> Casting should be safe as far as I know

I assume you're talking about this:

    long x = <whatever>
    double y = *(double*)&x;

Nope, it is undefined behavior, with few exceptions, even if sizeof double == sizeof long. See https://stackoverflow.com/questions/98650/what-is-the-strict...

If you got rid of this rule a huge number of optimizations would be lost, since the compiler would have to assume that any memory, of any type, could change if anything at all is modified through a pointer. You'd be surprised how slow everything would be. You'd also be surprised how much code violates this and gets away with it. Time bombs waiting to happen. ;-)

memcpy(), and more generally accessing bytes through char*, are how you avoid this kind of undefined behavior. It comes up so often that C++20 added std::bit_cast for this same use: https://en.cppreference.com/w/cpp/numeric/bit_cast