[/
  Copyright 2011 - 2020 John Maddock.
  Copyright 2013 - 2019 Paul A. Bristow.
  Copyright 2013 Christopher Kormanyos.

  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
]

[section:float128 float128]

`#include <boost/multiprecision/float128.hpp>`

   namespace boost{ namespace multiprecision{

   class float128_backend;

   typedef number<float128_backend, et_off>    float128;

   }} // namespaces

The `float128` number type is a very thin wrapper around GCC's `__float128` or Intel's `_Quad` data types
and provides an real-number type that is a drop-in replacement for the native C++ floating-point types, but with
a 113 bit mantissa, and compatible with FORTRAN's 128-bit QUAD real.

All the usual standard library and `std::numeric_limits` support are available, performance should be equivalent
to the underlying native types: for example the LINPACK benchmarks for GCC's `__float128` and
`boost::multiprecision::float128` both achieved 5.6 MFLOPS[footnote On 64-bit Ubuntu 11.10, GCC-4.8.0, Intel Core 2 Duo T5800.].

As well as the usual conversions from arithmetic and string types, instances of `float128` are
copy constructible and assignable from GCC's `__float128` and Intel's `_Quad` data types.

It's also possible to access the underlying `__float128` or `_Quad` type via the `data()` member
function of `float128_backend`.

Things you should know when using this type:

* Default constructed `float128`s have the value zero.
* This backend supports rvalue-references and is move-aware, making instantiations of `number` on this backend move aware.
* This type is fully `constexpr` aware - basic constexpr arithmetic is supported from C++14 and onwards, comparisons,
plus the functions `fabs`, `abs`, `fpclassify`, `isnormal`, `isfinite`, `isinf` and `isnan` are also supported if either
the compiler implements C++20's `std::is_constant_evaluated()`, or if the compiler is GCC.
* It is not possible to round-trip objects of this type to and from a string and get back
exactly the same value when compiled with Intel's C++ compiler and using `_Quad` as the underlying type: this is a current limitation of
our code.  Round tripping when using `__float128` as the underlying type is possible (both for GCC and Intel).
* Conversion from a string results in a `std::runtime_error` being thrown if the string can not be interpreted
as a valid floating-point number.
* Division by zero results in an infinity being produced.
* Type `float128` can be used as a literal type (constexpr support).
* Type `float128` can be used for full `constexpr` arithmetic from C++14 and later with GCC.  The functions `abs`, `fabs`, 
`fpclassify`, `isnan`, `isinf`, `isfinite` and `isnormal` are also `constexpr`, but the transcendental functions are not.
* When using the Intel compiler, the underlying type defaults to `__float128` if it's available and `_Quad` if not.  You can override
the default by defining either `BOOST_MP_USE_FLOAT128` or `BOOST_MP_USE_QUAD`.
* When the underlying type is Intel's `_Quad` type, the code must be compiled with the compiler option `-Qoption,cpp,--extended_float_type`.
* When compiling with `gcc`, you need to use the flag `--std=gnu++11/14/17`, as the suffix 'Q' is a GNU extension. Compilation fails with the flag `--std=c++11/14/17`
unless you also use `-fext-numeric-literals`.
* You will need to link to `libquadmath.dll` with the link command `-lquadmath` and ensure that the DLL is visible by the linker. 
If you are using the B2/bjam build system then commands`<linkflags>-lQUADMATH` and `<linkflags>-L"path/to/lib"` will be needed.
* The values shown by `std::numeric_limits<float128>` and extremely close ['but not identical]
to those from the equivalent precision and range multiprecision types `std::numeric_limits<cpp_bin_float_quad>` and `std::numeric_limits<cpp_dec_float_quad>`.

[h5 float128 example:]

[float128_eg]

Values for `std::numeric_limits<float128>` are:

[float128_numeric_limits]

[endsect] [/section:float128 float128]