How can I speed up ostringstream writing to an ofstream

[JonnoA]

I am using an ostringstream to hold text which is written to a file using an ofstream object, but it is unacceptably slow - it takes more than 10 secs to write 200kb on a fast pc. Please can you tell me how to speed this up significantly.

In the document's OnSave function(I'm using MFC) I create an ostringstream and pass it to a function to be filled with data, then I write the text string to the ofstream:

BOOL CMyDocument::OnFileSave(lpszPathName)
{
ofstream fout;
fout.open(lpszPathName);
ostringstream os;
pData->TextOut(os);
fout << os.str();
}

where pData is a pointer to a data object (CData*) held by the document. CData is a base class for CDataReal and CDataComplex, which hold real and complex data respectively. In the derived class I do the following to write each row of a 2d array on separate lines with a comma delimiter:

void CDataReal::TextOut(ostringstream& os)
{
   for (int i=0; i < someIndex; ++i) {
      for (int j=0; j < someIndex; ++j) {
         os << "," << m_TemplateData.begin()[j][i];
      }
      os << endl;
   }
}

I don't necessarily have to pass an ostringstream object - it just seemed like a neat solution. Could someone please explain why the above is SO slow.

Thank you