Double from long bits

Question

I have an unsigned long long (or uint64_t) value and want to convert it to a double. The double shall have the same bit pattern as the

Answer 1

The portable way to do this is with memcpy (you may also be able to conditionally do it with reinterpret_cast or a union, but those aren't certain to be portable because they violate the letter of the strict-alias rules):

// First, static assert that the sizes are the same
memcpy(&result, &bits, sizeof(bits));

But before you do make sure you know exactly what you're doing and what floating point representation is being used (although IEEE754 is a popular/common choice). You'll want to avoid all kinds of problem values like infinity, NaN, and denormal numbers.