C++ OO Callback Technique

Environments: C++, OO, callback, notification, threading, templates

So, you're writing C++ and are trying to keep it as OO as possible. You wish to have a server object callback a method in your client object. This could be for purpose of threading, notifications, and so forth. How can one do this without being C++ risky and getting messy?

The following technique is type safe, and does not rely on inheritence or static stubs. You just add the server object as one of the client's data members and init it with the name of the methods to be called.

The mechanism is based on the server exporting a template callback object. The client creates one using its own type and gives it to the server.

I have used a timer as a server in my example. Thus, setting up a timer to call one of my methods would be as simple as this:

/****************************************************************/
CMyClass::CMyClass()
{
    // * Create Callback
    CMyTimerCallback<CMyClass>* pCallback =
            new CMyTimerCallback<CMyClass>;

    // * Set up Callback
    pCallback->SetClass(this);
    pCallback->SetMethodTick( TimerNotify );

    // * Create Timer and attatch Callback to it
    m_pMyTimer = new CMyTimer;
    m_pMyTimer->InitCallback(pCallback);

    // * Set up Timer
    m_pMyTimer->SetTimer(1234);
}

/****************************************************************/
CMyClass::TimerNotify(int Id)
{
  // ...
}
/****************************************************************/

The server side implementation is a little complicated to grasp at first, but once you get it, it makes a lot of sense. It uses a little template magic.

/****************************************************************/
class CMyTimer
{
public:
    void InitCallback(CMyTimerCallbackBase* pCallback);
    void SetTimer(int iInterval);

private:
    CMyTimerCallbackBase* m_pCallback;
};


/****************************************************************/
class CMyTimerCallbackBase
{
public:
    virtual void MethodTick(int Id) = 0;
  // add other methods here

};


/****************************************************************/
template<class C>
class CMyTimerCallback : public CMyTimerCallbackBase
{
public:
    CMyTimerCallback()      { m_pClass = 0; m_pMethodTick = 0; }

    typedef void (C::*MethodTick_t)(int Id);

    void SetClass(C* pClass)                    { m_pClass =
                                                  pClass; }
    void SetMethodTick(MethodTick_t pMethod)    { m_pMethodTick =
                                                  pMethod; }
    // add other methods here

protected:

    void MethodTick(int Id)
    {
        if(m_pClass && m_pMethodTick) 
        {
            (m_pClass->*m_pMethodTick) (Id);
        }
    }

private:
    C*              m_pClass;
    MethodTick_t    m_pMethodTick;
};

This technique has proven to be quite useful, I've used it to recieve data from a comms link, receive notifications, and to run multiple threads within a single client object.

A download of C++ code is provided that can be compiled and stepped to aid your understanding.

Downloads

Download demo code - 3 Kb


Comments

  • Better using a functor

    Posted by Legacy on 11/06/2003 12:00am

    Originally posted by: Robert J. Liu

    It can be better handled using a functor - a function object.
    
    class CMyTimer
    {
    public:
    void MethodTick( int Id, Callback & timerNotify){ timerNotify(Id); }
    void SetTimer(int iInterval);
    ...

    };
    class Callback
    {
    public:
    int operator() (int Id) { ...;}
    };

    int main ()
    {
    Callback timerNotify;
    CMyTimer theTimer;
    theTimer.SetTimer(1234);
    theTimer.MethodTick(Id, timerNotify);
    ...
    }

    Reply
  • try using boost.function and boost.bind

    Posted by Legacy on 11/06/2003 12:00am

    Originally posted by: Shelby

    You could also look at using boost.function (see boost.org for more information)
    
    

    Basically you could do this:

    // create a callback to TimerNotify
    boost::function< void (int) > pCallback =
    boost::bind( CMyClass::TimerNotify, this );

    You would then make your CMyTimer function accept the same type "boost::function< void (int) >" have it call this at every tick.
    No need for any base classes such as "CMyTimerCallbackBase"

    To call this function:

    void CMyTimer::OnTick()
    {
    // call the stored callback
    m_pCallback();
    }

    I recommended check out www.boost.org. It has many other useful libraries worth checking out.

    Reply
Leave a Comment
  • Your email address will not be published. All fields are required.

Top White Papers and Webcasts

  • Live Event Date: December 11, 2014 @ 1:00 p.m. ET / 10:00 a.m. PT Market pressures to move more quickly and develop innovative applications are forcing organizations to rethink how they develop and release applications. The combination of public clouds and physical back-end infrastructures are a means to get applications out faster. However, these hybrid solutions complicate DevOps adoption, with application delivery pipelines that span across complex hybrid cloud and non-cloud environments. Check out this …

  • On-demand Event Event Date: October 29, 2014 It's well understood how critical version control is for code. However, its importance to DevOps isn't always recognized. The 2014 DevOps Survey of Practice shows that one of the key predictors of DevOps success is putting all production environment artifacts into version control. In this webcast, Gene Kim discusses these survey findings and shares woeful tales of artifact management gone wrong! Gene also shares examples of how high-performing DevOps …

Most Popular Programming Stories

More for Developers

RSS Feeds