Make a read only text file with C++

[dan.s]

Hi,
I would like to know how to create a read only text file on the fly using C++.
I have this code:

#include <iostream>
#include <fstream>
using namespace std;
int main(){

ofstream myFile("testfile.txt");
if(myFile.is_open()){
myFile << "Hi!";

myFile.close();
}
cin.get();
return 0;
}

And that generates the text file, but how do i make it readonly?
Thanks,
Dan