How to pass lambda function by pointer?

Question

I am a new in c++ maybe I miss something, but actually what I need to do is: I have a class that processing something in other thread, during this processing I need that it invoke a callback for progress.

How I see I can do it, I need to declarate pointer (maybe shared_ptr) for my callback function as a class member, than I have a setter in order to pass pointer to callback and then I can use it. A few issues here are how to pass it correctly? How to invoke pointer on function?

My implementation is:

class RobocopyCopy
{
    //Public members
public:
    typedef std::function<void(int)> TVoidIntCallback;

    RobocopyCopy * set_monitoring_done_callback(TVoidIntCallback monitoring_done_callback)
    {
       m_pMonitoring_done_callback = &monitoring_done_callback;
       return this;
    }

    //This method executes in background
    void execute()
    {
        ...
        //and here I need to invoke my callback
        (TVoidIntCallback *)m_pMonitoring_done_callback(777); //but this is not correct


private:
    TVoidIntCallback * m_pMonitoring_done_callback;


...
}

and final implementation of this should be like this (I think) :

RobocopyCopy robocopy;
    robocopy.set_monitoring_done_callback([this](int my_progress) {
        printf("Progress is :: %d", my_progress);
    });

So, as I mentioned above questions is :

how to pass this function callback as a lambda and save in Robocopy class as a pointer
How to invoke this function correctly, because this (TVoidIntCallback *)m_pMonitoring_done_callback(777); doesn't work.

m_pMonitoring_done_callback shouldn't be a pointer. Drop the star, make it TVoidIntCallback m_pMonitoring_done_callback; Then call it simply as m_pMonitoring_done_callback(777); — Igor Tandetnik
– Igor Tandetnik, Commented May 31, 2020 at 14:14

M.Furkan ARDOĞAN · Accepted Answer · 2020-05-31 14:54:58Z

1

I am using VC++ I hope this code will be successful for you.

class RobocopyCopy
{
   typedef std::function<void(int)> TVoidIntCallback;
   TVoidIntCallback evnt;
   public:

   RobocopyCopy* set_monitoring_done_callback(TVoidIntCallback 
   monitoring_done_callback)
   {
     //set callBack function from out side.
     evnt = monitoring_done_callback;
     return this;
   }

    void execute() {
       //invoke your callBack
      evnt(1000000);
    }
};

int main()
{
   RobocopyCopy obj;
   obj.set_monitoring_done_callback([](int data) {
    std::cout << data << "\n";
    })->execute();
}

answered May 31, 2020 at 14:54

M.Furkan ARDOĞAN

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