Changing the contents of a file is a bug in g ++ 4.7.2, or am I doing it wrong?

Question

While writing code to update the position in the binary, I noticed something strange. Consider this code example:

#include <iostream>
#include <fstream>
#include <string>
using namespace std;

int main() {
    char tmp;
    string s;
    fstream fs;

    fs.open("test.txt", fstream::out);
    fs << "blub" << endl;
    fs.close();
    fs.open("test.txt", fstream::in);
    fs >> s;
    cout << s << endl;
    fs.close();

    fs.open("test.txt", ios::in|ios::out|ios::binary);
    if (!fs.is_open() || !fs.good())
        cerr << "could not open!" << endl;

    fs.read(&tmp, 1);
    fs.read(&tmp, 1);
    //fs.tellg(); //<-- required to fix for old g++?
    const char *c = "ah";
    fs.write(&c[0], 1);
    fs.write(&c[1], 1);
    fs.close();

    fs.open("test.txt", fstream::in);
    fs >> s;
    cout << s << endl;
}

In recent g ++ versions (at least from 6.2.1) I can just read and then write a few bytes without any problems. In the example, you will get the correct output:

blub
blah

Then I compiled the code with g ++ 4.7.2 and suddenly the update has no effect, i.e. the second output is still "blub", unless I add fs.tellg () or fs.tellp (). I found this question, but as I understand it, this is a limitation under Windows, but I work under Linux.

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