How to extract number from json? - c++

Question

I want to extract a specific array from a json file:
{
"status": "OK",
"type": "startVehicle",
"info": {
"idTransit": 36612,
"timestampUTCTransit": {
"epochUTCTransitMS": 1606935562810,
"dateUTCTransit": "2020/12/2",
"hourUTCTransit": "18:59:22.810"
},
"timestampUTCReported": {
"epochUTCReportedMS": 1606935562810,
"dateUTCReported": "2020/12/2",
"hourUTCReported": "18:59:22.810"
},
"road": 0,
"lane": 0,
"xCoordinates": [
143.6,
-456.335
]
}
}
I want extract the values of xCoordinates. This is what I tried:
#include <rapidjson/document.h>
void Vdac::addStart(std::string json_str)
{
Document rjsondoc;
rjsondoc.Parse(json_str.c_str());
if(rjsondoc.HasParseError())
slog.getLogger()->debug("Invalid json");
else
{
auto coordinates = rjsondoc["info"]["xCoordinates"].GetArray();
for(SizeType i=0;i<coordinates.Size();i++)
slog.getLogger()->debug("The start coordinates are: {0:d}",coordinates[i].GetInt());
return;
}
}
But I have an error when the program accesses the value. The error is:
../include/rapidjson/document.h:1737: int rapidjson::GenericValue<Encoding, Allocator>::GetInt() const [with Encoding = rapidjson::UTF8<>; Allocator = rapidjson::MemoryPoolAllocator<>]: Assertion `data_.f.flags & kIntFlag' failed.
How can I extract the value?

Answer 1

Can't give you any help with rapidjson.
But an alternative is ThorsSerializer (note: I wrote it. So take that as you will). But it is designed to be easy to use. With zero (or very little) code that need to be written to parse the JSON into C++ objects.
Note: If you don't want to read a field just remove it.
#include <fstream>
#include "ThorSerialize/Traits.h"
#include "ThorSerialize/JsonThor.h"
struct TimeStamp
{
long epochUTCTransitMS;
std::string dateUTCTransit;
std::string hourUTCTransit;
};
// If you want to read a class then
// Add this declaration with the name of the class and the
// fields you want to read/write in JSON.
ThorsAnvil_MakeTrait(TimeStamp, epochUTCTransitMS, dateUTCTransit, hourUTCTransit);
struct Info
{
long idTransit;
TimeStamp timestampUTCTransit;
TimeStamp timestampUTCReported;
int road;
int lane;
std::vector<double> xCoordinates;
};
ThorsAnvil_MakeTrait(Info, idTransit, timestampUTCTransit, timestampUTCReported, road, lane, xCoordinates);
struct Object
{
std::string status;
std::string type;
Info info;
};
ThorsAnvil_MakeTrait(Object, status, type, info);
int main()
{
std::ifstream data("pl1.data");
Object object;
using ThorsAnvil::Serialize::jsonImporter;
// jsonImporter to read or jsonExporter to write:
data >> jsonImporter(object);
std::cout << object.info.xCoordinates[0] << "\n";
}
Easy to install via brew:
> brew install thors-serializer
Build your code with:
> g++ -std=c++17 <file>.cpp -lThorSerialize17 -lThorsLogging17
Note on new M1 machs you will also need -L /opt/homebrew/lib (still working on that).
All code on github