Writing a binary file in C++

The binary file stores data in binary format, and data can be written using the write() or fwrite() method in C++.

Writing to a Binary File

To write a binary file in C++, use write method like fwrite()/write() method. It is used to write a given number of bytes on the given stream, starting at the position of the "put" pointer. The file is extended if the put pointer is current at the end of the file. If this pointer points into the middle of the file, characters in the file are overwritten with the new data.

If any error has occurred during writing in the file, the stream is placed in an error state.

Syntax of write() method

Following is the basic syntax of writing a binary file in C++:

ostream& write(const char*, int);

Algorithm to Write a Binary File

Following is the algorithm is as follows:

Begin
   Create a structure Student to declare variables.
   Open binary file to write.
   Check if any error occurs in file opening.
   Initialize the variables with data.
   If file opens successfully, write the binary data using write method.
      Close the file for writing.
   Check if any error occurs.
   Print the data.
End.

C++ Program to Write a Binary File

This program defines a "Student" structure and stores 3 student records (roll number and name). It writes these records into a binary file (student.dat) using write(), and then prints the same data to the console. It uses a fixed-size character array as char name[50] to store and write names as raw binary:

#include<iostream>
#include<fstream>
#include<cstring>
using namespace std;
// Define structure for Student
struct Student {
    int roll_no;
    char name[50]; // Fixed-size character array for name
};
int main() {
   // Open binary file for writing
   ofstream wf("student.dat", ios::out | ios::binary);
   if (!wf) {
      cout<<"Cannot open file!"<<endl;
      return 1;
   }
   // Initialize student data
   Student wstu[3];
   wstu[0].roll_no = 1;
   strcpy(wstu[0].name, "Revathi");
   
   wstu[1].roll_no = 2;
   strcpy(wstu[1].name, "Vivek");
   
   wstu[2].roll_no = 3;
   strcpy(wstu[2].name, "Tapas");
   
   // Write each student record to the binary file
   for (int i = 0; i<3; i++) {
      wf.write(reinterpret_cast<char*>(&wstu[i]), sizeof(Student));
   }
   
   wf.close(); // Close file after writing
   
   // Check if write operation was successful
   if (!wf.good()) {
       cout<<"Error occurred at writing time!"<<endl;
       return 1;
   }  
   // Display student data
   cout<<"Student's Details:\n";
   for (int i = 0; i<3; i++) {
      cout<<"Roll No: "<<wstu[i].roll_no<<endl;
      cout<<"Name: "<<wstu[i].name<<endl;
   }
   return 0;
}

Following is the output to the above program:

Student's Details:
Roll No: 1
Name: Revathi
Roll No: 2
Name: Vivek
Roll No: 3
Name: Tapas

Writing to a Binary File Using fwrite() Function

You can also write data to a binary file using the fwrite() function. The fwrite() function write raw data (like structures) directly into a file in binary format.

Example

This example demonstrates how you can write data to a Binary file using the fwrite() function:

#include <cstdio>
#include <cstring>

// Define structure for Student
struct Student {
    int roll_no;
    char name[50]; // Fixed-size character array for name
};

int main() {
    // Open binary file for writing
    FILE* file = fopen("student.dat", "wb");
    if (file == nullptr) {
        printf("Cannot open file!\n");
        return 1;
    }

    // Initialize student data
    Student wstu[3];
    wstu[0].roll_no = 1;
    strcpy(wstu[0].name, "Revathi");

    wstu[1].roll_no = 2;
    strcpy(wstu[1].name, "Vivek");

    wstu[2].roll_no = 3;
    strcpy(wstu[2].name, "Tapas");

    // Write each student record to the binary file using fwrite()
    for (int i = 0; i < 3; i++) {
        if (fwrite(&wstu[i], sizeof(Student), 1, file) != 1) {
            printf("Error writing to file!\n");
            fclose(file);
            return 1;
        }
    }

    fclose(file); // Close the file

    // Display student data
    printf("Student's Details:\n");
    for (int i = 0; i < 3; i++) {
        printf("Roll No: %d\n", wstu[i].roll_no);
        printf("Name: %s\n", wstu[i].name);
    }

    return 0;
}

Following is the output of the above program:

Student's Details:
Roll No: 1
Name: Revathi
Roll No: 2
Name: Vivek
Roll No: 3
Name: Tapas