Answer 1

Because the behaviour is undefined it will give different results on different compilers.
You should not do more than one increment on one line.
https://www.quora.com/What-does-an-expression-involving-multiple-post-pre-decrement-increment-operators-evaluate-to-in-C-and-C++
https://stackoverflow.com/questions/949433/why-are-these-constructs-using-undefined-behavior-in-c