I was asked a question about fstream a few days:

How do you shorten the file length of a bidirectional io? A requirement is that we can only open the stream once.

I tried to write the following code, but it failed.
======================================

#include <iostream>
#include <string>
#include <fstream>

/** Original content of a.txt is
aba

After running the program, I want the modified content to be
aa

This means that I have to shorten the file size by 1 byte.
*/

using namespace std;

int main()
{
fstream fs("a.txt", ios::in | ios::out);
string s;

fs>>s;
cout<<s<<endl; // s is "aba"
s.replace(s.find("b"), 1, "");
cout<<s<<endl; // s is "aa" now

fs.seekp(ios::beg);
fs.clear();

fs<<s; // this gives "aaa" since we don't shorten the file length

fs.close();

return 0;
}

to wfpb:

thanks for your soln. Don't know if this also works for Linux system.

Win32 API

I guess it won't work for standard c++.

However, it is a platform-dependent soln.