RegisterPointPickingCallback (Pclviewer: :Callback_Test, (Void*)&Viewer); Returns C3687 Non-Standard Syntax; Use '&' to Create a Pointer to Member

I found the problem thanks to @Some programmer dude 's comments:

Solution:

1) I declared the callback_Test outside the PCLViewer class

2) I used two arguments as expected by code calling the callback

Header file:

#ifndef PCLVIEWER_H
#define PCLVIEWER_H

#include <iostream>

// Qt
#include <QMainWindow>
#include <QDebug>

// Point Cloud Library
 #include <pcl/point_cloud.h>
//#include <C:\Program Files\PCL 1.8.0\include\pcl-1.8\pcl\point_cloud.h>
#include <pcl/point_types.h>
//#include <C:\Program Files\PCL 1.8.0\include\pcl-1.8\pcl\point_types.h>
#include <pcl/visualization/pcl_visualizer.h>
//#include <C:\Program Files\PCL 1.8.0\include\pcl-1.8\pcl\visualization\pcl_visualizer.h>

// included this as well -- however it seems to not be needed
//#include <pcl/visualization/point_picking_event.h>

// Visualization Toolkit (VTK)
#include <vtkRenderWindow.h>
//#include <C:\Program Files\VTK\include\vtk-7.1\vtkRenderWindow.h>



typedef pcl::PointXYZRGBA PointT;
typedef pcl::PointCloud<PointT> PointCloudT;

namespace Ui
{
  class PCLViewer;
}

class PCLViewer : public QMainWindow
{
  Q_OBJECT

public:
  explicit PCLViewer (QWidget *parent = 0);
  ~PCLViewer ();



public slots:

protected:
  boost::shared_ptr<pcl::visualization::PCLVisualizer> viewer;
  PointCloudT::Ptr cloud;

  unsigned int red;
  unsigned int green;
  unsigned int blue;

private:
  Ui::PCLViewer *ui;

};
// ########################################################################################
// DECLARE OUTSIDE OF CLASS! WITH TWO ARGUMENTS

void callback_Test(const pcl::visualization::PointPickingEvent& event, void* viewer_void);

#endif // PCLVIEWER_H

Then I called the registerPointPickingCallback without &;

// NOW ITS OK and no & is needed since callback_Test is outside the PCLViewer class 
  viewer->registerPointPickingCallback (callback_Test,(void*)&viewer);

The below code works.

.CPP file:

#include "pclviewer.h"
#include "../build/ui_pclviewer.h"

PCLViewer::PCLViewer (QWidget *parent) :
  QMainWindow (parent),
  ui (new Ui::PCLViewer)
{
  ui->setupUi (this);
  this->setWindowTitle ("PCL viewer");

  // Setup the cloud pointer
  cloud.reset (new PointCloudT);
  // The number of points in the cloud
  cloud->points.resize (200);

  // The default color
  red   = 128;
  green = 128;
  blue  = 128;

  // Fill the cloud with some points
  for (size_t i = 0; i < cloud->points.size (); ++i)
  {
    cloud->points[i].x = 1024 * rand () / (RAND_MAX + 1.0f);
    cloud->points[i].y = 1024 * rand () / (RAND_MAX + 1.0f);
    cloud->points[i].z = 1024 * rand () / (RAND_MAX + 1.0f);

    cloud->points[i].r = red;
    cloud->points[i].g = green;
    cloud->points[i].b = blue;
  }

  // Set up the QVTK window
  viewer.reset (new pcl::visualization::PCLVisualizer ("viewer", false));
  ui->qvtkWidget->SetRenderWindow (viewer->getRenderWindow ());
  viewer->setupInteractor (ui->qvtkWidget->GetInteractor (), ui->qvtkWidget->GetRenderWindow ());
  ui->qvtkWidget->update ();

  viewer->addPointCloud (cloud, "cloud");
  viewer->setPointCloudRenderingProperties (pcl::visualization::PCL_VISUALIZER_POINT_SIZE, 2, "cloud");
  viewer->resetCamera ();
  ui->qvtkWidget->update ();

  // NOW ITS OK and no & is needed since callback_Test is outside the PCLViewer class 
  viewer->registerPointPickingCallback (callback_Test,(void*)&viewer);
}

PCLViewer::~PCLViewer ()
{
  delete ui;
}


void callback_Test (const pcl::visualization::PointPickingEvent& event, void* viewer_void)
{
    qDebug() << "callback_Test executed";
}

The actual answer to your question is that you needed to include the & like the compiler told you

viewer->registerPointPickingCallback (&PCLViewer::callback_Test,(void*)&viewer)

If anyone runs into this (and wants to use a member function). I solved it using the following. Just define your function as a member and use *this:

viewer->registerPointPickingCallback (&PCLViewer::callback_Test,*this)