Filter out messages from a ros bag

Hi, I'd like to filter out some "bad messages" from my rosbags. I'm using the C++ rosbag API. Should I open the bagfile in read mode and republish the good messages and record them in another bag using 'rosbag record'? Or is there a way to modify the bag in place when it's in read or write mode? I've gone through the C++ API of rosbag a bit, I couldn't find examples similar to what I want to do. Thanks. Update: Is there a method to find out the list of topics that are present in the bag and other statistics through the API? Is there a way to find out which topic, a particular message instance is from while iterating over them in a `boost foreach` loop as shown in the example [here](http://wiki.ros.org/rosbag/Code%20API#cpp_api)