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136 lines
3.0 KiB
Markdown
136 lines
3.0 KiB
Markdown
# nlohmann::basic_json::flatten
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```
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basic_json flatten() const;
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```
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The function creates a JSON object whose keys are JSON pointers (see [RFC 6901](https://tools.ietf.org/html/rfc6901)) and whose values are all primitive (see [`is_primitive()`](https://json.nlohmann.me/api/basic_json/is_primitive/index.md) for more information). The original JSON value can be restored using the [`unflatten()`](https://json.nlohmann.me/api/basic_json/unflatten/index.md) function.
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## Return value
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an object that maps JSON pointers to primitive values
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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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## 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 to `null` and will not be reconstructed correctly by the [`unflatten()`](https://json.nlohmann.me/api/basic_json/unflatten/index.md) function.
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## Examples
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Example: flatten a JSON object
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The following code shows how a JSON object is flattened to an object whose keys consist of JSON pointers.
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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 =
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{
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{"pi", 3.141},
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{"happy", true},
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{"name", "Niels"},
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{"nothing", nullptr},
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{
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"answer", {
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{"everything", 42}
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}
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},
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{"list", {1, 0, 2}},
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{
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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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}
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};
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// call flatten()
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std::cout << std::setw(4) << j.flatten() << '\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/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": null,
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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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```
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Example: empty objects and arrays are flattened to `null`
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The following code shows that an empty object and an empty array are both flattened to `null`, and that `unflatten()` restores them as `null` rather than as empty containers.
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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 a JSON value with an empty object and an empty array
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json j =
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{
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{"empty_object", json::object()},
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{"empty_array", json::array()},
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{"name", "Niels"}
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};
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// call flatten()
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json flattened = j.flatten();
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std::cout << std::setw(4) << flattened << "\n\n";
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// the empty containers cannot be restored by unflatten()
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std::cout << std::setw(4) << 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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"/empty_array": null,
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"/empty_object": null,
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"/name": "Niels"
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}
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{
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"empty_array": null,
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"empty_object": null,
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"name": "Niels"
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}
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```
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## See also
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- [unflatten](https://json.nlohmann.me/api/basic_json/unflatten/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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