What is the best way to check if a specific key exists in a JSON object?

Question

Ryan McCombe · Accepted Answer

There are a couple of ways to check if a key exists in a JSON object.

One option is to use the contains() method:

1#include <iostream>
2#include <json.hpp>
3using json = nlohmann::json;
4
5int main() {
6  using namespace nlohmann::literals;
7  json doc{R"(
8    {
9      "name": "Roderick",
10      "health": 100
11    }
12  )"_json};
13
14  if (doc.contains("name")) {
15    std::cout << "JSON object contains 'name'";
16  }
17}

1JSON object contains 'name'

contains() will return true if the key exists in the object, even if the corresponding value is null.

Another option is to use find():

1#include <iostream>
2#include <json.hpp>
3using json = nlohmann::json;
4
5int main() {
6  using namespace nlohmann::literals;
7  json doc{R"(
8    {
9      "name": "Roderick",
10      "health": 100
11    }
12  )"_json};
13
14  auto it = doc.find("health");
15  if (it != doc.end()) {
16    std::cout << "JSON object contains 'health'";
17  }
18}

1JSON object contains 'health'

find() returns an iterator to the key/value pair if the key exists. We can compare this iterator to end() to determine if the key was found or not.

A third way is to use the [] operator and check if the result is a null json value:

1#include <iostream>
2#include <json.hpp>
3using json = nlohmann::json;
4
5int main() {
6  using namespace nlohmann::literals;
7  json doc{R"(
8    {
9      "name": "Roderick",
10      "health": 100
11    }
12  )"_json};
13
14  if (doc["mana"].is_null()) {
15    std::cout << "JSON object contains does"
16      " not contain 'mana' key";
17  }
18}

1JSON object contains does not contain 'mana' key

However, this will actually add the key to the object if it doesn't already exist, so it's not always ideal.

In general, contains() is the most straightforward way to test for a key's existence in a JSON object.

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