CODEWITHSUNDEEP
c++boost
Xegara
Xegara

Reputation: 563

After specifying an argument placer in the boost::bind, why is it omitted when it was called?

I've been studying boost::bind and boost::function. I can best explain my understanding by giving an example.. Given this sample code snippet:

 void print(int x, int y){
    cout << x << "\t" << y << endl;
 }

 int main(){
       boost::function<void (int)> f = boost::bind (&print, _1, 2);
   f(5);
 }

Displays 5 2.. From my understanding, binding a function creates a function object that can have some of its arguments bound to some constant arguments (programmer's preference).

However, what I really can't understand is this code snippet of the source code posted below:

boost::function<void (const pcl::PointCloud<pcl::PointXYZ>::ConstPtr&)> f =
                 boost::bind (&SimpleOpenNIViewer::cloud_cb_, this, _1);
interface->registerCallback (f);

The argument placer is _1. Should not it be f(arg)? Why is the arg omitted? 

         #include <pcl/io/openni_grabber.h>
         #include <pcl/visualization/cloud_viewer.h>

         class SimpleOpenNIViewer
         {
           public:
             SimpleOpenNIViewer () : viewer ("PCL OpenNI Viewer") {}

             void cloud_cb_ (const pcl::PointCloud<pcl::PointXYZ>::ConstPtr &cloud)
             {
               if (!viewer.wasStopped())
                 viewer.showCloud (cloud);
             }

             void run ()
             {
               pcl::Grabber* interface = new pcl::OpenNIGrabber();

               boost::function<void (const pcl::PointCloud<pcl::PointXYZ>::ConstPtr&)> f =
                 boost::bind (&SimpleOpenNIViewer::cloud_cb_, this, _1);

               interface->registerCallback (f);

               interface->start ();

               while (!viewer.wasStopped())
               {
                 boost::this_thread::sleep (boost::posix_time::seconds (1));
               }

               interface->stop ();
             }

             pcl::visualization::CloudViewer viewer;
         };

         int main ()
         {
           SimpleOpenNIViewer v;
           v.run ();
           return 0;
         }

Upvotes: 0

Views: 112

Answers (1)

David G
David G

Reputation: 96790

No, the function is not attempting to be called on registerCallback(f). The function f is being passed to the parameter that accepts a boost::function. The arguments will eventually be given to f when it is called at some later point. For example:

typedef boost::function<void (int)> Function;

void h( Function f )
{
    f(5);
}

int main()
{
    auto cube = [] (int n) { std::cout << n * n * n; };

    h(cube); // cube is passed to the function, not called
}

Upvotes: 2

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