Error with libcurl + rapidjson for JSON parsing ( null dom ) - c++

Question

I can't parsing the string (Through the curl‘s post) to json by using rapidjson's API
NULL DOM
String from curl's post
As the Picture I have already get the json,but I can't parsing it to analyze the information,because the Dom had null data.As a beginner,I really need help,3Q~.
int main(int argc, const char * argv[]) {
// insert code here...
rapidjson::Document doc;
doc.SetObject(); //key-value 相当与map
//doc.Setvalue(); //数组型 相当与vector
rapidjson::Document::AllocatorType &allocator = doc.GetAllocator(); //获取分配器
doc.AddMember("answerId","1804201043079617296361988",allocator);
doc.AddMember("queryItems", true, allocator);
rapidjson::StringBuffer buffer;
rapidjson::PrettyWriter<rapidjson::StringBuffer> writer(buffer); //PrettyWriter是格式化的json，如果是Writer则是换行空格压缩后的json
//rapidjson::Writer<rapidjson::StringBuffer> writer(buffer);
//doc.Accept(writer);
//std::cout<<buffer.GetString()<<std::endl;
CURL *curl;
CURLcode res;
std::stringstream out;
//HTTP报文头
struct curl_slist* headers = NULL;
const char *url = "https://www.qingsuyun.com/h5/actions/exam/execute/find-exam.json";
curl = curl_easy_init();
if(curl)
{
//设置url
curl_easy_setopt(curl, CURLOPT_URL, url);
curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);
curl_easy_setopt(curl, CURLOPT_CUSTOMREQUEST, "POST");//自定义请求方式
// 设置要POST的params
curl_easy_setopt(curl, CURLOPT_POSTFIELDS,"answerId=1804201043079617296361988&queryItems=true");
// 设置接收数据的处理函数和存放变量
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data);//设置回调函数
curl_easy_setopt(curl, CURLOPT_WRITEDATA, &out);//设置写数据
res = curl_easy_perform(curl);//执行
curl_slist_free_all(headers); /* free the list again */
string str_json = out.str();//返回请求值
printf("%s",str_json.c_str());
int iStart = str_json.find("{");
int iEnd = str_json.rfind("}");
if ((iStart < 0) || (iEnd < 1) || (iStart > iEnd))
{
return -1;
}
std::string strTemp = str_json.substr(iStart, (iEnd - iStart + 1));
rapidjson::Document document;
document.Parse(strTemp.c_str());
if (document.HasParseError())
{
return -1;
}
rapidjson::Document::AllocatorType& allocator = document.GetAllocator();}
return 0;}