Image_transport using member function as callback function

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Hello!

I am trying to set up an image transport subscriber which should use a member function as callback method.

I have tried the following standard approach:

class A{
public:
    void camCallback1(const sensor_msgs::ImageConstPtr &msg) {
        // do something
    }  

int main(int argc, char **argv) {

    ros::init(argc, argv, "test_node");
    ros::NodeHandle nh;
    A a;
    image_transport::ImageTransport it(nh);

    image_transport::Subscriber subCam1 = it.subscribe("/raspi_cam1/image_raw", 2,
            &A::camCallback1,
            &a);
    ros::spin()
}

like here

This is the only of many methods i have tried that would compile for me. However the callback is never thrown.

I hope can somebody help me out here since I can not find anything regarding this particular problem in the image_transport documentation