read single message from bag-file (c++)

Hi,I'm new to this whole ROS stuff and have a problem which I can't solve on my own. I want to read a single message from a given bag-file using c++11. To do this I read the short tutorial and cookbook for rosbag. I get the message with iterating over all messages in the bag. But I want to know if there is a way to direct get the specified message (data)? The topic (m.getTopic()), timestamp (m.getTime()) and the structure of the message (m.MessageDefinition()) is known. Thanks for your help.