Quoting the latest draft of the ISO C++ standard, section 5.6 ([expr.mul]):
If the second operand of / or % is zero the behavior is undefined.
This applies to both integer and floating-point division.
A particular C++ implementation may conform to the IEEE floating-point standard, which has more specific requirements for division by zero, which which case the behavior may be well defined for that implementation. That's probably why floating-point division by zero yields Infinity in your implementation. But the C++ standard doesn't require IEEE floating-point behavior.
