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basic_json unflatten() const;

The function restores the arbitrary nesting of a JSON value that has been flattened before using the flatten() function. The JSON value must meet certain constraints:

  1. The value must be an object.
  2. The keys must be JSON pointers (see RFC 6901)
  3. The mapped values must be primitive JSON types.

Return value

the original JSON from a flattened version

Exception safety

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


The function can throw the following exceptions:


Linear in the size the JSON value.


Empty objects and arrays are flattened by flatten() to null values and can not unflattened to their original type. Apart from this example, for a JSON value j, the following is always true: j == j.flatten().unflatten().



The following code shows how a flattened JSON object is unflattened into the original nested JSON object.

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

using json = nlohmann::json;

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

    // call unflatten()
    std::cout << std::setw(4) << j_flattened.unflatten() << '\n';


    "answer": {
        "everything": 42
    "happy": true,
    "list": [
    "name": "Niels",
    "nothing": null,
    "object": {
        "currency": "USD",
        "value": 42.99
    "pi": 3.141

See also

Version history

  • Added in version 2.0.0.

Last update: May 1, 2022
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