When a double with an integer value is cast to an integer, is it guaranteed to do it 'properly'?

Question

When a double has an 'exact' integer value, like so:

double x = 1.0;
double y = 123123;
double z = -4.000000;

Is it guaranteed that it will round properly to 1, 123123, and -4 when cast to an integer type via (int)x, (int)y, (int)z? (And not truncate to 0, 123122 or -5 b/c of floating point weirdness). I ask b/c according to this page (which is about fp's in lua, a language that only has doubles as its numeric type by default), talks about how integer operations with doubles are exact according to IEEE 754, but I'm not sure if, when calling C-functions with integer type parameters, I need to worry about rounding doubles manually, or it is taken care of when the doubles have exact integer values.

Answer 1

As Daniel Fischer stated, if the value of the integer part of the double (in your case, the double exactly) is representable in the type you are converting to, the result is exact. If the value is out of range of the destination type, the behavior is undefined. “Undefined” means the standard allows any behavior: You might get the closest representable number, you might get zero, you might get an exception, or the computer might explode. (Note: While the C standard permits your computer to explode, or even to destroy the universe, it is likely the manufacturer’s specifications impose a stricter limit on the behavior.)