C++ Tutorial/File Stream/fstream

change a file by offset

<source lang="cpp">#include <iostream>

1. include <fstream>
2. include <cstdlib>

using namespace std; int main(int argc, char *argv[]) {

 fstream out("test.txt", ios::in | ios::out | ios::binary);
 if(!out) {
   cout << "Cannot open file.";
   return 1;
 }
 out.seekp(12, ios::beg);
 out.put("a");
 out.close();
 return 0;

}</source>

Open a file for output then write to that file

<source lang="cpp">#include <fstream>

1. include <iostream>

using namespace std;

int main () {

 char buffer[256];

 // open it for output then write to it
 fstream myfile;
 myfile.open("test.txt",ios::out | ios::trunc);
 if (myfile.is_open())   {
    myfile << "This outputting a line.\n";
    myfile.close();
 }

 return 0;

}</source>

Reverse a file

<source lang="cpp">#include <iostream>

1. include <fstream>
2. include <cstdlib>

using namespace std; int main(int argc, char *argv[]) {

 fstream inout("test.txt", ios::in | ios::out | ios::binary);
 if(!inout) {
   cout << "Cannot open input file.\n";
   return 1;
 }
 long e, i, j;
 char c1, c2;
 e = 5;
 for(i=0, j=e; i<j; i++, j--) {
   inout.seekg(i, ios::beg);
   inout.get(c1);
   inout.seekg(j, ios::beg);
   inout.get(c2);
   inout.seekp(i, ios::beg);
   inout.put(c2);
   inout.seekp(j, ios::beg);
   inout.put(c1);
 }
 inout.close();
 return 0;

}</source>

Use fstream to read and write a file.

<source lang="cpp">#include <iostream>

1. include <fstream>

using namespace std; int main() {

 char ch;
 fstream finout("test.dat");
 if(!finout) {
   cout << "Cannot open file for output.\n";
   return 1;
 }
 for(int i=0; i < 3; ++i) finout.put("X");
 if(!finout.good()) {
   cout << "Error occurred while writing to the file.\n";
   return 1;
 }
 finout.flush();
 cout << "Here are the next ten characters: ";
 for(int i=0; i < 10; ++i) {
   finout.get(ch);
   cout << ch;
 }
 cout << endl;
 if(!finout.good()) {
   cout << "Error occurred while reading from the file.\n";
   return 1;
 }
 finout.close();
 if(!finout.good()) {
   cout << "Error occurred while closing the file.\n";
   return 1;
 }
 return 0;

}</source>