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#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 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[3].

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:

float128 example:

#include <boost/multiprecision/float128.hpp>

using namespace boost::multiprecision;

// Operations at 128-bit precision and full numeric_limits support:
float128 b = 2;
// There are 113-bits of precision:
std::cout << std::numeric_limits<float128>::digits << std::endl;
// Or 34 decimal places:
std::cout << std::numeric_limits<float128>::digits10 << std::endl;
// We can use any C++ std lib function, lets print all the digits as well:
std::cout << std::setprecision(std::numeric_limits<float128>::max_digits10)
   << log(b) << std::endl; // print log(2) = 0.693147180559945309417232121458176575
// We can also use any function from Boost.Math:
std::cout << boost::math::tgamma(b) << std::endl;
// And since we have an extended exponent range we can generate some really large 
// numbers here (4.02387260077093773543702433923004111e+2564):
std::cout << boost::math::tgamma(float128(1000)) << std::endl;
// We can declare constants using GCC or Intel's native types, and the Q suffix,
// these can be declared constexpr if required:

constexpr float128 pi = 3.1415926535897932384626433832795028841971693993751058Q;

[3] On 64-bit Ubuntu 11.10, GCC-4.8.0, Intel Core 2 Duo T5800.