Json serialised differently depending on whether it was created via ctor or assignment

[ggegoge]

Hi

I have faced a very confusing behaviour from the json library, where depending on the way of instantiating a nlohmann::json member, it gets serialised differently

I have a json member json and depending on how I initialise it, it gets serialised differently. Using json{data} in the constructor initialisation list vs using json = data yields to different content being output from json.dump. data is of type std::unordered_map<std::string, MyType> with MyType having defined to_json conversion functions.

I'd expect json to output a standard object with key value pairs in both scenarios that correspond to the map's structure

instead, when I don't use the assignment operator, what gets created is an array with one item, the item being this aforementioned object

​struct MyClass
{
    std::unordered_map<std::string, MyType> data;
    nlohmann::json json;

    MyClass() : data{createData()}, json{data}
    {
        std::cout << json.dump(2);
        json = data;
        std::cout << json.dump(2)
    }
};

The output for for the first cout is

[
  {
    "key": { // value
      }
  }
]

and for the 2nd cout it is

{
  "key": { // value
    }
}

which is what I expected.

When I went through the code with a debugger it seemed like in the first case this constructor would be triggered first before going to basic_json

template <
        class... Args,
        enable_if_t<std::is_constructible<value_type, Args...>::value, int> = 0 >
    json_ref(Args && ... args)
        : owned_value(std::forward<Args>(args)...)
    {}

whereas with the assignment operator, it would go straight to basic_json constructor

Any ideas on what this is about, is this a bug?

[nlohmann]

First entry of the FAQ: https://json.nlohmann.me/home/faq/

[ggegoge]

indeed. didn't realise there is the FAQ once couldn't find it in world wide web and among GH issues, sorry for trouble and thanks for the answer!

[ggegoge]

I guess I didn't expect braces to be the culprit which derailed my search