c++ - Can't get ios::beg to go back to the beginning of the file -

it seems things should no problem cause problems me. don't it. :/

so i'm trying make sure understand how manipulate text files. i've got 2 files, "infile.txt" , "outfile.txt". "infile.txt" has 6 numbers in , nothing else. here code used manipulate files.

#include<fstream> using std::ifstream; using std::ofstream; using std::fstream; using std::endl; using std::ios;  int main() { ifstream instream; ofstream outstream;//create streams  instream.open("infile.txt", ios::in | ios::out); outstream.open("outfile.txt");//attach files  int first, second, third; instream >> first >> second >> third; outstream << "the sum of first 3 nums " << (first+second+third) << endl; //make 2 operations on 6 numbers instream >> first >> second >> third; outstream << "the sum of second 3 nums " << (first+second+third) << endl;  instream.seekg(0); //4 different ways force program go beginning of file //2. instream.seekg(0, ios::beg); //3. instream.seekg(0, instream.beg); //4. instream.close(); instream.open("infile.txt"); //i have tried 4 of these lines , #4 works.  //there has got more natural option  //closing , reopening file. right?  instream >> first >> second >> third; outstream << "and again, sum of first 3 nums " << (first+second+third) << endl;  instream.close(); outstream.close(); return 0; } 

maybe don't understand quite how stream works, i've seen few sources said seekg(0) should move index start of file. instead, out of it.

the sum of first 3 nums 8

the sum of second 3 nums 14

and again, sum of first 3 nums 14

it went back, not in way have hoped. idea why happened? why did first 3 attempts fail?

as bo persson states, may because input has encountered end of file; shouldn't, because in c++, text file defined being terminated '\n', practically speaking, if you're working under windows, lot of ways of generating file omit final '\n'—although formally required, practical considerations mean you'll make sure works if final '\n' missing. , can't think of other reason off hand why seekg's wouldn't work. instream.seekg( 0 ) is, of course, undefined behavior, in practice, work pretty everywhere. instream.seekg( 0, ios::beg ) guaranteed work if instream.good(), , is, imho, preferable first form. (the single argument form of seekg used results of tellg argument.) , of course, works if actual input source supports seeking: won't work if you're reading keyboard or pipe (but presumably, "infile.txt" neither).

in general, should check status of instream after each read, before using results. if problem file doesn't end '\n', it's probable status ok (!fail()) after final read, if you've encountered end of file. in case, you'll need clear() anyway.

note above comments valid c++-03 , precedent. c++11 has changed specification of single argument form of seekg, , requires reset eofbit before else. (why change single argument form of seekg, , not 2 argument form? oversight?)