...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
All the code in this library is inside namespace boost::math.
In order to use a distribution my_distribution you will need to include either the header <boost/math/distributions/my_distribution.hpp> or the "include everything" header: <boost/math/distributions.hpp>.
For example, to use the Students-t distribution include either <boost/math/distributions/students_t.hpp> or <boost/math/distributions.hpp>