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>
2.5 KiB
nlohmann::basic_json::unflatten
basic_json unflatten() const;
The function restores the arbitrary nesting of a JSON value that has been flattened before using the
flatten() function. The JSON value must meet certain constraints:
- The value must be an object.
- The keys must be JSON pointers (see RFC 6901)
- The mapped values must be primitive JSON types.
Return value
the original JSON from a flattened version
Exception safety
Strong exception safety: if an exception occurs, the original value stays intact.
Exceptions
The function can throw the following exceptions:
- Throws
type_error.314if value is not an object - Throws
type_error.315if object values are not primitive - Throws
type_error.313if a key (JSON pointer) leads to a conflicting nesting; example:"invalid value to unflatten" - Throws
parse_error.109if an array index in a key is not a number; example:"array index 'one' is not a number"
Complexity
Linear in the size of the JSON value.
Notes
Empty objects and arrays are flattened by flatten() to #!json null values and cannot unflattened to
their original type.
A flattened array and a flattened object whose keys are array indices are indistinguishable, because both are
described by the same JSON pointers. A value is therefore restored as an array if and only if one of its keys is the
reference token 0, and as an object otherwise: #!json {"2": 1} is restored unchanged, whereas #!json {"0": 1} is
restored as #!json [1]. This decision does not depend on the order in which the flattened object is iterated.
Apart from these two cases, for a JSON value j, the following is always true:
#!cpp j == j.flatten().unflatten().
Examples
??? example
The following code shows how a flattened JSON object is unflattened into the original nested JSON object.
```cpp
--8<-- "examples/unflatten.cpp"
```
Output:
```json
--8<-- "examples/unflatten.output"
```
See also
- flatten the reverse function
Version history
- Added in version 2.0.0.
- Made the array/object decision independent of the object's iteration order in version 3.13.0.