can two json objects be concatenated?

[CheyenneForbes]

for example:

json j1 = {
 {obj1, value1},
 {obj2, value2}
};
json j2 = {
 {obj3, value3},
 {obj4, value4}
};

json j3 = j1 + j2;

[nlohmann]

Duplicate of #428, implemented with the update function.

[gregmarr]

https://github.com/nlohmann/json/blob/develop/single_include/nlohmann/json.hpp#L14797-L14816

json j3 = j1;
j3.update(j2);

[p-i-]

Fantastic to find update.
Is there any particular reason not to overload + and +=?
That's the first thing I tried: j2 += j1
I think that would be a really neat syntax.

[jaredgrubb]

There's not a lot to gain by adding another way to do this as there are only a few characters difference between the two. I personally like the Python mantra: "There should be one -- and preferably only one -- obvious way to do it.". C++ doesn't always live up to this, but it's a nice rule in general.

But, I'd also expect addition operations to have familiar properties like commutivity (eg, "j1+j2" and "j2+j1" should give the same result, but for JSON they wouldn't!). Sure, std::string has a non-commutative "+" but that doesn't mean we should give up on that goal.

[DolphinDream]

Does this update method work with merging recursively?
j1 = { “attributes”: { “background-color”: “red”}, “prop1”: 2 }
j2 = { “attributes”: { “text-color”: “blue”}, “prop2”: 3 }

Merging j1 and j2 (concatenating, updating) I would expect to get:

{ “attributes”: { “background-color”: “red”, “text-color”: “blue”}, “prop1”: 2, “prop2”: 3 }

I’m not sure if any changes to merge occurred since 3.3.0 .. but in this older version when merging only the top level properties are concatenated.. namely, if there is a duplicate to be merged (albeit with different subtree values) the previous property is being replaced by the duplicate property in the other j object. Essentially merging j1.update(j2) creates this:

{ “attributes”: { "text-color”: “blue”}, “prop1”: 2, “prop2”: 3 }

which removes the previous attributes values.

  fifo_json j1;
  j1["attributes"]["background-color"] = "red";
  j1["prop1"] = 2;
  cout << "j1 = " << endl;
  cout << j1.dump(2) << endl;

  fifo_json j2;
  j2["attributes"]["text-color"] = "blue";
  j2["prop2"] = 3;
  cout << "j2 = " << endl;
  cout << j2.dump(2) << endl;

  // concatenate
  j1.update(j2);
  cout << "j1+j2 = " << endl;
  cout << j1.dump(2) << endl;

gives the output:

j1 = 
{
  "attributes": {
    "background-color": "red"
  },
  "prop1": 2
}
j2 = 
{
  "attributes": {
    "text-color": "blue"
  },
  "prop2": 3
}
j1+j2 = 
{
  "attributes": {
    "text-color": "blue"
  },
  "prop1": 2,
  "prop2": 3
}

[gregmarr]

@DolphinDream There is now a recursive parameter to the update function. https://github.com/nlohmann/json/blob/master/docs/mkdocs/docs/api/basic_json/update.md

[DolphinDream]

@DolphinDream There is now a recursive parameter to the update function. https://github.com/nlohmann/json/blob/master/docs/mkdocs/docs/api/basic_json/update.md

Yeah.. i just noticed "Added merge_objects parameter in 3.10.4” .. which explains my 3.3.0 issue :)

[nlohmann]

See https://json.nlohmann.me/api/basic_json/update/