iterate and retrieve nested object in JSON using rapidjson

Question

I am parsing a JSON structure which is similar as follows

{
    "item1" : "value1"
    "item2" : "value2"
    // ...
    "itemn" : {
        "outernestedItem1" : {
            "innerNestedItem1" : "valuen1"
            "innerNestedItem2" : "valuen2"
        }
        // ....
        "outernestedItemn" : {
            "innerNestedItem1" : "valuen1"
            "innerNestedItem2" : "valuen2"
        }
    }
}

The number of outer nested items is not fixed, so I was iterating using iterator from rapidjson, inner-nested objects variables are fixed, so I can get access to them using [].

const rapidjson::Value& itemn = document["itemn"];
for (rapidjson::Value::ConstMemberIterator itr = itemn.MemberBegin();
itr != itemn.MemberEnd(); ++itr)
{
    rapidjson::StringBuffer sb;
    rapidjson::Writer<rapidjson::StringBuffer> writer( sb );
    itr->value.Accept(writer);

    std::cout << sb["innerNestedItem1"].GetString();
    std::cout << sb["innerNestedItem2"].GetString();
}

but [] is not allowed with sb(string buffer), any idea how can I do this?

Edit1: I did it in very inefficient way, but just sharing the solution, so it might help someone to come up with efficient solution.

const rapidjson::Value& itemn = document["itemn"];
for (rapidjson::Value::ConstMemberIterator itr = itemn.MemberBegin();
itr != itemn.MemberEnd(); ++itr)
{
    rapidjson::StringBuffer sb;
    rapidjson::Writer<rapidjson::StringBuffer> writer( sb );
    itr->value.Accept(writer);

    //changed from here onwards
    rapidjson::Document for_outer_nested_item;
    std::string temp = sb.GetString();
    char buffer2[100000];
    strcpy_s(buffer2, temp.c_str());
    for_outer_nested_item.ParseInsitu(buffer2);
    std::cout << executive_command["innerNestedItem1"].GetString() << std::endl;
    std::cout << executive_command["innerNestedItem2"].GetString() << std::endl;
}

If `document["itemn"]` retrieves an element of the whole structure, then `document["itemn"]["innerNestedItem1"]` should give you the content of the nested item within. Your approach to first format the content of `*itr` to a string and then to try to retrieve elements from that nonstructured data is wrong, it would be easier to keep the structured data. — Ulrich Eckhardt, Jun 18 '15 at 05:22
@UlrichEckhardt document["itemn"] contains all undefined number of outernesteditems and outernesteditem is fixed type of object. — Ruturaj, Jun 18 '15 at 13:08

TGafford · Accepted Answer · 2016-01-20T22:49:25.380

First, let me provide credit to MiloYip at this link

Second-- here's what I did for my project:

rapidjson::Document document;
// document holds a json document retrieved from a http GET request
// I did not include all of that in this example.  I am only showing
// the part of iterating through a nested object and retrieving members.

std::vector<std::string> symbols;
// holds the values I retrieve from the json document

if (document.Parse<0>( symbol.c_str() ).HasParseError() )
    Log() << "ERROR: encountered a JSON parsing error" << std::endl;
else {
    // Get the nested object that contains the elements I want.
    // In my case, the nested object in my json document was results
    // and the values I was after were identified as "t"
    rapidjson::Value& results = document["results"];
    assert(results.IsArray());
    for (rapidjson::SizeType i = 0; i < results.Size(); i++) {
        // Store the value of the element in a vector.
        symbols.emplace_back(results[i]["t"].GetString());
}

I think this is a pretty clean/efficient approach.

This assumes that the JSON contains arrays, but I don't see any arrays in the question. Only nested key/value pairs. — Mark Ransom, Jul 31 '23 at 21:12

score 10 · Answer 2 · answered Mar 30 '17 at 14:10

This is something I recently worked on:

void enter(const Value &obj, size_t indent = 0) { //print JSON tree

if (obj.IsObject()) { //check if object
    for (Value::ConstMemberIterator itr = obj.MemberBegin(); itr != obj.MemberEnd(); ++itr) {   //iterate through object   
        const Value& objName = obj[itr->name.GetString()]; //make object value

        for (size_t i = 0; i != indent; ++i) //indent
            cout << " ";

        cout << itr->name.GetString() << ": "; //key name

        if (itr->value.IsNumber()) //if integer
            std::cout << itr->value.GetInt() ;

        else if (itr->value.IsString()) //if string
            std::cout << itr->value.GetString();


        else if (itr->value.IsBool()) //if bool
            std::cout << itr->value.GetBool();

        else if (itr->value.IsArray()){ //if array

            for (SizeType i = 0; i < itr->value.Size(); i++) {
                if (itr->value[i].IsNumber()) //if array value integer
                    std::cout << itr->value[i].GetInt() ;

                else if (itr->value[i].IsString()) //if array value string
                    std::cout << itr->value[i].GetString() ;

                else if (itr->value[i].IsBool()) //if array value bool
                    std::cout << itr->value[i].GetBool() ;

                else if (itr->value[i].IsObject()){ //if array value object
                    cout << "\n  ";
                    const Value& m = itr->value[i]; 
                    for (auto& v : m.GetObject()) { //iterate through array object
                        if (m[v.name.GetString()].IsString()) //if array object value is string
                            cout << v.name.GetString() << ": " <<   m[v.name.GetString()].GetString();
                        else //if array object value is integer
                            cout << v.name.GetString() << ": "  <<  m[v.name.GetString()].GetInt();

                       cout <<  "\t"; //indent
                    }
                }
                cout <<  "\t"; //indent
            }
        }

        cout << endl; 
        enter(objName, indent + 1); //if couldn't find in object, enter object and repeat process recursively 
        }     
 }
 }

This can handle any type of JSON tree. All you have to do is pass a Value as such:

Value v = document.GetObject();
Value& m= v;
enter(m);

And you're done!

I tried your solution, but: `Value v = document.GetObject();` gave me ` error: ‘rapidjson::Document {aka class rapidjson::GenericDocument >}’ has no member named ‘GetObject’` — user904542, Jul 26 '18 at 16:04

score 4 · Answer 3 · answered Sep 22 '17 at 12:35

void parseRecursive(std::string scope
                    , rapidjson::Value::ConstMemberIterator object
                    , std::unordered_map<std::string, std::string>& values)
{
    if (scope.empty())
    {
        scope = object->name.GetString();
    }
    else
    {
        scope = scope + "::" + object->name.GetString();
    }
    auto inElement = scope + "::";

    if (object->value.IsObject())
    {
        for (auto it = object->value.MemberBegin(); it != object->value.MemberEnd(); ++it)
        {
            parseRecursive(scope, it, values);
        }
    }
    else if (object->value.IsDouble())
    {
        values.emplace(inElement, std::to_string(object->value.GetDouble()));
    }
    else if (object->value.IsInt())
    {
        values.emplace(inElement, std::to_string(object->value.GetInt()));
    }
    else
    {
        LOGW("Unsuported: " << inElement << object->name.GetString());
    }
}

And start with document: rapidjson::Document document;

for (auto it = document.MemberBegin(); it != document.MemberEnd(); ++it)
{
    parseRecursive("", it, _values);
}

score 3 · Answer 4 · answered Oct 19 '17 at 22:17

a.raya203's post https://stackoverflow.com/a/43120359/6155053 didn't work for me out of the box (doesn't handle all types the current rapidjson version implements, and thus ran into errors when e.g. outputting gltfs containing doubles, etc.) but it brought me on the right track for understanding how rapidjson works for parsing a document, so I thought I'd leave my (updated) code here, maybe it helps out someone else…

#include <iostream>
#include <string>

class JsonNodePrinter final
{
public: 
    static void PrintNode(const rapidjson::Value &node, size_t indent = 0, unsigned int level = 0, const std::string& nodeName = "")
    {
        std::cout << GetIndentString(indent, level);

        if (!nodeName.empty())
            std::cout << nodeName << ": ";

        if (node.IsBool())
            std::cout << node.GetBool();

        else if (node.IsInt())
            std::cout << node.GetInt();

        else if (node.IsUint())
            std::cout << node.GetUint();

        else if (node.IsInt64())
            std::cout << node.GetInt64();

        else if (node.IsUint64())
            std::cout << node.GetUint64();

        else if (node.IsDouble())
            std::cout << node.GetDouble();

        else if (node.IsString())
            std::cout << node.GetString();

        else if (node.IsArray())
        {
            if (!nodeName.empty()) std::cout << "\n" << GetIndentString(indent, level);
            PrintArray(node, indent, level);
        }

        else if (node.IsObject())
        {
            if (!nodeName.empty()) std::cout << "\n" << GetIndentString(indent, level);
            PrintObject(node, indent, level);
        }

        std::cout << "\n";
    }

    static void PrintObject(const rapidjson::Value &node, size_t indent = 0, unsigned int level = 0)
    {
        std::cout << "{\n";

        for (rapidjson::Value::ConstMemberIterator childNode = node.MemberBegin(); childNode != node.MemberEnd(); ++childNode)
        {
            PrintNode(childNode->value, indent, level + 1, childNode->name.GetString());
        }

        std::cout << GetIndentString(indent, level) << "}";
    }

    static void PrintArray(const rapidjson::Value& node, size_t indent = 0, unsigned int level = 0)
    {
        std::cout << "[\n";

        for (rapidjson::SizeType i = 0; i < node.Size(); ++i)
        {
            PrintNode(node[i], indent, level + 1);
        }

        std::cout << GetIndentString(indent, level) << "]";
    }

    static std::string GetIndentString(size_t indent = 0, unsigned int level = 0)
    {
        return std::move(std::string(level * indent, ' '));
    }
};

Use it like

#include "3rdParty/rapidjson/document.h"

rapidjson::Document document;
{
    document.Parse(FileHelper::ReadString(filePath)->c_str());
}

if (!document.HasParseError())
{
    JsonNodePrinter::PrintNode(document, 4);
}

Thank you, I very appreciate your work. It is very useful, and elegant. — Barbara RB, Nov 21 '19 at 15:37

score 0 · Answer 5 · answered Apr 08 '18 at 14:01

I was wondering the same very recently, here is what i got :

#include "rapidjson\filereadstream.h"
#include "rapidjson\document.h"
#include "rapidjson\istreamwrapper.h"
#include <fstream>
#include <iostream>

using namespace rapidjson;

// Documentation : using file stream instead of C FILE pointers
// http://rapidjson.org/md_doc_stream.html#FileStreams
ifstream file_stream(filepath);
IStreamWrapper isw(file_stream);

Document doc;
doc.ParseStream(isw);
file_stream.close();

if(doc.HasMember(CONF_NODE)){
Value *config_node = &(doc[CONF_NODE]);
// Now I can use it like so:
std::cout << (*config_node)["My Other Json node"].GetString() << std::endl;
}

I used this trick several times to avoid using unterminably long accessing request like

 doc["Node1"]["Node2"]["Node3"]...["NodeX"].GetType()

and instead rely on pointers that could be used to virtually "split" the doc chain:

doc["Node1"]["Node2"]["Node3"]...["NodeX"].GetType()
                     |            |
                 pointer1         |
                                  pointer2
(*pointer_on_Node_N)["Node N+1"] = doc["Node1"][...]["NodeN"]["Node N+1]

That was particularly handy when I needed to iterate over my file (when facing arrays).

iterate and retrieve nested object in JSON using rapidjson

5 Answers5

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