read topic without callback

edit

What you're looking for is a polled topic which was proposed a while ago, but unfortunately there is currently no plan for it to be implemented.

You can get the same effect by having a variable that gets updated with the most recent message on the topic every time the callback is called, and then read that variable whenever you need the topic.

This could be a way around? Example code

#include ros/ros.h>
#include boost/thread.hpp>
#include tf/transform_broadcaster.h>
#include turtlesim/Pose.h>

tf::Transform tfA;
tf::Transform tfB;
static boost::mutex mutex;

void topicACallback(const turtlesim::PoseConstPtr& msg){
  tf::Transform transform;
  transform.setOrigin( tf::Vector3(msg->x, msg->y, 0.0) );
  tf::Quaternion q;
  q.setRPY(0, 0, msg->theta);
  transform.setRotation(q);
  mutex.lock();
  tfA = transform;
  mutex.unlock();
}

void topicBCallback(const turtlesim::PoseConstPtr& msg){
  tf::Transform transform;
  transform.setOrigin( tf::Vector3(msg->x, msg->y, 0.0) );
  tf::Quaternion q;
  q.setRPY(0, 0, msg->theta);
  transform.setRotation(q);
  mutex.lock();
  tfB = transform;
  mutex.unlock();
}

void timerCallback(const ros::TimerEvent& event){
  tf::Transform tfA_local,tfB_local;
  mutex.lock();
  tfA_local = tfA;
  tfB_local = tfB;
  mutex.unlock();
  ROS_INFO("doing the rest of computation");
}

int main(int argc, char** argv){
  ros::init(argc, argv, "test");
  ros::NodeHandle node;
  ros::Subscriber sub1 = node.subscribe("/turtle1/pose", 10, &topicACallback);
  ros::Subscriber sub2 = node.subscribe("/turtle2/pose", 10, &topicBCallback);
  ros::Timer timer = node.createTimer(ros::Duration(0.1),timerCallback);
  ros::spin();
  return 0;
};