C++/File/ofstream

Accessing Data in a File

<source lang="cpp">

1. include <algorithm>
2. include <cstdlib>
3. include <fstream>
4. include <functional>
5. include <iostream>
6. include <iterator>
7. include <vector>

using namespace std; template <class T> void print(T& c){

  for( typename T::iterator i = c.begin(); i != c.end(); i++ ){
     std::cout << *i << endl;
  }

} int main(){

  vector<int> output_data( 10 );
  generate( output_data.begin(), output_data.end(), rand );
  transform( output_data.begin(), output_data.end(),output_data.begin(), bind2nd( modulus<int>(), 10 ) );
  ofstream out( "data.txt" );
  if( !out )
  {
     cout << "Couldn"t open output file\n";
     return 0;
  }
  copy( output_data.begin(), output_data.end(),ostream_iterator<int>( out, "\n" ) );
  out.close();
  ifstream in( "data.txt" );
  if( !in )
  {
     cout << "Couldn"t open input file\n";
     return 0;
  }
  vector<int> input_data( (istream_iterator<int>( in )),istream_iterator<int>() );
  in.close();
  print( output_data );
  print( input_data );

}

 </source>

basic file operations

<source lang="cpp">

1. include <iostream>
2. include <fstream>

using namespace std; int main () {

 ofstream myfile;
 myfile.open ("example.txt");
 myfile << "Writing this to a file.\n";
 myfile.close();
 return 0;

}

 </source>

basic file operations: open and save

<source lang="cpp">

1. include <iostream>
2. include <fstream>

using namespace std; int main () {

 ofstream myfile;
 myfile.open ("example.txt");
 myfile << "Writing this to a file.\n";
 myfile.close();
 return 0;

}

 </source>

Connect ofstream and streambuf and operations with streambufs.

<source lang="cpp">

1. include <iostream>
2. include <fstream>

using namespace std; int main(void){

  int c;
  char *filename = "test.txt";
  ofstream outfile;
  streambuf *out, *input = cin.rdbuf();
  outfile.open( filename, ios::ate | ios::app);
  if (!outfile) 
  {
     cerr << "Could not open " << filename;
     return(-1);
  }
  out = outfile.rdbuf();  
  while ( (c = input -> sbumpc() ) != EOF){
     cout << char(c);                         // Echo to screen.
     if (out -> sputc(c) == EOF)
        cerr << "Output error";
  }

}

 </source>

eat the newline after the file name

<source lang="cpp">

1. include <iostream>
2. include <fstream>

using namespace std; int main() {

  char buffer[255];    
  ofstream fout("text.txt");  
  fout << "This line written directly to the file...\n";
  cout << "Enter text for the file: ";
  cin.ignore(1,"\n");
  cin.getline(buffer,255);
  fout << buffer << "\n";
  fout.close();         
  ifstream fin("text.txt");    
  char ch;
  while (fin.get(ch))
     cout << ch;
  fin.close();
return 0;

}

 </source>

ofstream.fail() Demo

<source lang="cpp">

1. include <fstream>
2. include <iostream>

using namespace std; int main () {

  ofstream outfile;
  outfile.open("file.dat");
  cout << "( outfile) = " << outfile << endl;
  cout << "( outfile.fail()) = " << outfile.fail() << endl;
  return 0;

}

 </source>

ofstream: Use read(), write(), ios::out | ios::binary

<source lang="cpp">

1. include <iostream>
2. include <fstream>

using namespace std;

int main() {

 int n[5] = {1, 2, 3, 4, 5}; 
 register int i; 

 ofstream out("test", ios::out | ios::binary); 
 if(!out) { 
   cout << "Cannot open file.\n"; 
   return 1; 
  } 

 out.write((char *) &n, sizeof n); 

 out.close(); 

 for(i = 0; i <5; i++) // clear array 
   n[i] = 0; 

 ifstream in("test", ios::in | ios::binary); 
 if(!in) { 
   cout << "Cannot open file.\n"; 
   return 1; 
 } 

 in.read((char *) &n, sizeof n); 

 for(i = 0; i <5; i++) // show values read from file 
   cout << n[i] << " "; 

 in.close(); 

 return 0; 

}

 </source>

Output file with ofstream

<source lang="cpp">

1. include <iostream>
2. include <fstream>

using namespace std; int main() {

 ofstream pout("test");
 if(!pout) {
   cout << "Cannot open file.\n";
   return 1;
 }
 pout << "A   444 555-4444\n";
 pout << "B   222 555-2222\n";
 pout << "Joe 333 555-1111\n";
 pout.close();
 return 0;

}

 </source>

put( ) to write all characters from zero to 255 to a file called CHARS.

<source lang="cpp">

1. include <iostream>
2. include <fstream>

using namespace std;

int main() {

 int i;
 ofstream out("CHARS", ios::out | ios::binary);
  
 if(!out) {
   cout << "Cannot open output file.\n";
   return 1;
 }
  
 // write all characters to disk
 for(i=0; i<256; i++) out.put((char) i);
  
 out.close();
 return 0;

}

 </source>

reads strings entered at the keyboard and writes them to disk

<source lang="cpp">

1. include <iostream>
2. include <fstream>

using namespace std;

int main(int argc, char *argv[]) {

 if(argc!=2) {
   cout << "Usage: output <filename>\n";
   return 1;
 }
  
 ofstream out(argv[1]); // output, normal file
  
 if(!out) {
   cout << "Cannot open output file.\n";
   return 1;
 }
  
 char str[80];
 cout << "Write strings to disk. Enter ! to stop.\n";
  
 do {
   cout << ": ";
   cin >> str;
   out << str << endl;
 } while (*str != "!");
  
 out.close();
 return 0;

}

 </source>

Setting the locale to German_Germany

<source lang="cpp">

1. include <iostream>
2. include <fstream>
3. include <locale>

using namespace std; int main() {

 ofstream fout("test.dat");
 if(!fout) {
   cout << "Cannot open file.\n";
   return 1;
 }
 cout << "The original locale is " << fout.getloc().name();
 locale loc("German_Germany");
 fout.imbue(loc);
 cout << "The current locale is now " << fout.getloc().name();
 cout << endl;
 if(has_facet<moneypunct<char, true> >(fout.getloc())) {
   const moneypunct<char, true> &mp = use_facet<moneypunct<char, true> >(fout.getloc());
   cout << "Money symbol: " << mp.curr_symbol() << endl;
   cout << "Thousands separator: " << mp.thousands_sep() << endl;
 }
 fout.close();
 if(!fout.good()) {
   cout << "Error closing file.\n";
   return 1;
 }
 return 0;

}

 </source>

Uses put() to write all characters from zero to 255 to a file

<source lang="cpp">

1. include <iostream>
2. include <fstream>

using namespace std; int main() {

 int i;
 ofstream out("CHARS_File", ios::out | ios::binary);
 if(!out) {
   cout << "Cannot open output file.\n";
   return 1;
 }
 // write all characters to disk
 for(i=0; i<256; i++) 
    out.put((char) i);
 out.close();
 return 0;

}

 </source>

writes formatted output to a file, using

<source lang="cpp">

1. include <fstream>

 #include <iostream>  
 #include <string>  
 using namespace std;  
   
 int main(){  
    char ch = "x";  
    int j = 77;  
    double d = 6.02;  
    string str1 = "test";        
    string str2 = "this is a test";
   
    ofstream outfile("fdata.txt"); 
   
    outfile << ch                  
            << j  
            << " "                 
            << d  
            << str1  
            << " "                 
            << str2;  
    cout << "File written\n";  
    return 0;  
 }
 
   
 </source>

Writes information inputted to a file

<source lang="cpp">

1. include <fstream>
2. include <iostream>

using namespace std; int main () {

  char data[80];
  ofstream outfile;
  outfile.open("file.txt");
  cout << "Writing to the file" << endl;
  cout << "Enter your name: "; 
  cin.getline(data, 80);
  outfile << data << endl;
  cout << "Enter your id number: "; 
  cin >> data;
  cin.ignore();
  outfile << data << endl;
  outfile.close();
  return 0;

}

 </source>