For simplicity sake, I want to store both floats and integers in the same structure element. I was wondering if I could just store everything as double. In other words, convert integers to doubles
before a save and convert the double to an integer after retrieval?

There won't be any precision lost?

Thanks.

doubles have 52 bits of mantissa precision, so they can losslessly represent any integer between -4503599627370496 and 4503599627370496. 4503599627370497 is unrepresentable.