Object deserialized as array

[martin-vitek]

What is the issue you have?

When initializing basic_json from std::string (or using basic_json::parse()), then main JSON object is parsed as array, not as object. Deserialization using std::stringstream with >> operator or with _json works as expected.

Please describe the steps to reproduce the issue.

1. Create std::string with specified JSON string
2. Create basic_json object and initialize it with this string
3. Dump json object, or try to access value via name

Can you provide a small but working code example?

std::string json_string = R"(
{
    "serviceVersion":"1.0",
    "resultCode":
    {
        "code":0,
        "text":"OK"
    },
    "challenge":"sdfg45sdg45",
    "authenticationType":0,
    "challengeId":"dsv1d51df5-dasfv551v1-asdv155551"
})";

auto json_object {nlohmann::json::parse(json_string)};
std::cout << json_object.dump() << '\n';
std::cout << json_object["challenge"] << '\n';

What is the expected behavior?

{"authenticationType":0,"challenge":"sdfg45sdg45","challengeId":"dsv1d51df5-dasfv551v1-asdv155551","resultCode":{"code":0,"text":"OK"},"serviceVersion":"1.0"}
"sdfg45sdg45"

And what is the actual behavior instead?

[{"authenticationType":0,"challenge":"sdfg45sdg45","challengeId":"dsv1d51df5-dasfv551v1-asdv155551","resultCode":{"code":0,"text":"OK"},"serviceVersion":"1.0"}]
terminate called after throwing an instance of 'nlohmann::detail::type_error'
  what():  [json.exception.type_error.305] cannot use operator[] with a string argument with array

Which compiler and operating system are you using?

• Compiler: GCC 10.1.1
• Operating system: Fedora 32
• Language version: C++20

Which version of the library did you use?

• latest release version 3.8.0
• other release - please state the version: ___
• the develop branch

[nlohmann]

The problem are the braces. Replace

auto json_object {nlohmann::json::parse(json_string)};

with

auto json_object = nlohmann::json::parse(json_string);

Explanation: Braces are interpreted as array or object, depending on the context.

json array = {1, 2, 3}; // [1,2,3]
json object = { {"key", "value"}, {"foo", "bar"} }; // {"key":"value","foo":"bar"}

[martin-vitek]

Ok, thanks for the explanation and proposed fix, that solved my problem.

[wtom76]

could you please mention it in "Read JSON from a file" section of readme.md explicitly?

[SimonMaracine]

I wasted a couple of hours trying to figure out why I couldn't extract anything from my simple json file, then why it was treated as an array, then why everything was treated as an array until I finally found this specific issue.

I personally use aggregate initialization for everything as a rule or convention in my C++ code to make it distinct from any sort of assignment. I don't know what to say if this is a mistake in this library's API design or an issue in C++'s design. If it is the former, I would greatly appreciate if you could fix it. I may open up an issue, if you wish.

[nlohmann]

See https://json.nlohmann.me/home/faq/

[SimonMaracine]

Thank you. I'm really sorry for this. I see now that lots of people already pointed out this "issue". How much patience do you have left?... :|

The problem was that I mostly looked at the examples in the README or looked at my past code (which was using copy initialization) with this library. My mistake.
Though a quick warning about this in the README might help.

[mold-boy]

This is actually a bug. The initializer list is completely valid with C++20.

[nlohmann]

Compilers behave differently, and I have no idea how to fix this. Any help appreciated.