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Niels LohmannandClaude Opus 5
7a37a27a67
unflatten() decided between array and object by looking at the first reference
token it happened to see for a node: it started an array only when that token
was 0. With a sorted object type the token 0 always arrives first, so the
result was correct by accident; with an object type whose iteration order is
unspecified, {"/c/2":3,"/c/1":2,"/c/0":1} unflattened to an object with the
keys "0", "1", and "2" instead of an array.
Collect the pointer prefixes that have a reference token 0 among their children
before building the result, and let get_and_create() consult that set. The
outcome is now independent of the iteration order and matches, for every input,
what a sorted object type produced before: a value is restored as an array if
and only if one of its keys is 0. Iterating the flattened object in a different
order would have been simpler, but it would have changed the key order of the
result for insertion-ordered object types.
The serializer, std::hash, and the UBJSON writer converted the elements of a
binary value to an integer implicitly, which does not compile for a BinaryType
whose value type is std::byte, and which made dump() write the bytes of a
signed value type as negative numbers. Convert to std::uint8_t explicitly in
all three places, so every byte type dumps as 0..255. The default
std::vector<std::uint8_t> configuration is unaffected.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018hxZxz8svM54c6ATEvXp5E
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
74 lines
2.5 KiB
Markdown
74 lines
2.5 KiB
Markdown
# <small>nlohmann::basic_json::</small>unflatten
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```cpp
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basic_json unflatten() const;
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```
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The function restores the arbitrary nesting of a JSON value that has been flattened before using the
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[`flatten()`](flatten.md) function. The JSON value must meet certain constraints:
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1. The value must be an object.
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2. The keys must be JSON pointers (see [RFC 6901](https://tools.ietf.org/html/rfc6901))
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3. The mapped values must be primitive JSON types.
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## Return value
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the original JSON from a flattened version
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## Exception safety
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Strong exception safety: if an exception occurs, the original value stays intact.
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## Exceptions
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The function can throw the following exceptions:
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- Throws [`type_error.314`](../../home/exceptions.md#jsonexceptiontype_error314) if value is not an object
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- Throws [`type_error.315`](../../home/exceptions.md#jsonexceptiontype_error315) if object values are not primitive
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- Throws [`type_error.313`](../../home/exceptions.md#jsonexceptiontype_error313) if a key (JSON pointer) leads to a
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conflicting nesting; example: `"invalid value to unflatten"`
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- Throws [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) if an array index in a key is not a
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number; example: `"array index 'one' is not a number"`
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## Complexity
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Linear in the size of the JSON value.
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## Notes
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Empty objects and arrays are flattened by [`flatten()`](flatten.md) to `#!json null` values and cannot unflattened to
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their original type.
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A flattened array and a flattened object whose keys are array indices are indistinguishable, because both are
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described by the same JSON pointers. A value is therefore restored as an array if and only if one of its keys is the
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reference token `0`, and as an object otherwise: `#!json {"2": 1}` is restored unchanged, whereas `#!json {"0": 1}` is
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restored as `#!json [1]`. This decision does not depend on the order in which the flattened object is iterated.
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Apart from these two cases, for a JSON value `j`, the following is always true:
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`#!cpp j == j.flatten().unflatten()`.
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## Examples
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??? example
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The following code shows how a flattened JSON object is unflattened into the original nested JSON object.
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```cpp
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--8<-- "examples/unflatten.cpp"
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```
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Output:
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```json
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--8<-- "examples/unflatten.output"
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```
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## See also
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- [flatten](flatten.md) the reverse function
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## Version history
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- Added in version 2.0.0.
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- Made the array/object decision independent of the object's iteration order in version 3.13.0.
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