how std::string and a Json::Value string convert, each other?

I read a json string，and convert to a Json::Value:
void xxx_json_param(const char* _jsondata, const char* _savepath)
{
Json::Value root;
Json::Reader reader;
reader.parse((const std::string)_jsondata, root);

    Json::Value applicationjson_j = root["Application"];
    //applicationjson_j  is also a json，but when i asString(), it broke 
    string applicationjson = root["Application"].asString();

}

Start from the provided xxx_json_param function and inspect how root["Application"] is represented before calling asString(). Read the Json::Value conversion behavior and existing parsing tests or documentation; done means establishing whether this conversion is supported and documenting or reproducing the expected result.