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nlohmann
103 lines
2.8 KiB
Markdown
103 lines
2.8 KiB
Markdown
# nlohmann::basic_json::unflatten
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```
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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 [`flatten()`](https://json.nlohmann.me/api/basic_json/flatten/index.md) function. The JSON value must meet certain constraints:
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1. The value must be an object.
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1. The keys must be JSON pointers (see [RFC 6901](https://tools.ietf.org/html/rfc6901))
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1. 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`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error314) if value is not an object
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- Throws [`type_error.315`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error315) if object values are not primitive
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- Throws [`type_error.313`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error313) if a key (JSON pointer) leads to a conflicting nesting; example: `"invalid value to unflatten"`
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- Throws [`parse_error.109`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error109) if an array index in a key is not a 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()`](https://json.nlohmann.me/api/basic_json/flatten/index.md) to `null` values and cannot unflattened to their original type. Apart from this example, for a JSON value `j`, the following is always true: `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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```
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#include <iostream>
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#include <iomanip>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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// create JSON value
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json j_flattened =
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{
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{"/answer/everything", 42},
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{"/happy", true},
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{"/list/0", 1},
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{"/list/1", 0},
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{"/list/2", 2},
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{"/name", "Niels"},
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{"/nothing", nullptr},
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{"/object/currency", "USD"},
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{"/object/value", 42.99},
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{"/pi", 3.141}
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};
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// call unflatten()
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std::cout << std::setw(4) << j_flattened.unflatten() << '\n';
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}
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```
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Output:
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```
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{
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"answer": {
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"everything": 42
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},
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"happy": true,
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"list": [
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1,
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0,
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2
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],
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"name": "Niels",
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"nothing": null,
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"object": {
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"currency": "USD",
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"value": 42.99
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},
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"pi": 3.141
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}
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```
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## See also
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- [flatten](https://json.nlohmann.me/api/basic_json/flatten/index.md) the reverse function
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## Version history
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- Added in version 2.0.0.
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