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

Are there such possible values for integer that pre-conditions is ok but post-condition not?

Yes there's a number of input values, that can cause the post condition to fail. If e.g.

int a = length*width;

overflows the positive int range (std::numeric_limits::max()) and the compiler implementation yields a negative value for this case.

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As others noted in their answers, the situation that length*width goes out of bounds from ]0-std::numeric_limits::max()[ is actually undefined behavior, and the post condition renders merely useless, because any value might need to be expected for a.

The key point to fix this, is given in @Deduplicator's answer, the pre-condition needs to be improved.

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As a lance for Bjarne Stroustrup's reasonings to give that example:

I assume he wanted to point out that such undefined behavior might lead to unexpected negative values in the post-condition and surprising results for a naive assumption checked with the pre-condition.