Finding Bufferd frames in tf2_ros::Buffer (c++)

I am creating a post-processing utility where I read all the transforms from a .bag file into a tf2_ros::Buffer. I would like to be able to ask the Buffer for all the frames it has cached. I don't see any **public** member functions in the [doccumentation](http://docs.ros.org/indigo/api/tf2_ros/html/c++/classtf2__ros_1_1Buffer.html#ab96886f1292cc3cd7452d65312ec94d0) that allow you to view cached frames. What is the best way to access cached all the frames in a tf2_ros::Buffer?