short lifetime of object?

[icegood]

Have code

std::ostringstream lout;

lout << ...;

if (curl_easy_setopt(m_curl, CURLOPT_POSTFIELDS, 
  lout.str().c_str()) != CURLE_OK) //CURLOPT_COPYPOSTFIELDS right_string
  return 1;
// Perform the HTTP POST.
    res =curl_easy_perform(m_curl) != CURLE_OK;

In this case curl_easy_setopt obtains wrong value (might be empty) while curl_easy_perform returns success

But in case

std::ostringstream lout;
string right_string;

lout << ...;

right_string = lout.str().c_str();
if (curl_easy_setopt(m_curl, CURLOPT_POSTFIELDS, 
  right_string) != CURLE_OK) //CURLOPT_COPYPOSTFIELDS right_string
  return 1;
// Perform the HTTP POST.
    res =curl_easy_perform(m_curl) != CURLE_OK;

everything seems to be OK. Is it due short lifetime of lout.str() object?