Positive integers that multiply to a negative value

Question

I am learning C++ by reading Stroustrup\'s \"Principles and Practice Using C++\".

In the section about pre- and post-conditions there is the following example of functio

Answer 1

The answer is that his precondition-check is incomplete. Even though it is too restrictive.
He failed to include a check that the product can be represented instead of resulting in UB:

int area(int length, int width) {
    // calculate area of a rectangle
    assert(length >= 0 && width >= 0 && (!width
        || std::numeric_limits::max() / width >= length));
    int a = length * width;
    assert(a >= 0); // Not strictly neccessary - the math is easy enough
    return a;
}