C++、JsonCpp、LIGURL和UTF-8困境

我在用C++ JSONCPP库进行LIBCURL工作时遇到了一些问题,经过大量的研究,我提出了这个代码:

int post(const string& call, const string& key, const string& value) {
  // (...)

  char* char_data=NULL; 
  struct curl_slist *headers=NULL;

  headers = curl_slist_append(headers, "Content-Type: application/json; charset=UTF-8");

  Json::Value root;
  root[key] = value;

  Json::StyledWriter writer;
  string data = writer.write(root);

  char_data = (char*) malloc((strlen(data.c_str())+1) * sizeof(char));
  strcpy(char_data, data.c_str());

  curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers); 
  curl_easy_setopt(curl, CURLOPT_POSTFIELDS, char_data);
  curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, (long)strlen(char_data));

  // (...)
}

只要 data (C++) std::string (包含JSON字符串表示)没有任何非ascii字符。当它出现时,我从后端得到一个错误(用Rails 3编写):

Started POST "/deployments/4c904f607d7c4249cf00002c/log.json" for 67.23.79.89 at Wed Sep 15 00:45:40 -0400 2010
  Processing by DeploymentsController#log as JSON
  Parameters: {"log"=>"0 upgraded, 0 newly \214\211K########talled, 0 to remove and 46 not upgraded.\n", "id"=>"4c904f607d7c4249cf00002c"}
Completed   in 6ms

BSON::InvalidStringEncoding (String not valid UTF-8):
  app/models/deployment.rb:161:in `log'
  app/models/deployment.rb:160:in `each'
  app/models/deployment.rb:160:in `log'
  app/controllers/deployments_controller.rb:54:in `log'

什么是最好的方式采取C++ SCTRIN(在这种情况下数据),并安全地转换它UTF-8,然后到 *char 可以很好地使用libcurl的变量?