The answer, taken from @PlasmaHH's comment, and quite obvious in retrospect, is that no, boost::multiprecision::cpp_int
is not a POD.
To quote from PlasmaHH:
Think about it for a moment, where could it possibly extend into when it grows? Also what could maybe "Determines the number of Bits to store directly within the object before resorting to dynamic memory allocation" in the documentation you have linked mean?