Tellg（）函数C ++的奇怪行为

Question

I got a problem with tellg() function from std::fstream class. Typically it should return the position of the current character in the input stream. However, it works very strange for me. Below is some short sample code:

#include <iostream>
#include <fstream>

using namespace std;

int main(void) {
char c;
ifstream czytaj;
czytaj.open("test_file.txt");

cout << czytaj.tellg() << endl;     //is 0 should be 0
czytaj.peek();                      //is 0 should be 0
cout << czytaj.tellg() << endl;     //is 2 should be 0    !!
czytaj.get(c);                      //is 3 should be 3
czytaj.get(c);                      //is 4 should be 4
cout << czytaj.tellg() << endl;     //is 6 should be 4    !!

int r; cin >> r;
return 0;
}

While the txt file looks like follows:

abcdefghij
kturjbkfvd

After compilation I get output like:

0
2
6

First use of tellg() works properly, it returns position 0 as its the beginning of the file. Unfortunatelly, each next use works like it adds +2 to the position. As the result I get letters 'c' and 'd' extracted from the stream. Both tellg() and peek() are supposed not to change the position, so I should get letters 'a' and 'b', while the correct result should be:

0
0
2

Such things happen if I use encoding ANSI in txt file. When I change it for Unicode, it works as it should. Also if I use ANSI and additionally binary mode ios::binary, it works properly as well. Strange fact is, on my other computer it works fine even with ANSI and without ios::binary. Why does it happen?

EDIT: Forgot to mention about very important fact. If I remove from this sample code all lines containing tellg() , the extracting is correct - I get letters 'a' and 'b'.

Answer 1

tellg() tells you where the next "get" position in the file is. Since files in for example Windows that use CR+LF ( '\\r','\\n' ) as newlines have two characters as a newline, where the C++ (and C) standard requires that a newline is LF '\\n' as a single character, when your program reads a CR+LF sequence, the C runtime counts that as one character, but the file position where you get the next character from is two steps forward.

Answer 2

had the same issue. try to read the filestream binary:

    czytaj.open("test_file.txt",ios::binary);

it helped for me