Boost.Multiprecision was a development from the e_float (TOMS Algorithm 910) library by Christopher Kormanyos.

e_float can still be used with Boost.Math library via the header:

<boost/math/bindings/e_float.hpp>

And the type boost::math::ef::e_float: this type is a thin wrapper class around ::e_float which provides the necessary syntactic sugar to make everything "just work".

There is also a concept checking test program for e_float support here.

New projects are recommended to use Boost.Multiprecision with cpp_float backend instead.