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Niels Lohmann
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ordered_map keeps its elements in a std::vector<std::pair<const Key, T>>. With a std::string key, that pair is not nothrow move constructible (the const key has to be copied), so std::vector copies every element when it reallocates. For ordered_json, this deep-copies every member value an object already holds, including whole nested subtrees, on each growth step. Grow the storage in ordered_map instead, copying the keys and moving the values. This happens in two phases, so the strong exception guarantee is kept without try/catch. The first phase may throw, but only touches a temporary buffer: it copies the keys, value-initializes the values, and constructs the new element. The second phase moves the values (noexcept) and swaps the buffers. Because the new element is constructed before any value is moved, arguments that refer to elements of the container stay valid, as with std::vector. Types that cannot take this path keep the std::vector behavior. Parsing into ordered_json (ParseStringOrdered, Apple M1 Max, clang -O3): twitter 3.20 -> 1.70 ms, citm_catalog 7.73 -> 3.67 ms, jeopardy 219 -> 177 ms, canada unchanged. The number of allocations for twitter and citm_catalog drops by two thirds. Also add ParseStringOrdered rows to the benchmarks, and document the growth behavior and the exception safety of ordered_map. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
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