As of C++23, the C++ language defines a header that contains fixed-width floating-point types. Implementations of these are optional, but if supported, std::float128_t corresponds to quadruple precision. The fast_float should support it.

Additional possibilities:
For older standards, we can use the GCC quad-precision math library, libquadmath, which provides __float128 operations or cross-platform implementation from the Boost multiprecision library Boost.Multiprecision provides unified cross-platform C++ interface for __float128 and _Quad types and includes a custom implementation of the standard math library.

See also #88