Question

If I have an int, convert it to a double, then convert the double back to an int, am I guaranteed to get the same value back that I started with? In other words, given this function:

int passThroughDouble(int input)
{
  double d = input;
  return d;
}

Am I guaranteed that passThroughDouble(x) == x for all ints x?

Answer 1

No it isn’t. The standard says nothing about the relative sizes of int and double.

If int is a 64-bit integer and double is the standard IEEE double-precision, then it will already fail for numbers bigger than 2^53.

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That said, int is still 32-bit on the majority of environments today. So it will still hold in many cases.