Parsing JSON Responses Of Unknown Format For Single Value - c++

Question

I am creating a stock screener that sources data from ultiple apis. I would like to parse api responses from various apis for a single value.
I am using stock data apis that return JSON in different nested formats. Since the format of the JSON can be different, e.g. root could be an object or an array.. I am having trouble understanding how to go about this. I have successfully parsed JSON responses when the format is known. I am using qt, with no 3rd party libs, which seems to require that you parse these responses explcitly as I have done previously. How do I create a generic JSON parser that? Is this even possible?
Example: For this JSON response.. I would like to parse for "value"
{
"historical_data": [
{
"date": "2019-06-28",
"value": 197.92
}
],
"security": {
"id": "sec_agjrgj",
"company_id": "com_NX6GzO",
"stock_exchange_id": "sxg_ozMr9y",
"name": "Apple Inc",
"code": "EQS",
"currency": "USD",
"ticker": "AAPL",
"composite_ticker": "AAPL:US",
"figi": "BBG000B9Y5X2",
"composite_figi": "BBG000B9XRY4",
"share_class_figi": "BBG001S5N8V8"
},
"next_page": null
}
I also want to parse this JSON repsone for "value"
{
"date": "2019-06-28",
"value": 197.92
}
I am trying to not write a parser function for each api I use. I would like to be able to find if the JSON has a "value" and if so.. return its value.

Answer 1

You can use JsonPath for this. For example, using the jsoncons implementation of JsonPath, you can write
#include <iostream>
#include <jsoncons/json.hpp>
#include <jsoncons_ext/jsonpath/json_query.hpp>
using namespace jsoncons;
int main()
{
std::string data = /* your JSON response */;
json root = json::parse(data);
// json_query returns an array of items that match the JsonPath expression
json result = jsonpath::json_query(root, "$..value");
for (const auto& item : result.array_range())
{
std::cout << item << "\n";
}
}
Output:
197.92