C++ provides the following classes to perform output and input of characters to/from files:

  • ofstream: Stream class to write on files
  • ifstream: Stream class to read from files
  • fstream: Stream class to both read and write from/to files.

These classes are derived directly or indirectly from the classes istream, and ostream. We have already used objects whose types were these classes: cin is an object of class istream and cout is an object of class ostream. Therfore, we have already been using classes that are related to our file streams. And in fact, we can use our file streams the same way we are already used to use cin and cout, with the only difference that we have to associate these streams with physical files. Let’s see an example:

// basic file operations
#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main () {
  ofstream myfile;
  myfile.open ("example.txt", ios::binary);
  myfile << "Writing this to a file.\n";
  myfile.close();

  string line;

  ifstream infile("example.txt", ios::binary);
  while(!infile.eof() )
  {
	getline(infile, line);
	cout<< line <<endl;
  }

  getchar();
  return 0;
}

This code creates a file called example.txt and inserts a sentence into it in the same way we are used to do with cout, but using the file stream myfile instead.

But let’s go step by step:

Opening a file

The first operation generally performed on an object of one of these classes is to associate it to a real file. This procedure is known as to open a file. An open file is represented within a program by a stream object (an instantiation of one of these classes, in the previous example this was myfile) and any input or output operation performed on this stream object will be applied to the physical file associated to it.

In order to open a file with a stream object we use its member function open():

open (filename, mode);

Where filename is a null-terminated character sequence of type const char * (the same type that string literals have) representing the name of the file to be opened, and mode is an optional parameter with a combination of the following flags:

Mode  Meaning
ios::in Open for input operations.
ios::out Open for output operations.
ios::binary Open in binary mode.
ios::ate Set the initial position at the end of the file.
If this flag is not set to any value, the initial position is the beginning of the file.
ios::app All output operations are performed at the end of the file, appending the content to the current content of the file. This flag can only be used in streams open for output-only operations.
ios::trunc If the file opened for output operations already existed before, its previous content is deleted and replaced by the new one.

All these flags can be combined using the bitwise operator OR (|). For example, if we want to open the file example.bin in binary mode to add data we could do it by the following call to member function open():

ofstream myfile;
myfile.open ("example.bin", ios::out | ios::app | ios::binary);

Each one of the open() member functions of the classes ofstream, ifstream and fstream has a default mode that is used if the file is opened without a second argument:

class

default mode parameter

ofstream ios::out
ifstream ios::in
fstream ios::in | ios::out

File streams opened in binary mode, perform input and output operations independently of any format considerations. Non-binary files are known as text files, and some translations may occur due to formatting of some special characters (like newline and carriage return characters)Since the first task that is performed on a file stream object is generally to open a file, these three classes include a constructor that automatically calls the open() member function and has the exact same parameters as this member. Therefore, we could also have declared the previous myfile object and conducted the same opening operation in our previous example by writing:

ofstream myfile ("example.bin", ios::out | ios::app | ios::binary);

Combining object construction and stream opening in a single statement. Both forms to open a file are valid and equivalent.

To check if a file stream was successful opening a file, you can do it by calling to member is_open() with no arguments. This member function returns a bool value of true in the case that indeed the stream object is associated with an open file, or false otherwise:

if (myfile.is_open()) { /* ok, proceed with output */ }

Closing a file

When we are finished with our input and output operations on a file we shall close it so that its resources become available again. In order to do that we have to call the stream’s member function close(). This member function takes no parameters, and what it does is to flush the associated buffers and close the file:

myfile.close();

Once this member function is called, the stream object can be used to open another file, and the file is available again to be opened by other processes.

In case that an object is destructed while still associated with an open file, the destructor automatically calls the member function close().

Objects and the files

The following example illustrates that how to write objects to the files and how to read objects from the files.

class emp{
private:
	int id;
	string name, dept;
	double sal;
public:
	emp(int i=0, string n="", double s=0, string d="")
	{ id = i; name = n; sal = s; dept = d;}

	void set( int i, string n, double s, string d)
	{ id = i; name = n; sal = s; dept = d;}

	void print( )
	{
        cout << id << '' <<name << ' ' << sal << ' ' << dept << endl;
	}
	void StoreInFile(ofstream &out)
	{
        out << id << ' ' << name << ' ' << sal << ' ' << dept << endl;
        }
	void StoreFromFile(ifstream &in)
	{
        in >> id >> name >> sal >> dept;
        }
};
//////////////// main() ////////////////////////////////////
int main () {
	emp alice(10, "alice", 15000, "CS"), abbott, john, temp;
	abbott.set(20, "abbott", 25000, "ES");
	john.set(30, "john", 35000, "SWE");
	ofstream outfile ("example.txt", ios::binary);
	if (outfile.is_open())
	{
	  ////// stores data in file using our own function //////
	  // alice.StoreInFile(outfile);
	  // abbott.StoreInFile(outfile);
	  // john.StoreInFile(outfile);
	  ////// stores data in file using built in function /////
	  outfile.write((const char *) (&alice), sizeof(emp) );
	  outfile.write((const char *) (&abbott), sizeof(emp) );
	  outfile.write((const char *) (&john), sizeof(emp) );
          // close file
	  outfile.close();
	}
	else cout << "Unable to open file";

////////////// reading the stored file //////////////////////////
	ifstream infile ("example.txt", ios::binary);
	if (infile.is_open())
	{
		while (! infile.eof() )
		{
		  //retrieves data from file using our own function
		  //temp.StoreFromFile(infile);

		  //retrieves data from file using built in function
		  infile.read ( (char *) (&temp), sizeof(emp) );

		  temp.print();
		  //getchar();
		}
		infile.close();
	}
  else cout << "Unable to open file";
  getchar();
  return 0;
}

Built in functions

1- write( (char *) (&ObjName), sizeof(class_name) ); 

Takes the first argument a char pointer for the data to be stored and the second argument as size of the memory to be stored in file.

2- read( (char *) (&ObjName), sizeof(data type) );

Takes the first argument a char pointer for the variable / object in which retrieved data (from file) is to be stored and the second argument as size of the memory to be read.

Acknowledgement:

Most of the contents above have been taken directly from here solely for the purpose of education.

Exercise

Write a program that will read from a file in string and write that string in another file in reverse order. Also program the same code so that it will read reverse from a file and write this to another file.

Tagged with: C/C++ languageObject OrientedProgramming
 

One Response to File Input-Output (I/O) in C++

  1. Abdela Ayalew says:

    Really I will Appreciate you. Every Computer science professionalism and students should see this web site.

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