Here is a new version of signbit, taking input from Peter Dimov into account: template<class T, class S> T binary_cast(const S& s) { BOOST_STATIC_ASSERT(sizeof(S) == sizeof(T)); T t; memcpy(&t, &s, sizeof(S)); return t; } const boost::uint32_t signbit_mask = binary_cast<boost::uint32_t>(1.0f) ^ binary_cast<boost::uint32_t>(-1.0f); inline bool signbit(float x) { return binary_cast<boost::uint32_t>(x) & signbit_mask; } inline bool signbit(double x) { return signbit(static_cast<float>(x)); } inline bool signbit(long double x) { return signbit(static_cast<float>(x)); } The code should work on any platform with 1. IEEE 754 or some similar representation 2. sizeof(float) == 4 3. casts between different numeric types preserve the sign bit, also for non-finite numbers. The function is intended to be used when there is no predefined signbit function or macro. --Johan Råde