playing with system_error and error codes

fenv.hpp

#include <system_error>

#include <cfenv>
#include <cassert>

namespace fe {
enum class rounding_mode : int {
    downward = FE_DOWNWARD,
    to_nearest = FE_TONEAREST,
    toward_zero = FE_TOWARDZERO,
    upward = FE_UPWARD
};

inline rounding_mode get_rounding(std::system_error &ec) {
    int mode = fegetround();
    if (mode < 0) {
        ec = std::make_error_code(static_cast<std::errc>(mode));
        return static_cast<rounding_mode>(-1);
    }
    assert(mode == FE_DOWNWARD || mode == FE_TONEAREST || mode == FE_TOWARDZERO || mode == FE_UPWARD);
    return static_cast<rounding_mode>(mode);
}
    
inline rounding_mode get_rounding() {
    int mode = fegetround();
    if (mode < 0) {
        throw std::system_error(std::make_error_code(static_cast<std::errc>(mode)), "no such rounding mode or the current rounding mode cannot be determined");
    }
    assert(mode == FE_DOWNWARD || mode == FE_TONEAREST || mode == FE_TOWARDZERO || mode == FE_UPWARD);
    return static_cast<rounding_mode>(mode);
}
    
inline void set_rounding(rounding_mode m, std::error_code &ec) {
    int result = fesetround(static_cast<int>(m));
    if (result != 0) {
        ec = std::make_error_code(static_cast<std::errc>(result));
    }
}

inline void set_rounding(rounding_mode m) {
    std::error_code ec;
    set_rounding(m, ec);
    if (ec) {
        throw std::system_error(ec, "failed to set rounding mode");
    }
}
    
struct rounding_mode_setter {
    rounding_mode saved_mode;
    rounding_mode_setter(rounding_mode m) : saved_mode(get_rounding()) { set_rounding(m); }
    ~rounding_mode_setter() noexcept(false) {set_rounding(saved_mode);}
    
    rounding_mode_setter(rounding_mode_setter const &) = delete;
    rounding_mode_setter(rounding_mode_setter &&) = delete;
};
}