As a commenter noted as well, you can perform the swap using two std::rotate calls vs. three (less than 2N operations). This said, Raymond’s use of reverse is still most efficient at N operations (not considering paging/caching issues).
To swap B and D, with intervening C (i.e. B C D), what he his doing is individually reversing each of B C, and D (= total N swaps), then reversing the combined B' C' D' (= another N swaps).
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As a commenter noted as well, you can perform the swap using two std::rotate calls vs. three (less than 2N operations). This said, Raymond’s use of reverse is still most efficient at N operations (not considering paging/caching issues).
Isn't he also using 2N operations?
To swap B and D, with intervening C (i.e. B C D), what he his doing is individually reversing each of B C, and D (= total N swaps), then reversing the combined B' C' D' (= another N swaps).