Trick Question

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Originally Posted by @nthony

Thanks for clearing that up all. I'm going to need to re-read this a couple times to fully understand why the post-increment results in undefined behaviour.

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Simply put: Because the standard says "you must only use a variable with a modification ONCE between sequence points (sequence points is essentially the semicolon at the end of the line - although it's not strictly true, that's the simplified view of it). The REASON the standard says so is that the compiler is ALLOWED to perform the ++ part of the calculation at any time within the two sequence points.

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Mats

I think the problem was the misconception that a post-fix expression is to be evaluated exactly after the statement in which it appears and before the next. But from what you've said, it seems apparent now.

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Originally Posted by ISO:9989 - 6.5.2.4 Postfix increment and decrement

The result of the postfix ++ operator is the value of the operand. After the result is
obtained, the value of the operand is incremented. ... The side effect of updating the stored value of the operand shall occur between
the previous and the next sequence point.

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