How to Merge two Json files into one using C++?

I have 2 Json files having 100,000 records and 5 columns each. My goal is to merge both files to get a single file with 200k records using C++.

My json files look like this:

[ 
    { 
    "city":"Lisbon",
    "name":"Shannon Ward",
    "job":"Ambulatory Nurse",
    "email":"[email protected]",
    "salary":"2308593.84555454"
    },
    { 
    "city":"Lancaster",
    "name":"Kurt Hopkinson",
    "job":"Lecturer",
    "email":"[email protected]",
    "salary":"8564088.47550575"
    },
    { 
    "city":"Glendale",
    "name":"John Yates",
    "job":"Accountant",
    "email":"[email protected]",
    "salary":"9003114.81075051"
    },
    { 
    "city":"Sacramento",
    "name":"Camila Wallace",
    "job":"Budget Analyst",
    "email":"[email protected]",
    "salary":"1287392.46738351"
    }
]

which is basically an array holding structure (or map).

I am using JsonCpp library .

What I have tried:

Json::Value records1; 
Json::Value records2;

Json::Reader reader;  

reader.parse(file1, records1);    //file1 is input stream from 1st json file
reader.parse(file2, records2);    //file2 is input stream from 2nd json file

for (auto& itr : records2)
    records1.append(itr);

So, basically I am parsing json data from both files into Json::Value objects, and then later trying to append 1 object with elements of other object 1 by 1.

But, It's not working and compiler is giving this error:

C++ no instance of overloaded function matches the argument list and object (the object has type qualifiers that prevent a match)
argument types are: (const Json::Value)
object type is: const Json::Value

Please let me know what I am doing wrong or what's the right approach to append 2 json files in C++

Solution

Unless you're really set on using jsoncpp, I'd consider using ThorSerializer (installable via brew) library instead. For a quick demo:

#include <sstream>
#include <iostream>
#include <vector>
#include <map>
#include <string>

#include "ThorSerialize/JsonThor.h"

using namespace std::string_literals;

std::stringstream a( R"(
[
    {
        "city":"Lisbon",
        "name":"Shannon Ward",
        "job":"Ambulatory Nurse",
        "email":"[email protected]",
        "salary":"2308593.84555454"
    },
    {
        "city":"Lancaster",
        "name":"Kurt Hopkinson",
        "job":"Lecturer",
        "email":"[email protected]",
        "salary":"8564088.47550575"
    }
]
)"s);

std::stringstream b( R"(
[
    {
        "city":"Glendale",
        "name":"John Yates",
        "job":"Accountant",
        "email":"[email protected]",
        "salary":"9003114.81075051"
    },
    {
        "city":"Sacramento",
        "name":"Camila Wallace",
        "job":"Budget Analyst",
        "email":"[email protected]",
        "salary":"1287392.46738351"
    }
])"s);

int main() {

    using ThorsAnvil::Serialize::jsonImporter;
    using ThorsAnvil::Serialize::jsonExporter;

    std::vector<std::map<std::string, std::string>> data;

    a >> jsonImporter(data);
    b >> jsonImporter(data);

    std::cout << jsonExporter(data) << "\n";
}

The advantage of ThorsSerializer over other libraries is that it parses data directly into the C++ types that you are using, rather than into a generic JSON like structure that must then be iterated over to convert into your C++ types.

All the standard types and containers are handled by default. There is no code declaration style used to add new user defined types.

Example parsing JSON into C++ class.