Binary files

BINARY FILES:

                           Binary files are the file streams opened in the mode ios::binary. They perform input and output operations independently of any format considerations. There is no need to format any data, and data may not use the separation codes (like space, newline, etc…) used by text files, to separate elements. Hence, in binary files, input and output data with the extraction and insertion operators (<< and >>) and functions like getline are not efficient.

                    Binary files are used to store programs and data. Binary files can be classified in the way they are accessed – Sequential access files and Direct (or random) access files. To input and output binary data sequentially, file streams contain two member functions write() and read(). The function write() is a member function of ostream inherited by ofstream and read() is a member function of istream that is inherited by ifstream. Objects of class fstream contain both the functions. Their prototypes are:

                                             write(memory_block, size);

                                              read(memory_block, size);

Where memory block is of type “pointer to char” (char*), and represents the address of an array of bytes where the read data elements are stored from where the data elements to be written are taken. The size parameter is an integer value that specifies the number of characters to be read form the memory block or the number of characters to be written to the memory block. One can also use these functions to write and read class objects in a file.

CLASSES AND FILE OPERATIONS:

In order to write an object obj of a class into a file the following format is used.

           mfile.write( (char*) &obj,sizeof(obj) );

where mfile is a fstream object.         

Similarly, in order read an object obj of a class from a file the following format is used.

             mfile.write( (char*) &obj,sizeof(obj) );

Program: This program illustrates writing class objects into a file and reading the same.

#include<iostream.h>

#include<fstream.h>

#include<iomanip.h>

#include<conio.h>

class studentinfo

{

      protected:

                  char name[20],sex;

                  int age;

                  float height,weight;

       public:

                  void getdata();

                  void display();

};

void studentinfo : : getdata()

{

      cout<<”enter the following information: \n”;

      cout<<”name:”;                        cin>>name;

      cout<<”Age:”; cin>>age;

      cout<<”Sex:”; cin>>sex;

      cout<<”height:”; cin>>height;

      cout<<”weight:”; cin>>weight;

}

void studentinfo : : display()

{

cout<<name<<setw(5)<<age<<<<endl;

cout<<sex<<setw(5)<<height<<setw(5)<<weight<<endl;

} 

void main()

{

clrscr();

studentinfo obj;

fstream mfile;

char fname[10];

cout<<”enter a file name: \n:”;

cin>>fname;

obj.getdata();                 //reading an object from the keyboard

mfile.open(fname, ios : : out);

cout<<”writing data into the file……….\n”

mfile.write( (char*) &obj , sizeof(obj) ); //writing the object into the file

mfile.close();

mfile.open(fname, ios : : in);

cout<<”reading data from the file………..\n”;

mfile.read( (char*) & obj, sizeof(obj) ); //reading the object from the file

obj.display();

mfile.close();

getch();

}

The above program contains a class studentinfo with two functions getdata() and display(). An object obj of type studentinfo is created and the data in the object, received from the keyboard, is written in a file opened in output mode ios : : out, the name of the file being chosen at runtime.

        The file is then closed and opened again in the input mode ios : : in.  The data in the object obj stored earlier is then read from the file and displayed.