Size of struct containing double field

Question

Firstly, I understand byte padding in structs. But I have still a small test contain a double field in struct and I don\'t know how to explain this :

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Answer 1

It's sixteen bytes so that if you have an array of datas, the double values can all be aligned on 8-byte boundaries. Aligning data properly in memory can make a big difference in performance. Misaligned data can be slower to operate on, and slower to fetch and store.