pointer like unique_pointer, so that once the object goes out of the scope, automatically it will be freed in unique_poiter destructor

2. You can create your own smart pointer, it will be kind of wrapper around Cmd class. And has to overload a few operators. This smart pointer will also be taking care of dynamically allocated memory in the destructor. Client code should use this smart pointer while creating Cmd object dynamically.