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Reading and writing of files in C++

Opening a File:

A file must be opened before you can read from it or write to it. Either the ofstream or fstreamobject may be used to open a file for writing and ifstream object is used to open a file for reading purpose only.

Following is the standard syntax for open() function, which is a member of fstream, ifstream, and ofstream objects.

#d6d6d6;">void open(const char *filename, ios::openmode mode);

Here, the first argument specifies the name and location of the file to be opened and the second argument of the open() member function defines the mode in which the file should be opened.

You can combine two or more of these values by ORing them together. For example if you want to open a file in write mode and want to truncate it in case it already exists, following will be the syntax:

#d6d6d6;">ofstream outfile;
outfile.open("file.dat", ios::out | ios::trunc );

Similar way, you can open a file for reading and writing purpose as follows:

#d6d6d6;">fstream  afile;
afile.open("file.dat", ios::out | ios::in );

Closing a File

When a C++ program terminates it automatically closes flushes all the streams, release all the allocated memory and close all the opened files. But it is always a good practice that a programmer should close all the opened files before program termination.

Following is the standard syntax for close() function, which is a member of fstream, ifstream, and ofstream objects.

#d6d6d6;">void close();

Writing to a File:

While doing C++ programming, you write information to a file from your program using the stream insertion operator (ofstream or fstream object instead of the cout object.

Reading from a File:

You read information from a file into your program using the stream extraction operator (>>) just as you use that operator to input information from the keyboard. The only difference is that you use an ifstream or fstream object instead of the cin object.

Read & Write Example:

Following is the C++ program which opens a file in reading and writing mode. After writing information inputted by the user to a file named afile.dat, the program reads information from the file and outputs it onto the screen:

#d6d6d6;">#include 
#include 
using namespace std;
 
int main ()
{
    
   char data[100];

   // open a file in write mode.
   ofstream outfile;
   outfile.open("afile.dat");

   cout  "Writing to the file"  endl;
   cout  "Enter your name: "; 
   cin.getline(data, 100);

   // write inputted data into the file.
   outfile  data  endl;

   cout  "Enter your age: "; 
   cin >> data;
   cin.ignore();
   
   // again write inputted data into the file.
   outfile  data  endl;

   // close the opened file.
   outfile.close();

   // open a file in read mode.
   ifstream infile; 
   infile.open("afile.dat"); 
 
   cout  "Reading from the file"  endl; 
   infile >> data; 

   // write the data at the screen.
   cout  data  endl;
   
   // again read the data from the file and display it.
   infile >> data; 
   cout  data  endl; 

   // close the opened file.
   infile.close();

   return 0;
}

When the above code is compiled and executed, it produces the following sample input and output:

#d6d6d6;">$./a.out
Writing to the file
Enter your name: Zara
Enter your age: 9
Reading from the file
Zara
9

Above examples make use of additional functions from cin object, like getline() function to read the line from outside and ignore() function to ignore the extra characters left by previous read statement.

Credits:

http://www.tutorialspoint.com/cplusplus/cpp_files_streams.htm

#TC1017 #Mastery28