How to extract specific data returned from web::json::value::serialize() with cpprestsdk/casablanca

问题

I have this code:

wcout << jvalue.serialize() << endl;

which prints out the whole body of json data:

{
    "data": {
        "timestamp": [
            {
                "_id": "5ad93fc48084e1089cd9667b",
                "staff_id": 172,
                "staff_name": "Michael Edano",
                "time_in": "2018-04-20T01:17:56.787Z",
                "time_out": null,
                "status": ['Leave','Vacation','Absent'],
                "createdAt": "2018-04-20T01:17:56.790Z",
                "updatedAt": "2018-04-20T01:17:56.790Z",
                "__v": 0
            }
        ],
        "success": true
    }
}

Can someone give me an example on how to get let say the _id field and single data of status[] field (which is array)

from data return by web::json::value::serialize() ?

Thank you.

回答1:

You don't have to call serialize to access the json values. Once you have a json::value, which holds a json object, you can traverse it to obtain inner objects and arrays as json::value's:

json::value jvalue; //this is your initial value

// this new value will hold the whole 'data' object:
json::value data = jvalue.at(U("data")); 

// read `timestamp` array from `data` and directly read 
// the item at index 0 from it:
json::value timestamp = data.at(U("timestamp")).at(0);

// from `timestamp` (first item of timestmap array`)
json::value id = timestamp.at(U("_id"));

// print `id` as string
std::wcout << id.as_string() << std::endl;

// read `status` array first item and print it as string
json::value status = timestamp.at(U("status"));
std::wcout << status.at(0).as_string() << std::endl;

As you can guess from the code above, calls to at can be chained together:

// read the `_id` value at once and print it as string
std::wcout << jvalue.at(U("data")).at(U("timestamp")).at(0).at(U("_id")).as_string() << std::endl;

Everything is well explained here.

来源：https://stackoverflow.com/questions/49933541/how-to-extract-specific-data-returned-from-webjsonvalueserialize-with-cp

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