#include <boost/multiprecision/cpp_double_fp_backend.hpp>

namespace boost { namespace multiprecision {

template <class FloatingPointType>
class cpp_double_fp_backend;

typedef number<cpp_double_fp_backend<float>, et_off> cpp_double_float;
typedef number<cpp_double_fp_backend<double>, et_off> cpp_double_double;
typedef number<cpp_double_fp_backend<long double>, et_off> cpp_double_long_double;
typedef number<cpp_double_fp_backend<boost::float128_type>, et_off> cpp_double_float128; // Only when boost::float128_type is available

} } // namespaces

The cpp_double_fp_backend back-end is the sum of two IEEE floating-point numbers combined to create a type having a composite width rougly twice that as one of its parts. The cpp_double_fp_backend back-end is used in conjunction with number and acts as an entirely C++ header only floating-point number type.

The implementation relies on double-word arithmetic which is a technique used to represent a real number as the sum of two floating-point numbers. Other commonly used names for this include double-double or double-word arithmetic.

The cpp_double_fp_backend types have fixed width and do not allocate. The type cpp_double_double, for instance, is composed of two built-in double components. On most common systems, built-in double is a double-precision IEEE floating-point number. This results in a cpp_double_double that has 106 binary digits and approximately 32 decimal digits of precision.

The exponent ranges of the types are slightly limited (on the negative side) compared to those of the composing type. Consider again the type cpp_double_double, which is built from two double-precision IEEE double-precision floating-point numebers. On common systems, this type has a maximum decimal exponent of 308 (the same as one single double-precision floating point number). The negative minimum exponent, however, is about -291, which is less range than -307 from standalone double. The reason for the limitation is because the composite lower-limb has lower value than its upper limb. The composite type would easily underflow or become subnormal if the upper limb had its usual minimum value.

There is full standard library and std::numeric_limits support available for this type.

Note that the availability of cpp_double_float128 depends on the availability of boost::float128_type, which can be queried at compile-time via the configuration macro BOOST_HAS_FLOAT128. This is available at the moment predominantly with GCC compilers in GNU-standard mode and (with GCC 14 and later) also in strict ANSI mode.

Run-time performance is a top-level requirement for the cpp_double_fp_backend types. The types still do, however, support infinities, NaNs and (of course) zeros. Signed negative zero, however, is not supported (in favor of efficiency). All zeros are treated as positive.

The cpp_double_fp_backend types interoperate with Boost.Math and Boost.Math.Constants. This offers the wealth of Boost-related mathematical tools instantiated with the cpp_double_fp_backend types.

Things you should know when using the cpp_double_fp_backend types:

The cpp_double_fp_backend back-end has been inspired by original works and types. These include the historical doubledouble and more, as listed below.