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# nlohmann::basic_json::flatten

    basic_json flatten() const;

The function creates a JSON object whose keys are JSON pointers (see RFC 6901) and whose values are all primitive (see `is_primitive()` for more information). The original JSON value can be restored using the `unflatten()` function.

## Return value

an object that maps JSON pointers to primitive values

## Exception safety

Strong exception safety: if an exception occurs, the original value stays intact.

## Complexity

Linear in the size the JSON value.

## Notes

Empty objects and arrays are flattened to `null` and will not be reconstructed correctly by the `unflatten()` function.

## Examples

Example

The following code shows how a JSON object is flattened to an object whose keys consist of JSON pointers.

    #include <iostream>
    #include <iomanip>
    #include <nlohmann/json.hpp>

    using json = nlohmann::json;

    int main()
    {
        // create JSON value
        json j =
        {
            {"pi", 3.141},
            {"happy", true},
            {"name", "Niels"},
            {"nothing", nullptr},
            {
                "answer", {
                    {"everything", 42}
                }
            },
            {"list", {1, 0, 2}},
            {
                "object", {
                    {"currency", "USD"},
                    {"value", 42.99}
                }
            }
        };

        // call flatten()
        std::cout << std::setw(4) << j.flatten() << '\n';
    }

Output:

    {
        "/answer/everything": 42,
        "/happy": true,
        "/list/0": 1,
        "/list/1": 0,
        "/list/2": 2,
        "/name": "Niels",
        "/nothing": null,
        "/object/currency": "USD",
        "/object/value": 42.99,
        "/pi": 3.141
    }

## See also

- unflatten the reverse function

## Version history

- Added in version 2.0.0.

Last update: May 1, 2022
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