flatten() appended the entries to an ordered_map one by one, so its vector
grew by doubling and moved every entry about once in total. The number of
entries is known after a walk over the value that only counts (it needs no
path strings), so objects with a reserve() are given that capacity first.
std::map has no reserve() and is unchanged.
flatten() of an ordered_json (instructions on macOS, Linux Ir GCC 13 / Clang 18):
citm_catalog 36.8M -> 23.1M, 32.7M -> 17.1M / 31.7M -> 16.8M; twitter 20.5M ->
13.6M, 17.6M -> 10.0M / 17.0M -> 9.8M. Output is identical.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Niels Lohmann
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