Extract data from .bag without messages built

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The message definitions that the bag file was recorded with are stored in the bag file.

You can use the rosbag command-line tool to list the topics and their message types from a bag file: rosbag info my_data.bag

You can also use the -b option to rostopic echo to print messages from a single topic in a bag file.

The python API to rosbag dynamically constructs messages based on the message definitions that are stored in the bag file, so you can use the python API to read and introspect all of the messages in a bag file.

Since C++ is statically typed, you have to have headers from your message definitions in order to compile a program which reads those messages - this means that you can't use the C++ API unless you have the message definitions.