fstream

From Wikipedia, the free encyclopedia

This article or section contains instructions, advice, or how-to content. The purpose of Wikipedia is to present facts, not to teach subject matter. Please help improve this article by removing or rewriting the how-to content, which may qualify for a move to http://www.wikihow.com/ or http://howto.wikia.com/. This article has been tagged since April 2008.

fstream is a c++ library that enables reading and writing to files either in text or in binary.

[edit] Text Files in fstream

For text files, a common method includes using the << and >> syntax that is followed with cout in the iostream library. The following example creates a file called 'file.txt' and puts the text 'Hello World' into it.

#include <fstream>
 
int main()
{
        ofstream file;
        file.open("file.txt");           
        file << "Hello world!\n";    
        file.close();               
}

[edit] Binary Files in fstream

Reading from and writing to binary files uses the following functions:

fstream::write(char *buff,int size);
fstream::read(char *buff, int size);

where buff is the buffer to write to or read from, and size is an integer number of bytes. www.cplusplus.com explains that these functions are necessary as the use of "the extraction and insertion operators (<< and >>) and functions like getline is not efficient, since we do not need to format any data, and data may not use the separation codes used by text files to separate elements (like space, newline, etc...)."

The following example creates a file called 'file.bin', creates a 100 element buffer, and writes 20 bytes from the buffer into the file.

#include <fstream.h>
 
int main()
{
        char buffer[100];
 
        ofstream file;
        file.open("file.bin", ios::out | ios::binary);      //opens the file for binary output       
 
        file.write(buffer, 20);                             //this writes 20 bytes from buffer to file.bin
        file.close();               
}

Reading from a binary file is very similar. The following example opens the file called 'file.bin', creates a 100 element buffer, and reads 20 bytes from the file into the buffer.

#include <fstream.h>
 
int main()
{
        char buffer[100];
 
        ifstream file;
        file.open("file.bin", ios::in | ios::binary);      //opens the file for binary input 
 
        file.read(buffer, 20);                             //this reads 20 bytes from file.bin to the buffer
        file.close();               
}

Note: it is a good idea to use the c++ operator sizeof to indicate how many bytes to write to the file. The following would write 4 bytes from the buffer to the file, as the size of the type int is 4 bytes.

file.write(buffer, sizeof(int));

Here comes the program that will update the binary file.

1. include<iostream.h>
2. include<fstream.h>
3. include<conio.h>

class inventory {

char *name;
int code;
float cost;

public:

inventory() { name=new char[20];}
void getData() { cout<<"Enter name , code and cost\n";
                 cin>>name>>code>>cost;}
int getCode() {return code;}
void disData() { cout<< "Name Code Cost\n";
               cout<<name<<"  "<<code<<"  "<<cost<<endl;
               }
}I[5],S;
int main()
{
fstream file;
file.open("stock.txt",ios::out);
clrscr();
for(int i=0;i<3;i++)
  {
  I[i].getData();
  file.write((char*)&I[i],sizeof(inventory));
  }
 file.close();
 clrscr();
 file.open("stock.txt",ios::in);
 cout<<"\n Stock items are...\n\n";
 file.read((char*)&S,sizeof(inventory));
 while(file)
  {
   S.disData();
  file.read((char*)&S,sizeof(inventory));
  }

    cout<<"To update product Enter the Code number\n";
  int cc;
  cin>>cc;
  file.close();

 file.open("stock.txt",ios::out|ios::in|ios::app);
  file.read((char*)&S,sizeof(inventory));
 while(file)
  {
   if(cc==S.getCode())
      {

      S.getData();
      int y=file.tellg();
      file.seekp(y,ios::beg);
      file.write((char*)&S,sizeof(inventory));
     cout<<"Bytes"<<"   "<<file.tellp()/sizeof(inventory);
     break;
      }
      else
      {
      file.read((char*)&S,sizeof(inventory));
      }
 }
 file.close();
 cout<<"\n Stock items are...\n\n";
 file.open("stock.txt",ios::in);
 file.read((char*)&S,sizeof(inventory));
 while(file)
  {
   S.disData();
  file.read((char*)&S,sizeof(inventory));
  }
 }

[edit] initializing files

When initializing a file, there are several modes you can choose, depending on what operation or function you are using the file for. The different modes you can use are as follows:

The following is a snippet of code that shows the function of these modes.

#include <fstream>
 
int main()
{
        ofstream ofile;
        ofile.open("ex.txt", ios::out); //opens file as output
        int x=1;           
        ofile<<x;
        ofile.close();
        //ex.txt is now "1" 
        ifstream ifile;
        ifile.open("ex.txt",ios::in); //opens file as input
        int a;
        ifile>>a;//a now equals 1
        a=2;
        ifile.close();
        ofile.open("ex.txt", ios::out|ios::trunc); //opens file as output
        //ex.txt is now empty due to ios::trunc
        ofile<<x;//write 1 to file
        //ex.txt is now "1"
        ofile.close();
        ofile.open("ex.txt", ios::out|ios::app); //open file as output
        ofile<<a;//write 2 to file
        //ex.txt is now "12"
        ofile.close();
        ofile.open("ex.txt", ios::in|ios::out|ios::ate); //open file as output
        ofile<<x;//write 1 to file
        //ex.txt is now "121"        
        ofile.close();                         
}

• • NOTE: for ios::ate, simply using this line of code: "ofile.open("ex.txt",ios::out|ios::ate);" will result in the file being overwritten as if ios::ate was not included in the function. Adding ios::in will enable this to function as described in the above table.

[edit] Member functions

What follows is an incomplete list of functions in iostream library:

See also ifstream, which is the C++ standard library class that enables input of data from files.

[edit] External References

• fstream reference
• http://www.cplusplus.com/doc/tutorial/files.html
• http://www.functionx.com/cpp/articles/filestreaming.htm