A conforming implementation executing a well-formed program shall
produce the same observable behavior as one of the possible executions
of the corresponding instance of the abstract machine with the
same program and the same input.
However, if any such execution contains an undefined operation, this document places no
requirement on the implementation executing that program with that input
(not even with regard to operations preceding the first undefined
The input and output dynamics of interactive devices shall take
place in such a fashion that prompting output is actually delivered before a program waits for input. What constitutes an interactive device is
This provision is
sometimes called the “as-if” rule, because an implementation is free to
disregard any requirement of this document as long as the result
is as if the requirement had been obeyed, as far as can be determined
from the observable behavior of the program.
For instance, an actual
implementation need not evaluate part of an expression if it can deduce that its
value is not used and that no
side effects affecting the
observable behavior of the program are produced.