json/api/ordered_json/index.md

nlohmann::ordered_json

using ordered_json = basic_json<ordered_map>;

This type preserves the insertion order of object keys.

Iterator invalidation

The type is based on ordered_map which in turn uses a std::vector to store object elements. Therefore, adding object elements can yield a reallocation in which case all iterators (including the end() iterator) and all references to the elements are invalidated. Also, any iterator or reference after the insertion point will point to the same index, which is now a different value.

Examples

Example

The example below demonstrates how ordered_json preserves the insertion order of object keys.

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

using ordered_json = nlohmann::ordered_json;

int main()
{
    ordered_json j;
    j["one"] = 1;
    j["two"] = 2;
    j["three"] = 3;

    std::cout << j.dump(2) << '\n';
}

Output:

{
  "one": 1,
  "two": 2,
  "three": 3
}

See also

• ordered_map
• Object Order

Version history

Since version 3.9.0.