Problems with JSON for Modern C++ Multidimentional Array

Question

So I have a JSON string I get using cURL that I'm trying to parse for data using JSON for Modern C++ (nlohmann::json). Here is my code:

double retValue(string data) {
    string str;
    double value = 0;
    try {
        auto jsonData = json::parse(data.c_str());
        str = jsonData["layer"][1]["Page"]["Number"];
        value = stoi(str);
    }
    catch(json::parse_error& e) {
        cout << "Error: " << e.what() << endl;
        return 0;
    }

    return value;
}

So In PHP json_decode works fine to decode into an array and the values can be easily parsed in this way, but I am having trouble with C++ and this library. I get the following error at run time but compiles fine:

terminate called after throwing an instance of 'nlohmann::detail::type_error' what(): [json.exception.type_error.305] cannot use operator[] with object Aborted (core dumped)

The JSON data is similar to this I'm trying to parse and I figure being multidimensional is the problem and that I'm not handling the data properly.

{
    "layer": {
        "1": {
            "Page": {
                "Number": 3.14 
                }
            } 
        }
}

Can anybody point me in the right direction?

shawn · Accepted Answer · 2018-09-25 08:37:25Z

2

C++ is a strong-typed language, you must use correct data type:

str = jsonData["layer"]["1"]["Page"]["Number"];

But in PHP, you access data[1], which is the same as data["1"]

answered Sep 25, 2018 at 8:37

shawn

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6 Comments

Jarod42 Over a year ago

data[1] would be possible with array: {"layer": [{}, {"Page": {"Number":314}}]}.

BudgetBaller Over a year ago

When I add quotes around "1" I get another error: terminate called after throwing an instance of 'nlohmann::detail::type_error' what(): [json.exception.type_error.302] type must be string, but is number Aborted (core dumped)

alter_igel Over a year ago

@BudgetBaller you mention the JSON structure is similar to what you've provided. You should dig deeper to find out if layer is really a JSON array or number-keyed JSON object like you claim.

Shawn Over a year ago

@BudgetBaller That one might be caused by trying to assign a JSON number (3.14) to a C++ string instead of a double.

BudgetBaller Over a year ago

@alterigel "layer" is a string like that, "1" is the number one like that but with quotes when returned. The only time it doesnt have quotes around the number is the equivelent of "number" : 3.14. Where "1", "Page" and "Number" are in layer of my actual JSON there are multiple branches.

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