nlohmann and copying maps

Question

Anyone has idea how to quickly copy nlohmann::json MAP values to an other nlohmann::json?

nlohmann::json j1;
nlohmann::json j2;

j1["MAP"]["value1"] = 1;
j2["MAP"]["value2"] = 2;
j2["MAP"] += j1["MAP"];

This will throw because += will think that I am adding a value to a list.

[json.exception.type_error.308] cannot use push_back() with object

I can enumerate j1["MAP"] and add them into j2["MAP"] but I was wondering if there is a straightforward method.

user_501250 · Answer

The json::update() method is the built-in version of this operation.

j2["MAP"].update(j1["MAP"]);

user_982161 · Answer

Another alternative to @cdhowie 's answer is to merge those objects, here the method that you need: merge_patch

so your code should look like:

nlohmann::json j1;
nlohmann::json j2;

j1["MAP"]["value1"] = 1;
j2["MAP"]["value2"] = 2;
//j2["MAP"] += j1["MAP"];

j2.merge_patch(j1);

std::cout << "j2: " << j2.dump().c_str() << std::endl;

and will produce a result like this:

j2:  {"MAP":{"value1":1,"value2":2}}

NOTE: even though the result is the same using update or merge_patch there are some differences in the behavior of those methods: I quote the sir nlohmann, who developed the lib

JSON Merge Patch (https://www.rfc-editor.org/rfc/rfc7396) is an official standard, whereas update is just a function that combines JSON objects.
JSON Merge Patch allows to delete, whereas update would change existing values to null.
Both only work on objects, whereas JSON Patch (https://github.com/nlohmann/json#json-pointer-and-json-patch) works on any JSON type.

so in the end is up to you to choose the method that better fits your applications requirements

nlohmann and copying maps

2 Answers