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Afonso Januário
c66be708eb
operator== converts between number_integer, number_unsigned, and number_float before comparing, so json(0), json(0U), and json(0.0) all compare equal. hash() folded the specific value_t into the result for each of the three numeric cases, giving each a distinct hash and breaking the standard Hash requirement that a == b implies hash(a) == hash(b). A std::unordered_set could therefore hold all three as separate elements even though they compare equal. hash() now treats all three numeric variants the same way: it converts the value to number_float_t and combines it with a single shared type tag, so any two numbers operator== considers equal hash identically regardless of which internal type actually holds them. Updated the accompanying test to check this consistency directly (including via an actual unordered_set) instead of asserting that 0, 0U, and 0.0 hash differently, since that assumption was the bug. Also corrected the function's own doc comment and the std::hash API docs, which described the old behavior as intended. Fixes #5400 Signed-off-by: Afonso Januário <afonso-januario@hotmail.com> Signed-off-by: Niels Lohmann <mail@nlohmann.me>
1.0 KiB
1.0 KiB
std::hashnlohmann::basic_json\
namespace std {
struct hash<nlohmann::basic_json>;
}
Return a hash value for a JSON object. The hash function tries to rely on std::hash where possible. Furthermore, the
type of the JSON value is taken into account, so #!json null, #!cpp false, and numbers all hash differently from
each other. Numbers that compare equal under operator== always hash equally, regardless of
whether they are stored as #!cpp 0, #!cpp 0U, or #!cpp 0.0.
Examples
??? example
The example shows how to calculate hash values for different JSON values.
```cpp
--8<-- "examples/std_hash.cpp"
```
Output:
```json
--8<-- "examples/std_hash.output"
```
Note the output is platform-dependent.
See also
- operator== compares two JSON values for equality, consistent with equal hash values
Version history
- Added in version 1.0.0.
- Extended for arbitrary basic_json types in version 3.10.5.