c++ file pointers not working properly - c++

Question

I'm trying to write a program that replaces a specific number with an 'x' character. The task requires every number to be in its own line, but it seems like '\n' is causing the read/write pointers to behave out of this world. Here's a picture of the output.
My questions are:
why are the pointers behaving this way?
How far do I need to move the write pointer backwards to overwrite a line to make this work?
is there an easier workaround?
Here's my code:
void input(int n)
{
fstream file;
file.open("numbers.txt", ios::out);
while(n --> 0)
{
file << n;
file << '\n';
}
file.close();
}
void read()
{
fstream file;
string tmp;
file.open("numbers.txt", ios::in);
while(true)
{
getline(file,tmp);
if(file.eof())
break;
cout << tmp << endl;
cout << "tellg: " << file.tellg() << " tellp: " << file.tellp() << endl;
}
file.close();
}
void replace()
{
fstream file;
string tmp;
file.open("numbers.txt", ios::in | ios::out);
while(true)
{
file >> tmp;
if(tmp == "6")
{
//cout << file.tellg() << endl;
file.seekp(file.tellg() - tmp.length()-1);
file << "x";
}
if(file.eof())
break;
}
file.close();
}
int main()
{
input(10);
replace();
read();
return 0;
}

Answer 1

Since you open your file in text mode, you need to account for the potential that the underlying stream may use a line end sequence (\r\n) rather than just a \n. I guess, this is the primary problem. The easiest remedy is probaly to open the file in binary mode:
file.open("numbers.txt", std::ios_base::binary | std::ios_base::in | std::ios_base::out);
That said, since you switch from writing to reading without intervening seek, your code is undefined behavior, i.e., anything can happen. You should seek to the current location between writing and reading.
Personally, I'd refrain from rewriting files in-place. It generally gets unnecessary trick. If I were to rewrite files in place, I'd use seekg() to get the current position before a read, saving the position and restoring it prior to the write (I essentially never use the seek operations, i.e., I may have got the signatures wrong):
for (std::streampos pos = (in >> std::ws).tellg();
in >> tmp; pos = (in >> ws).tellg()) {
if (need_to_overwrite) {
in.seekp(pos);
// ...
in.seekg(0, std::ios_base::cur);
}
}
The use of in >> std::ws is to make sure that whitespace is skipped before storing the position.
Also note that your check for file.eof() is wrong: the last line is processed twice. When reading from a file the result shall be tested before using the read string, e.g.:
while (in >> tmp) {
// ...
}