Why have undefined behavior?

The obvious question that arises from the last section is, why does the standard have UB at all? Why doesn't it specify the result for every situation? A slightly subtler question that acknowledges the reality that C++ is used on a wide variety of hardware with very different properties is this: why doesn't the standard fall back on implementation-defined behavior instead of leaving it undefined?

The last example from the previous section provides us with a perfect demonstration vehicle for the rationale behind the existence of UB. The statement is that an infinite loop is UB; another way of saying that is that the standard does not require a specific outcome from a program that enters an infinite loop (the standard is more nuanced than that, and some forms of infinite loops will cause the program to hang, but these details are not important at the moment). To understand why the rule is there, consider the following code:

size_t n1 = 0,...