@无名啊，我刚看到C++20的std::bit_cast，这应该是在C++中实现类型双关的唯二安全方法（另一种安全方法是memcpy）。C的话，联合应该是安全方法。
https://en.wikipedia.org/wiki/Fast_inverse_square_root 给出的安全版本：
// C
# include <stdint.h> // uint32_t
float q_rsqrt(float number)
{
union {
float f;
uint32_t i;
} conv = { .f = number };
conv.i = 0x5f3759df - (conv.i >> 1);
conv.f *= 1.5F - (number * 0.5F * conv.f * conv.f);
return conv.f;
}
// C++20
# include <bit>
# include <limits>
# include <cstdint>
constexpr float Q_rsqrt(float number) noexcept
{
static_assert(std::numeric_limits<float>::is_iec559); // (enable only on IEEE 754)
float const y = std::bit_cast<float>(
0x5f3759df - (std::bit_cast<std::uint32_t>(number) >> 1));
return y * (1.5f - (number * 0.5f * y * y));
}
