File I/O in C++

Hi

I am witing a program to change the the text in a file from "This is an apple" to This is a sample". I need to open the file in append mode too for adding text afterward.
The code for program is:-

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

int main (int argc, char *argv)
{
long pos;
fstream outfile;
outfile.open ("test.txt",ios:out|ios::ate|ios::app);
if (outfile.fail())
{
cout << "Error opening file, errno: " << errno ;
}
pos=outfile.tellp();
cout<<"\n pos is " << pos;
outfile.seekp (pos-7);
cout<<"\n pos is " << outfile.tellp();
outfile.write (" sam",4);

outfile.close();
return 0;
}

The problem is instead of changing the text it adds the word sam in the file. Please help.

Thanks.

you should not add "ios::app" in this code;
use this :

Code:

---

outfile.open ("test.txt",ios::out|ios::ate);

---

ios::app means add content after the end of the file.

Thank you for replying.

I tried this way too. if i open the file like
outfile.open ("test.txt",ios::out|ios::ate);
or
outfile.open ("test.txt",ios::ate);

it overwrites the contents of existing file and displays just the sam in the file.

Any other suggestions are welcome.

Thansks.

i have tried this code, it works fine.
make sure the content of text.txt is "this is an apple" only.

Code:

---

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

int main (int argc, char *argv)
{
long pos;
fstream outfile;
outfile.open ("test.txt",ios::out|ios::ate);
if (outfile.fail())
{
cout << "Error opening file, errno: " << endl ;
}
pos=outfile.tellp();
cout<<"\n pos is " << pos;
outfile.seekp (pos-7);
cout<<"\n pos is " << outfile.tellp();
outfile.write (" sam",4);

outfile.close();
return 0;
}

---

Hi

I tried it again. This is not working for me. I checked the text.txt file. It contains "This is an apple" only.
What could be the problem?

Thanks.

Code:

---

std::ofstream outfile("test.txt", std::ios::out | std::ios::in); //ios::in - don't overwrite
outfile.seekp(9);
outfile << " sam";

---

Though I'll have to doublecheck, I believe ios::app specifies that all characters written will be appended to the end of the file (perhaps it sets the position 0 to be at the end of the file). ios::ate specifies that the starting position is at the end of the file, but it's kind of unclear how it operates; from what I remember from my compiler's docs, it said that ate causes the first write operation to write at the end of the file, but after that all write operations write from the beginning of the file; other online references just say that it seeks to EOF. Consequently I just try to avoid it altogether ;) But the ios:: out flag is implicitly included in ofstream's constructor, which deletes everything anyway. To prevent truncation, you have to specify the ios::in flag (or ios::app, but that won't work in our case).

Hope this helps :)

Quote:

---

believe ios::app specifies that all characters written will be appended to the end of the file (perhaps it sets the position 0 to be at the end of the file)

---

ios::app causes the write position to be set to the end of file previous to each write operation. In other words, it effectively issues a
stream.seek(0, ios::end);
before each write.

Quote:

---

ios::ate specifies that the starting position is at the end of the file, but it's kind of unclear how it operates;

---

ios:ate issues a
stream.seek(0, ios::end);
directly after opening the file and has no effect whatsoever afterwards, meaning that writes go whereever you currently are.

Right, my mistake.
http://msdn.microsoft.com/library/de...a.ofstream.asp

Also, simran, if you want to seek from the end of the file you shouldn't use ios::ate and seek to (position - bytes), you should just do

Code:

---

std::ofstream outfile("test.txt", std::ios::out | std::ios::in); //ios::in - don't overwrite
outfile.seekp(-7, std::ios::end);
outfile << " sam";

---

Hi

Code:

---

outfile.open ("test.txt",ios::in | ios::out |ios::ate );

---

works. It does not overwrite the existing contents plus it replaces the text from "an apple" to "a sample"

Thansk a lot.