CODEWITHSUNDEEP
c++jsonjansson
zero-one
zero-one

Reputation: 196

Unable to distinguish between JSON Object/Array/String

I am trying to use the Jansson library to parse a JSON string. I am unable to parse it correctly. Here is my code in C++

std::string JSONString = "{\"Hostages\": [{\"Rescue\": \"help me!\",\"confidence\": 0.01}]}";

json_t *JsonTable, *Rescue, *Hostages;
json_error_t JsonError;

if (JSONString.c_str()) {
        JsonTable = json_loads(JSONString.c_str(), 0, &JsonError);
        if (!JsonTable) {
            printf("JSON parsing error: on line %d: %s\n", JsonError.line,
                    JsonError.text);
        }
        if (!json_is_object(JsonTable)) {
            printf("JSON Pased root is not an array : Invalid response received\n");
            json_decref(JsonTable);
        }

        Hostages = json_object_get(JsonTable, "Hostages");
        if (!json_is_array(Hostages)) {
            printf("error: Hostages is not array\n");
            json_decref(JsonTable);
            return 1;
        } else {
            Hostages = json_array_get(Hostages, json_array_size(Hostages));
            Rescue = json_object_get(Hostages,"Rescue");
            if (!json_is_string(Rescue)) {
                printf("error: Rescue is not string\n");
                json_decref(JsonTable);
                return 1;
            } else {

            }
        }
}

I dont understand whether Rescue is a string, object or an array. I tried all three options as if (!json_is_string(Rescue)), if (!json_is_array(Rescue)) & if (!json_is_object(Rescue)) but it always prints "error: Rescue is not string".

Any help please ?

Upvotes: 0

Views: 437

Answers (2)

Miles Budnek
Miles Budnek

Reputation: 30494

Hostages is an array of (one) objects, each of which contain a string and a real number.

You need to get the object out of the array before you try to get the string out of the object:

// C++17 raw string literal, just to nicely format the json string
std::string JSONString = R"EOF(
    {
        "Hostages": [
            {
                "Rescue": "help me!",
                "confidence": 0.01
            }
        ]
    }
)EOF";

json_t* JsonTable = json_loads(JSONString.c_str(), 0, &JsonError);
assert(json_is_object(JsonTable));

json_t* Hostages = json_object_get(JsonTable, "Hostages");
assert(json_is_array(Hostages));

for (int i = 0; i < json_array_size(Hostages); ++i) {
    json_t* Hostage = json_array_get(Hostages, i);
    assert(json_is_object(Hostage));

    json_t* Rescue = json_object_get(Hostage, "Rescue");
    assert(json_is_string(Rescue));

    // ...
}

You'll need to replace all of those asserts with your error-handling code. I've added them purely to show which conditions should hold true given your example.

Upvotes: 1

RC0993
RC0993

Reputation: 958

You are trying to access the incorrect element of the array.

Take for example an array A = [{},{},{}]; of 3 elements, the size of this array is 3 so you can access the places as 0, 1, 2 only.

in your post you are accessing A[sizeof(A)] as Hostages = json_array_get(Hostages, json_array_size(Hostages));

You can run a loop over it to access all the elements of your array, in your case just one. so you may access it as Hostages = json_array_get(Hostages, 0);

Upvotes: 1

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