pparescasellas pparescasellas - 5 months ago 78
JSON Question

Retrieving a nested object inside a JSON string using rapidjson

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

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.