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62 lines
1.6 KiB
Markdown
62 lines
1.6 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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## Complexity
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Linear in the size 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 can not unflattened to
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their original type. Apart from this example, 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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