Retrieving a nested object inside a JSON string using rapidjson

Question

I need to retrieve a nested object inside a JSON string and I'm trying to do it using rapidjson. All I've found is how to retrieve arrays and basic types, but not sub-objects. I have created the following toy example which gives an error:

rapidjson::Document document;
std::string test =  " { \"a\": { \"z\" : 21 } } ";
std::cout << test << std::endl;
if ( document.Parse<0>( test.c_str() ).HasParseError() ) {
    std::cout << "Parsing error" << std::endl;
} else {
    if ( document[ "a" ].IsObject() ) {
        std::cout << "OK" << std::endl;
        std::cout << document[ "a" ].GetString() << std::endl;
    }
}

This is the output when executed:

{ "a": { "z" : 21 } } 
OK
JSONTest: ../rapidjson/document.h:441: const typename Encoding::Ch* rapidjson::GenericValue<Encoding, Allocator>::GetString() const [with Encoding = rapidjson::UTF8<char>, Allocator = rapidjson::MemoryPoolAllocator<rapidjson::CrtAllocator>]: Assertion `IsString()' failed. Aborted

How do I retrieve the inner object to continue my parsing? Thanks.

Edit: What I need is to obtain the string representation of the inner object so I can call another function that is going to parse it.

Edit 2: code that allows to retrieve the inner object as a string:

rapidjson::Document document;
std::string test =  "{\"a\":{\"z\":21}} ";
if ( document.Parse<0>( test.c_str() ).HasParseError() ) {
    std::cout << "Error parsing" << std::endl;
} else {
    if ( document[ "a" ].IsObject() ) {
        rapidjson::StringBuffer sb;
        rapidjson::Writer<rapidjson::StringBuffer> writer( sb );
        document[ "a" ].Accept( writer );
        std::cout << sb.GetString() << std::endl;
    }
}

Does it mean that RapidJson does not support hierarchical objects? so it only parse the root level?!?! — Paolo Vigori, Mar 01 '17 at 17:19
Yes edit2 is very useful for recursion, can convert the inner json object to a string, which can be recursively parsed. — Paulus, Feb 26 '20 at 13:05

score 10 · Accepted Answer · answered Oct 05 '12 at 09:39

You need to iterate through object's members manually, as GetString() only works on string members, while document["a"] is an Object. You need to iterate through that object's members using MemberIterator variable. I had no practice in C* for more than 15 years, so I can only give a general idea of how it should work:

for (MemberIterator m = document["a"].MemberBegin(); m != document["a"].MemberEnd(); ++m) {
    std::cout << m.name << " " << (m.IsNumber()?m.GetNumber():m.GetString()) << endl;
}

Also, you might want to look at Accept() method, it seems to return a JSON string of an object you give it.

Yeah, "Accept()" was the answer. It allows to put the inner object into a writer which fills a stringbuffer. Then, it is possible to get the string from the buffer. I have edited the original question with a working code. Thanks a lot! — pparescasellas, Oct 05 '12 at 10:34

score 4 · Answer 2 · answered Jul 30 '14 at 14:53

4

If element is an object you can just access subproperties with []:

for (SizeType i = 0; i < layers.Size(); i++){   
  cout << layers[i]["name"].GetString() << endl;
}

answered Jul 30 '14 at 14:53

lasote

591
2
12

score 2 · Answer 3 · answered Jul 22 '18 at 12:09

There is another great approach realized in rapidjson - JSON Pointers. They have experimental status and according to the documentation shall be included in v.1.1. Anyway this approach looks like XPATH for XML so to get nested value we can use syntax like

Value* tmpValue = GetValueByPointer(doc, "/result/job/blob");

I tried this functionality and in my opinion this is better than iterators.

score 0 · Answer 4 · edited Aug 15 '18 at 00:26

0

You can use a pointer to get the subobject:

Value& a = *GetValueByPointer(document, "/a");
int z = a["z"].GetInt();

edited Aug 15 '18 at 00:26

Community

1
1

answered Aug 14 '18 at 12:18

Michael

1

Welcome to Stack Overflow! Please don't answer just with source code. Try to provide a nice description about how your solution works. See: [How do I write a good answer?](https://stackoverflow.com/help/how-to-answer). Thanks – sɐunıɔןɐqɐp Aug 14 '18 at 12:36

Kvae Kvae22 · Answer 5 · 2021-01-29T00:39:15.093

0

GenericObject doc2 = document["a"].GetObjectW();
int z = doc2["z"].GetInt();

an easy way to parse out nested objects inside json with rapidJson lib for c++..

edited Jan 29 '21 at 00:39

answered Jan 28 '21 at 00:58

Kvae Kvae22

111
1
5

score -1 · Answer 6 · answered Oct 05 '15 at 16:41

Here is one example code to get the nested object as rapidjson::Document object.

Document get_nested(Document &d, std::string key){
rapidjson::StringBuffer buffer;
const char *key_ctr = key.c_str();

assert(d[key_ctr].IsObject());

rapidjson::Writer<rapidjson::StringBuffer> writer(buffer);
d[key_ctr].Accept(writer);

rapidjson::Document result;
rapidjson::StringStream s(buffer.GetString());
result.ParseStream(s);

return result;
}

Shubham Trigunait · Answer 7 · 2016-10-07T09:10:09.170

-2

you can also use the pointer of Document:

Document *document= new Document();
document->parse( test.c_str());

and putting in the Value pointer and use it

Value *val= document;
val = &(*val)["a"];
val = &(*val)["z"];
cout << val->GetString();

edited Oct 07 '16 at 09:10

answered Oct 07 '16 at 09:00

Shubham Trigunait

19
2

score -3 · Answer 8 · answered Mar 30 '17 at 14:04

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!

are you just pasting your same answer on all rapidjson questions? downvote — Gillis Haasnoot, Apr 01 '17 at 19:56
I looked on the internet forever for this and couldn't find it. Just trying to help out. Calm down — a.raya203, Apr 05 '17 at 14:14

Retrieving a nested object inside a JSON string using rapidjson

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