When to use reinterpret_cast?

Question

I am little confused with the applicability of reinterpret_cast vs static_cast. From what I have read the general rules are to use static cast when

Answer 1

One use of reinterpret_cast is if you want to apply bitwise operations to (IEEE 754) floats. One example of this was the Fast Inverse Square-Root trick:

https://en.wikipedia.org/wiki/Fast_inverse_square_root#Overview_of_the_code

It treats the binary representation of the float as an integer, shifts it right and subtracts it from a constant, thereby halving and negating the exponent. After converting back to a float, it's subjected to a Newton-Raphson iteration to make this approximation more exact:

float Q_rsqrt( float number )
{
    long i;
    float x2, y;
    const float threehalfs = 1.5F;

    x2 = number * 0.5F;
    y  = number;
    i  = * ( long * ) &y;                       // evil floating point bit level hacking
    i  = 0x5f3759df - ( i >> 1 );               // what the deuce? 
    y  = * ( float * ) &i;
    y  = y * ( threehalfs - ( x2 * y * y ) );   // 1st iteration
//  y  = y * ( threehalfs - ( x2 * y * y ) );   // 2nd iteration, this can be removed

    return y;
}

This was originally written in C, so uses C casts, but the analogous C++ cast is the reinterpret_cast.