By default, Boost.Asio is a header-only library. However, some developers may prefer to build Boost.Asio using separately compiled source code. To do this, add #include <boost/asio/impl/src.hpp> to one (and only one) source file in a program, then build the program with BOOST_ASIO_SEPARATE_COMPILATION defined in the project/compiler settings. Alternatively, BOOST_ASIO_DYN_LINK may be defined to build a separately-compiled Boost.Asio as part of a shared library.