Reading and writing binary file in C++

This tutorial gives the description as well as the steps for programmers to learn how reading and writing binary file in C++ works. The process is simple and is divided into two main parts, writing the file first and then reading the same.

Reading and writing binary file in C++

The tutorial consists of two main parts. We will first see how to write to a binary file and then see how to read from it.

1. Libraries

Code :

#include<iostream>
#include<fstream>

iostream: input and output functions

stream : file stream.

2. Structure to store data to be written

Code :

struct Student {
   int roll_no;
   string name;
};

int main() {

   Student write_stu[2];
   write_stu[0].roll_no = 1;
   write_stu[0].name = "Akash";

   write_stu[1].roll_no = 2;
   write_stu[1].name = "Arjun";

   return 0;

}

The data for a student database is created and will be used to write to the binary file.

3. Writing to the Binary file

Code :

ofstream wf("student.dat", ios::out | ios::binary);
   
if(!wf) {
   cout << "Cannot open file!" << endl;
   return 1;
}

for(int i = 0; i < 2; i++)
   wf.write((char *) &write_stu[i], sizeof(Student));

wf.close();

if(!wf.good()) {
   cout << "Error occurred at writing time!" << endl;
   return 1;
}

ofstream wf: Creates Output stream class object (here ‘wf’) to operate on files.

wf(“student.dat”, ios::out | ios::binary);

student.dat: file name
ios::out: allows output (write operations) to a stream
ios::binary: Open in binary mode

if wf does not return a pointer, the user is informed that the file cannot be opened.
if the pointer is successfully returned, the data is written using write function.

wf.write((char *) &write_stu[i], sizeof(Student));

(char *) &write_stu[i]: Refers to the data to be written
sizeof(Student): The size of data to be written at a time.

The loop is run for the required times of iterations to write the complete data to the file.

wf.close(): Used to close the file
wf.good(): Used to determine if the process was successful or if there was any error and if so, the error occurrence is informed to the user.

(Note : Whenever any error occurs as mentioned above, the code process is stopped.)

4. Reading from a binary file

Code :

Student read_stu[2];
    
ifstream rf("student.dat", ios::out | ios::binary);
if(!rf) {
   cout << "Cannot open file!" << endl;
   return 1;
}

for(int i = 0; i < 2; i++)
   rf.read((char *) &read_stu[i], sizeof(Student));

rf.close();

if(!rf.good()) {
   cout << "Error occurred at reading time!" << endl;
   return 1;
}

Another student class object array ‘read_stu’ is declared to store the read data.

ifstream rf : Creates Input stream class object (here ‘rf’) to operate on files.

rf(“student.dat”, ios::out | ios::binary);

student.dat  : file name
ios::out : allows output (write operations) to a stream
ios::binary : Open in binary mode

if rf does not return a pointer, the user is informed that the file cannot be opened.
if the pointer is successfully returned, the data is read using read function.

rf.read((char *) &read_stu[i], sizeof(Student));

(char *) &write_stu[i] : Refers to the data to be read
sizeof(Student) : The size of data to be read at a time.

The loop is run for the required times of iterations to read the complete data from the file.

rf.close() : Used to close the file
rf.good() : Used to determine if the process was successful or if there was any error and if so, the error occurrence is informed to the user.

(Note : Whenever any error occurs as mentioned above, the code process is stopped.)

5. Display

Code :

cout<<"Student Details:"<<endl;

for(int i=0; i < 2; i++) {
   cout << "Roll No: " << read_stu[i].roll_no << endl;
   cout << "Name: " << read_stu[i].name << endl;
   cout << endl;
}

The data which is read from the file and stored in the read_stu array is displayed.

Output :

Student Details:
Roll No: 1
Name: Akash

Roll No: 2
Name: Arjun

Complete Code

#include<iostream>
#include<fstream>
using namespace std;

struct Student {
   int roll_no;
   string name;
};
int main() {

   Student write_stu[2];
   write_stu[0].roll_no = 1;
   write_stu[0].name = "Akash";

   write_stu[1].roll_no = 2;
   write_stu[1].name = "Arjun";

   ofstream wf("student.dat", ios::out | ios::binary);
   
   if(!wf) {
      cout << "Cannot open file!" << endl;
      return 1;
   }

   for(int i = 0; i < 2; i++)
      wf.write((char *) &write_stu[i], sizeof(Student));

   wf.close();

   if(!wf.good()) {
      cout << "Error occurred at writing time!" << endl;
      return 1;
   }

   Student read_stu[2];
    
   ifstream rf("student.dat", ios::out | ios::binary);
   if(!rf) {
      cout << "Cannot open file!" << endl;
      return 1;
   }

   for(int i = 0; i < 2; i++)
      rf.read((char *) &read_stu[i], sizeof(Student));

   rf.close();

   if(!rf.good()) {
      cout << "Error occurred at reading time!" << endl;
      return 1;
   }

   cout<<"Student Details:"<<endl;
   for(int i=0; i < 2; i++) {
      cout << "Roll No: " << read_stu[i].roll_no << endl;
      cout << "Name: " << read_stu[i].name << endl;
      cout << endl;
   }
   return 0;
}

Output :

Student Details:
Roll No: 1
Name: Akash

Roll No: 2
Name: Arjun

Therefore, we have learned how to read and write a binary file in C++.

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