What's a proper way of type-punning a float to

Question

The code below performs a fast inverse square root operation by some bit hacks. The algorithm was probably developed by Silicon Graphics in early 1990's and it's appeared in Quake 3 too. more info

However I get the following warning from GCC C++ compiler: dereferencing type-punned pointer will break strict-aliasing rules

Should I use static_cast, reinterpret_cast or dynamic_cast instead in such situations?

float InverseSquareRoot(float x)
{
    float xhalf = 0.5f*x;
    int32_t i = *(int32_t*)&x;
    i = 0x5f3759df - (i>>1);
    x = *(float*)&i;
    x = x*(1.5f - xhalf*x*x);
    return x;
}

Answer 1

The only cast that will work here is reinterpret_cast. (And even then, at least one compiler will go out of its way to ensure that it won't work.)

But what are you actually trying to do? There's certainly a better solution, that doesn't involve type punning. There are very, very few cases where type punning is appropriate, and they all are in very, very low level code, things like serialization, or implementing the C standard library (e.g. functions like modf). Otherwise (and maybe even in serialization), functions like ldexp and modf will probably work better, and certainly be more readable.