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api/basic_json/flatten/index.md

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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](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.
+
+## 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()`](https://json.nlohmann.me/api/basic_json/unflatten/index.md) 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](https://json.nlohmann.me/api/basic_json/unflatten/index.md) the reverse function
+
+## Version history
+
+- Added in version 2.0.0.