[ros-users] read value of a topic from the callback of anoth…

Hello,

sorry if this is a basic question but I couldn't find anything in the wiki.
I would like to retrieve the current value of a topic from the callback of
another topic. How is this possible? In more detail I have:

public:
    ImageConverter(ros::NodeHandle &n) : n_(n), it_(n_)
    {
        cvNamedWindow("Image window");
        image_sub_ = it_.subscribe("/camera_sim/image_raw", 1,
&ImageConverter::imageCallback, this);
    }

void imageCallback(const sensor_msgs::ImageConstPtr& msg_ptr)
{
        // Image obtained from subscribed node and converted to IplImage
        IplImage *img = NULL;
        try {
            img = bridge_.imgMsgToCv(msg_ptr, "bgr8"); // reads the image
            // I WOULD LIKE TO READ THE VALUE OF ANOTHER TOPIC
(std_msgs::String) HERE
        }
        catch (sensor_msgs::CvBridgeException error) {
            ROS_ERROR("error");
        }
}

Many Thanks