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Niels Lohmann
c3219fdc30
The binary entry points end with
return res ? result : basic_json(value_t::discarded);
The condition operator's second operand is an lvalue, so this is not a case
where the return value can be elided or implicitly moved from: every
successful from_cbor(), from_msgpack(), from_ubjson(), from_bjdata() and
from_bson() call deep-copies the value it just parsed, and then destroys the
original.
The copy is not cheap, and it is not incidental: basic_json's copy
constructor walks the whole value. Parsing a 2 MB CBOR document with 60,000
objects, median of 25 runs, clang 17 -O3:
from_cbor 26.99 ms -> 14.65 ms
from_msgpack 26.82 ms -> 14.82 ms
Moving instead of copying is the entire change; the parsed value is not used
again after the return expression is evaluated.
There is a second reason to prefer the move. The copy constructor recurses
once per nesting level, so the copy is also a stack-overflow path on the
return side, on a value the reader has already accepted. That is currently
masked because the readers themselves recurse and overflow first (#5104), but
it has to be fixed for making them iterative to have any effect.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>