C / C++ / MFC

CString MyCString = "The Text";
CStdioFile csf("FileName.txt",
    CFile::modeCreate | CFile::modeWrite | CFile::shareDenyNone);
csf.WriteString(MyCString);

    #inclide <iostream>
    #include <fstream>
    using namespace std;
std::ofstream of;
CString m_str;
m_str = "Writing this text";
of.open("test.txt", ios::out);
of << m_str.GetBuffer(sizeof(m_str));
m_str.ReleaseBuffer(0);
of.close();

#include <iostream>
#include <fstream>

std::ofstream of ("test.txt"); // ios::out is implicit, it's an ostream.  And why did you use all of namespace std and then scope it explicitly as well ???

CString m_str("Some string");
of << m_str.GetBuffer(m_str.GetLength());
m_str.ReleaseBuffer(0);

// you can close it if you like, but being a C++ class, it will do it itself when it goes out of scope.