Ftyjtk

I am using nlohmann::json for parsing json in the program.

given a json there is an array with several objects, according to one of the object members I want to get other members of the same object.

like in the json below

{

 "arr":[

    {"a":1, "b":11, "c":111, ...},

    {"a":2, "b":22, "c":222, ...},

    {"a":3, "b":33, "c":333, ...},

    ...

   ]

}

for instance if the value of a is 2, I want to get the values of b,c,... of the same index/object.

currently I am using a for loop and at the index that j["arr"][i]["a"].get<int> == 2 going for the rest of members. As the array might have hundreds of members it is nonsense.

what is the best approach in this case?

It's a JSON array, you need to iterate over it. So your approach is the simple and direct one.

x2struct(https://github.com/xyz347/x2struct) can load json to struct with condition.

#include "x2struct.hpp"

#include <iostream>



using namespace std;



struct Item {

    int a;

    int b;

    int c;

    XTOSTRUCT(O(a,b,c));

    XTOSTRUCT_CONDITION() { // load only this return true

        int a = -1;

        if (obj.has("a")) {

            obj["a"].convert(a);

        }

        return a==2;

    }

};



struct Test {

    Item arr;

    XTOSTRUCT(O(arr));

};





int main(int argc, char *argv) {

    string jstr = "{"arr":[{"a":1, "b":11, "c":111},{"a":2, "b":22, "c":222},{"a":3, "b":33, "c":333}]}";

    Test t;

    x2struct::X::loadjson(jstr, t, false);

    cout<<t.arr.b<<','<<t.arr.c<<endl;

}

output is:

22,222

Let's call the C++ type of the elements of arr Thing, you can convert arr to a std::vector<Thing>.

void to_json(nlohmann::json & j, const Thing & t) {

    j = nlohmann::json{{"a", t.a}, {"b", t.b}, {"c", t.c}}; // similarly other members ...

}



void from_json(const nlohmann::json & j, Thing & t) {

    j.at("a").get_to(t.a);

    j.at("b").get_to(t.b);

    j.at("c").get_to(t.c); // similarly other members ...

}



std::vector<Thing> things = j["arr"];

auto it = std::find_if(things.begin(), things.end(), (const Thing & t){ return t.a ==2; });

// use it->b etc