Parse an array of objects in C++ using rapidjson

Question

I'm trying to parse the following JSON file in c++. I would like to iterate over the 'attributes' array and obtain the value of the string:'value' for a specific value of a string:'name' of that attribute object. For ex: I would like to parse this JSON file and obtain the 'value' for 'mass'.

{
"active": true,
"apiTier": 0,
"attributes": [
    {
        "description": "Given in the datasheet as '0.33 kg (0.73 lbm)'.",
        "maximumValue": "",
        "measurementUnit": "kg",
        "minimumValue": "",
        "name": "mass",
        "productConfiguration": "base",
        "value": "0.33"
    },    
    {
        "description": "",
         "maximumValue": "",
        "measurementUnit": "",
        "minimumValue": "",
        "name": "propellant-type",
        "productConfiguration": "base",
        "value": "hydrazine"
    },
    {
        "description": "Given in the datasheet as 'Thrust/Steady State' and the specified value is also reported as '(0.05-0.230) lbf'.",
        "maximumValue": "1.02",
        "measurementUnit": "N",
        "minimumValue": "0.22",
        "name": "thrust",
        "productConfiguration": "base",
        "value": ""
    }
]

I am trying to do this in C++ using rapidjson library to parse the JSON file. Below is my implementation for the parsing of the JSON file. What I would like to do is within the for loop, for a specific 'value' for a string 'name' (for ex: mass) obtain the other 'values' for string such as 'maximumValue', 'minimumValue', 'value' etc.

#include <fstream>
#include <sstream>
#include <string>
#include <rapidjson/document.h>

int main()
{
   std::ifstream inputFile( "/path/to/json/file/" );
   std::stringstream jsonDocumentBuffer;
   std::string inputLine;

   while ( std::getline( inputFile, inputLine ) )
   {
      jsonDocumentBuffer << inputLine << "\n";
   }
   rapidjson::Document config;
   config.Parse( jsonDocumentBuffer.str( ).c_str( ) );



   assert(config.IsObject()); 

   const rapidjson::Value& attributes   = config["attributes"];
   assert(attributes.IsArray()); 

   int counter = 0; 

   for (rapidjson::Value::ConstValueIterator itr = attributes.Begin(); itr != attributes.End(); ++itr) 
   {
      const rapidjson::Value& attribute = *itr;
      assert(attribute.IsObject()); // each attribute is an object
      for (rapidjson::Value::ConstMemberIterator itr2 = attribute.MemberBegin(); itr2 != attribute.MemberEnd(); ++itr2) 
      {
         std::cout << itr2->name.GetString() << " : " << itr2->value.GetString() << std::endl;
      }
}

Edit 1: I found a solution on how to iterate over the attributes array and access each string and its value for each one of the objects in the attributes array. However, what I would like to do is obtain value of any string (Ex: 'maximumValue','minimumValue', 'Value') etc for a specific value (Ex: mass) of a string 'name'.

Roman Podymov · Accepted Answer · 2018-05-08T13:42:42.847

If you want to get only specified properties of a selected attribute you can use the following function:

#include <iostream>
#include <vector>
#include <map>
#include "rapidjson/document.h"

std::map<std::string, std::string> mapForAttributeThatMatchesName(const rapidjson::Value& attributes, const std::string& findMemberName, const std::string& findMemberValue, const std::vector<std::string>& keysToRetrieve) {

    std::map<std::string, std::string> result;
    for (rapidjson::Value::ConstValueIterator itr = attributes.Begin(); itr != attributes.End(); ++itr) {

        const rapidjson::Value::ConstMemberIterator currentAttribute = itr->FindMember(findMemberName.c_str());
        if (currentAttribute != itr->MemberEnd() && currentAttribute->value.IsString()) {

            if (currentAttribute->value == findMemberValue.c_str()) {

                for (auto &keyToRetrieve : keysToRetrieve) {

                    const rapidjson::Value::ConstMemberIterator currentAttributeToReturn = itr->FindMember(keyToRetrieve.c_str());
                    if (currentAttributeToReturn != itr->MemberEnd() && currentAttributeToReturn->value.IsString()) {

                        result[keyToRetrieve] = currentAttributeToReturn->value.GetString();
                    }
                }
                return result;
            }
        }
    }
    return result;
}

And you can test it like that:

const rapidjson::Value& attributes = config["attributes"];
assert(attributes.IsArray());

std::vector<std::string> keysToRetrieve = {"maximumValue", "minimumValue"};
std::map<std::string, std::string> mapForResult = mapForAttributeThatMatchesName(attributes, "name", "mass", keysToRetrieve);
for (auto &mapItem : mapForResult) {

    std::cout << mapItem.first << ":" << mapItem.second << "\n";
}

Parse an array of objects in C++ using rapidjson

1 Answers1