undefined reference to boost::system::system_category() when compiling

The boost library you are using depends on the boost_system library. (Not all of them do.)

Assuming you use gcc, try adding -lboost_system to your compiler command line in order to link against that library.

Linking with a library that defines the missing symbol (-lboost_system) is the obvious solution, but in the particular case of Boost.System, a misfeature in the original design makes it use boost::system::generic_category() and boost::system::system_category() needlessly. Compiling with the flag -DBOOST_SYSTEM_NO_DEPRECATED disables that code and lets a number of programs compile without requiring -lboost_system (that link is of course still needed if you explicitly use some of the library's features).

Starting from Boost 1.66 and this commit, this behavior is now the default, so hopefully fewer and fewer users should need this answer.

As noticed by @AndrewMarshall, an alternative is to define BOOST_ERROR_CODE_HEADER_ONLY which enables a header-only version of the code. This was discouraged by Boost as it can break some functionality. However, since 1.69, header-only seems to have become the default, supposedly making this question obsolete.

Another workaround for those who don't need the entire shebang: use the switch

-DBOOST_ERROR_CODE_HEADER_ONLY.

If you use CMake, it's add_definitions(-DBOOST_ERROR_CODE_HEADER_ONLY).