c++ - 如何防止截止时间计时器调用已删除类中的函数？

Question

我在一段真实代码中遇到问题，其中属于已删除类的函数被 boost::asio::deadline_timer 调用，偶尔会导致段错误。

我遇到的问题是 deadline_timer 的删除是从同一 io_service 上的另一个计时器运行的。删除第一个 deadline_timer 将触发对要运行的函数的最终调用，并出现 boost::asio::error::operation_aborted 错误。然而，这只能在删除完成后安排在(相同的)io_service 上，但到那时对象已经被删除，因此不再有效。

所以我的问题是:如何防止这种情况发生？

以下是同样故障的简化示例:

//============================================================================
// Name        : aTimeToKill.cpp
// Author      : Pelle
// Description : Delete an object using a timer, from a timer
//============================================================================

#include <iostream>
#include <boost/function.hpp>
#include <boost/bind.hpp>
#include <boost/asio.hpp>
#include <boost/thread.hpp>

using namespace std;
using namespace boost;

struct TimeBomb
{
    bool m_active;
    asio::deadline_timer m_runTimer;

    TimeBomb(boost::asio::io_service& ioService)
    : m_active(true)
    , m_runTimer(ioService)
    {
        cout << "Bomb placed @"<< hex << (int)this << endl;
        m_runTimer.expires_from_now(boost::posix_time::millisec(1000));
        m_runTimer.async_wait(boost::bind(&TimeBomb::executeStepFunction, this, _1));
    }

    ~TimeBomb()
    {
        m_active = false;
        m_runTimer.cancel();
        cout << "Bomb defused @"<< hex << (int)this << endl;
    }

    void executeStepFunction(const boost::system::error_code& error)
    {
        // Canceled timer
        if (error ==  boost::asio::error::operation_aborted)
        {
            std::cout << "Timer aborted: " << error.message() << " @" << std::hex << (int)this << std::endl;
            return;
        }
        if (m_active)
        {
            // Schedule next step
            cout << "tick .." <<endl;
            m_runTimer.expires_from_now(
                    boost::posix_time::millisec(1000));
            m_runTimer.async_wait(boost::bind(&TimeBomb::executeStepFunction, this, _1));
        }
    }
};

struct BomberMan
{
    asio::deadline_timer m_selfDestructTimer;
    TimeBomb* myBomb;

    BomberMan(boost::asio::io_service& ioService)
    : m_selfDestructTimer(ioService)
    {
        cout << "BomberMan ready " << endl;
        myBomb = new TimeBomb(ioService);
        m_selfDestructTimer.expires_from_now(boost::posix_time::millisec(10500));
        m_selfDestructTimer.async_wait(boost::bind(&BomberMan::defuseBomb, this, _1));
    }

    void defuseBomb(const boost::system::error_code& error)
    {
        cout << "Defusing TimeBomb" << endl;
        delete myBomb;
    }
};


int main()
{
    boost::asio::io_service m_ioService;
    BomberMan* b = new BomberMan(m_ioService);
    m_ioService.run();
    return 0;
}


./aTimeToKill
BomberMan ready 
Bomb placed @9c27198
tick ..
tick ..
tick ..
tick ..
tick ..
tick ..
tick ..
tick ..
tick ..
tick ..
Defusing TimeBomb
Bomb defused @9c27198
Timer aborted: Operation canceled @9c27198

删除后打印最后一行，说明我的问题。

Answer 1

解决这个问题的典型方法是使用shared_ptr

#include <boost/asio.hpp>
#include <boost/bind.hpp>
#include <boost/enable_shared_from_this.hpp>
#include <boost/shared_ptr.hpp>

#include <iostream>

using namespace std;

struct TimeBomb : public boost::enable_shared_from_this<TimeBomb>
{
    bool m_active;
    boost::asio::deadline_timer m_runTimer;

    TimeBomb(boost::asio::io_service& ioService)
    : m_active(true)
    , m_runTimer(ioService)
    {
        cout << "Bomb placed @"<< hex << this << endl;
        m_runTimer.expires_from_now(boost::posix_time::millisec(1000));
    }

    void start()
    {
        m_runTimer.async_wait(boost::bind(&TimeBomb::executeStepFunction, shared_from_this(), _1));
    }

    void stop()
    {
        m_runTimer.cancel();
    }

    ~TimeBomb()
    {
        m_active = false;
        m_runTimer.cancel();
        cout << "Bomb defused @"<< hex << this << endl;
    }

    void executeStepFunction(const boost::system::error_code& error)
    {
        // Canceled timer
        if (error ==  boost::asio::error::operation_aborted)
        {
            std::cout << "Timer aborted: " << error.message() << " @" << std::hex << this << std::endl;
            return;
        }
        if (m_active)
        {
            // Schedule next step
            cout << "tick .." <<endl;
            m_runTimer.expires_from_now(
                    boost::posix_time::millisec(1000));
            m_runTimer.async_wait(boost::bind(&TimeBomb::executeStepFunction, shared_from_this(), _1));
        }
    }
};

struct BomberMan
{
    boost::asio::deadline_timer m_selfDestructTimer;
    boost::shared_ptr<TimeBomb> myBomb;

    BomberMan(boost::asio::io_service& ioService)
    : m_selfDestructTimer(ioService)
    {
        cout << "BomberMan ready " << endl;
        myBomb.reset( new TimeBomb(ioService) );
        myBomb->start();
        m_selfDestructTimer.expires_from_now(boost::posix_time::millisec(10500));
        m_selfDestructTimer.async_wait(boost::bind(&BomberMan::defuseBomb, this, _1));
    }

    void defuseBomb(const boost::system::error_code& error)
    {
        cout << "Defusing TimeBomb" << endl;
        myBomb->stop();
    }
};


int main()
{
    boost::asio::io_service m_ioService;
    BomberMan* b = new BomberMan(m_ioService);
    m_ioService.run();
    return 0;
}