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 of the JSON value.

Notes

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

Examples

Example: flatten a JSON object

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
}

Example: empty objects and arrays are flattened to null

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.

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

using json = nlohmann::json;

int main()
{
    // create a JSON value with an empty object and an empty array
    json j =
    {
        {"empty_object", json::object()},
        {"empty_array", json::array()},
        {"name", "Niels"}
    };

    // call flatten()
    json flattened = j.flatten();
    std::cout << std::setw(4) << flattened << "\n\n";

    // the empty containers cannot be restored by unflatten()
    std::cout << std::setw(4) << flattened.unflatten() << '\n';
}

Output:

{
    "/empty_array": null,
    "/empty_object": null,
    "/name": "Niels"
}

{
    "empty_array": null,
    "empty_object": null,
    "name": "Niels"
}

Version history

Added in version 2.0.0.

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