Synchronizer.h Errors

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Hi guys,

I've been trying to do approximate time synchronization, and have been adapting tutorial examples for my own needs. I was facing some errors with sync_policies not having a type, synchronizer not being declared in the scope, and so on. Seeing some other forum posts, I added

using namespace message_filters

and got a wall of errors:

/home/pi/ros_ws/src/link/src/roscomm.cpp: In function ‘int main(int, char**)’:   
/home/pi/ros_ws/src/link/src/roscomm.cpp:222:85: error: ‘IMU’ is not a member of ‘sensor_msgs’                     
   typedef sync_policies::ApproximateTime<sensor_msgs::NavSatFix, std_msgs::Float64, sensor_msgs::IMU> NavSyncPolicy;
                                                                                     ^~~~~~~~~~~
/home/pi/ros_ws/src/link/src/roscomm.cpp:222:85: error: ‘IMU’ is not a member of ‘sensor_msgs’                   
/home/pi/ros_ws/src/link/src/roscomm.cpp:222:101: error: template argument 3 is invalid
   typedef sync_policies::ApproximateTime<sensor_msgs::NavSatFix, std_msgs::Float64, sensor_msgs::IMU> NavSyncPolicy;
                                                                                                     ^                                                                                 
In file included from /home/pi/ros_ws/src/link/src/roscomm.cpp:21:0:                                                                                                                                        
/opt/ros/kinetic/include/message_filters/synchronizer.h: In instantiation of ‘class message_filters::Synchronizer<int>’:
/home/pi/ros_ws/src/link/src/roscomm.cpp:223:49:   required from here
/opt/ros/kinetic/include/message_filters/synchronizer.h:68:7: error: base type ‘int’ fails to be a struct or class type
 class Synchronizer : public boost::noncopyable, public Policy
       ^~~~~~~~~~~~
/opt/ros/kinetic/include/message_filters/synchronizer.h:71:37: error: ‘int’ is not a class, struct, or union type
   typedef typename Policy::Messages Messages;
                                     ^~~~~~~~
/opt/ros/kinetic/include/message_filters/synchronizer.h:72:35: error: ‘int’ is not a class, struct, or union type
   typedef typename Policy::Events Events;
                                   ^~~~~~
/opt/ros/kinetic/include/message_filters/synchronizer.h:73:35: error: ‘int’ is not a class, struct, or union type
   typedef typename Policy::Signal Signal;
                                   ^~~~~~
/opt/ros/kinetic/include/message_filters/synchronizer.h:74:49: error: ‘int’ is not a class, struct, or union type
   typedef typename mpl::at_c<Messages, 0>::type M0;
                                                 ^~
/opt/ros/kinetic/include/message_filters/synchronizer.h:75:49: error: ‘int’ is not a class, struct, or union type
   typedef typename mpl::at_c<Messages, 1>::type M1;
                                                 ^~
/opt/ros/kinetic/include/message_filters/synchronizer.h:76:49: error: ‘int’ is not a class, struct, or union type
   typedef typename mpl::at_c<Messages, 2>::type M2;
                                                 ^~
/opt/ros/kinetic/include/message_filters/synchronizer.h:77:49: error: ‘int’ is not a class, struct, or union type
   typedef typename mpl::at_c<Messages, 3>::type M3;
                                                 ^~
/opt/ros/kinetic/include/message_filters/synchronizer.h:78:49: error: ‘int’ is not a class, struct, or union type
   typedef typename mpl::at_c<Messages, 4>::type M4;
                                                 ^~
/opt/ros/kinetic/include/message_filters/synchronizer.h:79:49: error: ‘int’ is not a class, struct, or union type
   typedef typename mpl::at_c<Messages, 5>::type M5;
                                                 ^~
/opt/ros/kinetic/include/message_filters/synchronizer.h:80:49: error: ‘int’ is not a class, struct, or union type
   typedef typename mpl::at_c<Messages, 6>::type M6;
                                                 ^~
/opt/ros/kinetic/include/message_filters/synchronizer.h:81:49: error: ‘int’ is not a class, struct, or union type
   typedef typename mpl::at_c<Messages, 7>::type ...