Is floating point multiplication by zero guaranteed to produce zero?

Question

I understand floating point has rounding errors, but I\'m wondering if there are certain situations where the error does not apply, such as multiplication by zero .

Answer 1

False:

0f * NAN == NAN
0f * INFINITY == NAN

and ...

0f * -1f == -0f (negative 0f), with 0f == -0f :-)

(on Intel, VC++, and probably on any platform that uses IEEE 754-1985 floating points)

Example on ideone (that uses GCC on some Intel compatible platform probably)

Answer 2

In addition to @xanatos fine answer, consider some of OP's middle-of-the-post concerns:

I'm wondering if there are certain situations where the (rounding) error does not apply

Candidates include some_double_y = some_double_x * 1.0 and some_double_y = some_double_x + 0.0 may never incur a rounding error.

Yet even those are suspect due to a compiler may evaluate double at higher precision considering the FLT_EVAL_METHOD == 2 where "evaluate all operations and constants to the range and precision of the long double type." In that case, an intermediate some_double_x may exist as a long double differing from an apparent double value of 0.0 or 1.0.