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131 lines
4.5 KiB
Markdown
131 lines
4.5 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.106`](../../home/exceptions.md#jsonexceptionparse_error106) if an array index in a key begins
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with '0'; example: `"array index '01' must not begin with '0'"`
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- Throws [`parse_error.107`](../../home/exceptions.md#jsonexceptionparse_error107) if a key is not empty and does not
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begin with a slash (`/`); example: `"JSON pointer must be empty or begin with '/' - was: 'a'"`
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- Throws [`parse_error.108`](../../home/exceptions.md#jsonexceptionparse_error108) if a tilde (`~`) in a key is not
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followed by `0` or `1`; example: `"escape character '~' must be followed with '0' or '1'"`
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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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- Throws [`out_of_range.404`](../../home/exceptions.md#jsonexceptionout_of_range404) if a level becomes an array
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(because one of its keys is `0`) and another key at that level begins with a digit but is not a valid array index
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(such as `1a`), or is `-`; example:
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`"unresolved reference token '-'"`
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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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#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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```json
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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](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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- Throws [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) instead of
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[`out_of_range.404`](../../home/exceptions.md#jsonexceptionout_of_range404) for a one-character array index that is
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not a digit (e.g., `/x`) in version 3.13.0, as it already did for longer ones.
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