Subscribe to /tf for position?

edit

hi all

I used the diffDrive_plugin and publish the position data onto the rostopic "\tf". using

           rostopic echo tf

I got:

transforms: 

header: 
  seq: 0
  stamp: 
    secs: 22
    nsecs: 164000000
  frame_id: /odom
child_frame_id: /base_footprint
transform: 
  translation: 
    x: -0.0680770173496
    y: 0.00835326625379
    z: -0.00216662328182
  rotation: 
    x: 0.0
    y: 0.0
    z: -0.172377366036
    w: 0.985030986151

The question is how to use the tf::TransformListener listener to lookup and store such value? thank you very much!