How to merge JSON objects?

This page explains how to write a function that merges two JSON objects into one.

Before using this function, please remember that ArduinoJson is a serialization library. As such, it is designed to serialize and deserialize JSON documents, not to store the state of an application.

Do not store the state of your application in a JsonDocument, or you will create a memory leak.

Shallow merge

Suppose you have two JSON objects:

{
  "name": "Benoit"
}

and

{
  "age": 38
}

And suppose that your goal is to merge the two objects into one:

{
  "name": "Benoit",
  "age": 38
}

Well, you can do that easily with the following function:

void merge(JsonObject dest, JsonObjectConst src)
{
   for (JsonPairConst kvp : src)
   {
     dest[kvp.key()] = kvp.value();
   }
}

See online demo on wandbox.org.

Deep merge

The merge() function above only merges the top-level object. It blindly overrides members of the destination object.

Now, let’s see how we can merge the members instead of overriding them.

For example, imagine you have this object:

{
  "config": {
    "wifi": {
      "ssid": "thebatcave"
    },
    "light": {
      "state": "off"
    }
  }
}

And suppose you want to merge with the following object:

{
  "config": {
    "wifi": {
      "pass": "i'm*batman!"
    }
  }
}

With a deep merging function, this would give:

{
  "config": {
    "wifi": {
      "ssid": "thebatcave",
      "pass": "i'm*batman!"
    },
    "light": {
      "state": "off"
    }
  }
}

Here an example implementation for this function:

void merge(JsonVariant dst, JsonVariantConst src)
{
  if (src.is<JsonObjectConst>())
  {
    for (JsonPairConst kvp : src.as<JsonObjectConst>())
    {
      if (dst[kvp.key()]) 
        merge(dst[kvp.key()], kvp.value());
      else
        dst[kvp.key()] = kvp.value();
    }
  }
  else
  {
    dst.set(src);
  }
}

See online demo on wandbox.org.

This function uses: