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Fix std::hash contract violation for numeric types

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Fixes #5256: json(42) == json(42u) is true, but their hashes differed,
violating the std::hash contract. This also applied to float comparisons:
json(42) == json(42.0) is true, but they hashed differently.

Solution: Normalize numeric type hashing to ensure equal values hash equal.
- Signed/unsigned integers: normalize unsigned to signed via static_cast,
  matching the existing operator== behavior (lines 3711-3717 in json.hpp)
- Integer/float bridging: for values exactly representable as the float type,
  hash via the float form to collide correctly with float values
- All numeric types share a single type tag to ensure hash collision

The fix is rigorous for the reported issue (int/uint, any magnitude) with zero
gaps. For int/float comparisons, there's a documented edge case at extreme
magnitudes due to float precision limits, mirroring limitations already
present in operator==.

Changes:
- include/nlohmann/detail/hash.hpp: core fix with new
  is_exactly_representable_as_float helper
- tests/src/unit-hash.cpp: update expected hash counts (21 -> 19 distinct),
  add explicit std::hash contract verification
- docs/mkdocs/docs/api/basic_json/std_hash.md: update description
- docs/mkdocs/docs/examples/std_hash.cpp/.output: show the fix in action
- single_include/nlohmann/json.hpp: regenerated via amalgamate

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-07-09 21:12:31 +02:00
parent fe0299545a
commit 68c87ad9de
6 changed files with 273 additions and 86 deletions
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tests/src/unit-hash.cpp
+22 -6
View File
@@ -35,10 +35,10 @@ TEST_CASE("hash<nlohmann::json>")
// number
hashes.insert(std::hash<json> {}(json(0)));
hashes.insert(std::hash<json> {}(json(static_cast<unsigned>(0))));
hashes.insert(std::hash<json> {}(json(static_cast<unsigned>(0)))); // now same hash as json(0)
hashes.insert(std::hash<json> {}(json(0.0))); // now same hash as json(0)
hashes.insert(std::hash<json> {}(json(-1)));
hashes.insert(std::hash<json> {}(json(0.0)));
hashes.insert(std::hash<json> {}(json(42.23)));
// array
@@ -60,7 +60,16 @@ TEST_CASE("hash<nlohmann::json>")
// discarded
hashes.insert(std::hash<json> {}(json(json::value_t::discarded)));
CHECK(hashes.size() == 21);
// Note: json(0), json(0U), and json(0.0) now hash to the same value
// (to satisfy the std::hash contract: equal values must hash equally)
// So we expect 19 distinct hashes instead of 21
CHECK(hashes.size() == 19);
// Verify the std::hash contract: equal values must hash equally
CHECK(std::hash<json>{}(json(0)) == std::hash<json>{}(json(static_cast<unsigned>(0))));
CHECK(std::hash<json>{}(json(0)) == std::hash<json>{}(json(0.0)));
CHECK(std::hash<json>{}(json(42)) == std::hash<json>{}(json(42u)));
CHECK(std::hash<json>{}(json(42)) == std::hash<json>{}(json(42.0)));
}
TEST_CASE("hash<nlohmann::ordered_json>")
@@ -84,10 +93,10 @@ TEST_CASE("hash<nlohmann::ordered_json>")
// number
hashes.insert(std::hash<ordered_json> {}(ordered_json(0)));
hashes.insert(std::hash<ordered_json> {}(ordered_json(static_cast<unsigned>(0))));
hashes.insert(std::hash<ordered_json> {}(ordered_json(static_cast<unsigned>(0)))); // now same hash as ordered_json(0)
hashes.insert(std::hash<ordered_json> {}(ordered_json(0.0))); // now same hash as ordered_json(0)
hashes.insert(std::hash<ordered_json> {}(ordered_json(-1)));
hashes.insert(std::hash<ordered_json> {}(ordered_json(0.0)));
hashes.insert(std::hash<ordered_json> {}(ordered_json(42.23)));
// array
@@ -109,5 +118,12 @@ TEST_CASE("hash<nlohmann::ordered_json>")
// discarded
hashes.insert(std::hash<ordered_json> {}(ordered_json(ordered_json::value_t::discarded)));
CHECK(hashes.size() == 21);
// Note: ordered_json(0), ordered_json(0U), and ordered_json(0.0) now hash to the same value
CHECK(hashes.size() == 19);
// Verify the std::hash contract for ordered_json as well
CHECK(std::hash<ordered_json>{}(ordered_json(0)) == std::hash<ordered_json>{}(ordered_json(static_cast<unsigned>(0))));
CHECK(std::hash<ordered_json>{}(ordered_json(0)) == std::hash<ordered_json>{}(ordered_json(0.0)));
CHECK(std::hash<ordered_json>{}(ordered_json(42)) == std::hash<ordered_json>{}(ordered_json(42u)));
CHECK(std::hash<ordered_json>{}(ordered_json(42)) == std::hash<ordered_json>{}(ordered_json(42.0)));
}