Niels Lohmann fc52e59c73 Hash deeply nested values without recursing per nesting level ...

std::hash<basic_json> hashed an array or object by hashing each element,
which called detail::hash again once per nesting level. A value nested
deeply enough - 50,000 levels of objects on an 8 MiB stack - exhausted
the call stack and terminated the process. parse() accepts such values
without complaint, since the parser is iterative, and a parsed value is
hashed wherever it is used as a key in an unordered container.

Bound the descent the same way dump() does: detail::hash takes the
nesting level, and once hash_depth_limit() (128) levels have been entered,
hash_iteratively() hashes what is left on an explicit stack. It combines
the seeds in exactly the same order, so hash values are unchanged. A value
nested less deeply than the bound is hashed by the same code as before,
without allocating, and is as fast as before.

Tests check that every depth up to twice the bound hashes exactly like
the recursive definition of the hash, and that values nested 100,000
levels deep hash without crashing.

Fixes #5545 for std::hash.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

2026-09-23 21:53:09 +02:00

..

nlohmann

Hash deeply nested values without recursing per nesting level

2026-09-23 21:53:09 +02:00