使用 Rapidjson 检索 JSON 字符串中的嵌套对象

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;
    }
}

Answer 1

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.

Answer 2

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;
}

Answer 3

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.

Answer 4

You can use a pointer to get the subobject:

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

Answer 5

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++..

Answer 6

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;
}

Answer 7

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();

Answer 8

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!