<cfenv> support

<cfenv>

IEEE 754 defines 5 rounding modes for Decimal Floating Point Types.

  1. Downward

  2. To nearest (with ties to even)

  3. To nearest from zero

  4. Toward zero

  5. Upward

The default rounding mode is to nearest with ties to even (#2) as specified in IEEE 754 Section 4.3.3

Using the following enum class and functions you can change the rounding mode from the default at RUNTIME only if BOOST_DECIMAL_NO_CONSTEVAL_DETECTION is not defined.

#include <boost/decimal/cfenv.hpp>

namespace boost {
namespace decimal {

enum class rounding_mode : unsigned
{
    fe_dec_downward = 1 << 0,
    fe_dec_to_nearest = 1 << 1,
    fe_dec_to_nearest_from_zero = 1 << 2,
    fe_dec_toward_zero = 1 << 3,
    fe_dec_upward = 1 << 4,
    fe_dec_default = fe_dec_to_nearest
};

rounding_mode fegetround() noexcept;

// Returns the rounding mode that has been set
//
// If your compiler defines BOOST_DECIMAL_NO_CONSTEVAL_DETECTION,
// this function will return the default rounding mode
// to alert you that the rounding mode has NOT changed
rounding_mode fesetround(rounding_mode round) noexcept;

} //namespace decimal
} //namespace boost

You can similarly change the default rounding mode at compile time with any compiler (unlike at runtime) using similarly named macros:

namespace boost {
namespace decimal {

static constexpr rounding_mode _boost_decimal_global_rounding_mode {
    #if defined(BOOST_DECIMAL_FE_DEC_DOWNWARD)
    rounding_mode::fe_dec_downward
    #elif defined(BOOST_DECIMAL_FE_DEC_TO_NEAREST)
    rounding_mode::fe_dec_to_nearest
    #elif defined(BOOST_DECIMAL_FE_DEC_TO_NEAREST_FROM_ZERO)
    rounding_mode::fe_dec_to_nearest_from_zero
    #elif defined (BOOST_DECIMAL_FE_DEC_TOWARD_ZERO)
    rounding_mde::fe_dec_toward_zero
    #elif defined(BOOST_DECIMAL_FE_DEC_UPWARD)
    rounding_mode::fe_dec_upward
    #else
    rounding_mode::fe_dec_default
    #endif
};

} // namespace decimal
} // namespace boost

The default rounding mode for compile time is the same as the default run-time.

There is no requirement to explicitly define a compile-time nor run-time rounding mode
Prior to v5.2.0 this header was <boost/decimal/fenv.hpp>, but has been changed to <boost/decimal/cfenv.hpp> for consistency with the STL.
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