I am trying to use your library to decode AIS in C++. I can’t seem to figure out how to actually decode the body into meaningful string data, or if your library has that functionality.
The libais library is designed to convert the NMEA to bits and then break down the bits into a C++ struct. From python you get a dict that you can print. Otherwise, you can extend the stream operators to print more for C++, but it's really application specific what format you might want for the output: xml, json, msgpack, csv, sql for database insertion, etc. plus variations on those themes like gpsd json. And do you want to mix message 5 data into 1,2,3 position reports. It's up to you how you want to print them.
libais just tries to nail the details of the low level packets in C++. Everything else is considered out of scope. Pull requests welcome if you want to add a C++ layer above that does more. I only added the python layer at this point.
If you just want to get a look at a json for quickly, you can pass the data through gpsd. It's missing some of the messages and field components (e.g. commstate), but it does make an assumption about output format.