Reading/Writing Binary Files with read() and write()

Intermediate13 min read•Updated: 2026-10-22

💾 Reading and Writing Binary Files with read() and write()

Introduction

Text files store human-readable ASCII/UTF-8 strings. While easy to read in a text editor, converting numeric types to text strings incurs formatting overhead and consumes extra space. Binary files dump exact in-memory bytes directly to disk, providing maximum I/O throughput.

⚙️ Binary Serialization Syntax

To read or write binary memory, open files with the flag std::ios::binary and use:

stream.write(reinterpret_cast<const char*>(&obj), sizeof(obj));
stream.read(reinterpret_cast<char*>(&obj), sizeof(obj));

💻 Full Working Example

1#include <iostream>
2#include <fstream>
3 
4struct CourseRecord {
5    int courseId;
6    double fee;
7    int durationWeeks;
8};
9 
10int main() {
11    CourseRecord recordOut{105, 3499.0, 8};
12 
13    // 1. Write binary data to disk
14    {
15        std::ofstream binaryOut("courses.dat", std::ios::binary | std::ios::out);
16        if (binaryOut) {
17            binaryOut.write(reinterpret_cast<const char*>(&recordOut), sizeof(CourseRecord));
18            std::cout << "Wrote " << sizeof(CourseRecord) << " raw bytes to courses.dat\n";
19        }
20    }
21 
22    // 2. Read binary data back into memory
23    CourseRecord recordIn{};
24    {
25        std::ifstream binaryIn("courses.dat", std::ios::binary | std::ios::in);
26        if (binaryIn) {
27            binaryIn.read(reinterpret_cast<char*>(&recordIn), sizeof(CourseRecord));
28            std::cout << "Read back -> ID: " << recordIn.courseId 
29                      << " | Fee: ₹" << recordIn.fee 
30                      << " | Duration: " << recordIn.durationWeeks << " weeks\n";
31        }
32    }
33 
34    return 0;
35}

⚠️ Important Constraint

Only Plain Old Data (POD) structs or Trivially Copyable types can be serialized using raw byte dumps. Classes containing pointers, std::string, or virtual tables (vptr) cannot be dumped with reinterpret_cast because pointer addresses become invalid once the process restarts.

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String Streams with std::stringstream for Parsing

Reading/Writing Binary Files with read() and write()

Intermediate13 min read•Updated: 2026-10-22

Study Materials

💾 Reading and Writing Binary Files with read() and write()

Introduction

⚙️ Binary Serialization Syntax

To read or write binary memory, open files with the flag std::ios::binary and use:

stream.write(reinterpret_cast<const char*>(&obj), sizeof(obj));
stream.read(reinterpret_cast<char*>(&obj), sizeof(obj));

💻 Full Working Example

1#include <iostream>
2#include <fstream>
3 
4struct CourseRecord {
5    int courseId;
6    double fee;
7    int durationWeeks;
8};
9 
10int main() {
11    CourseRecord recordOut{105, 3499.0, 8};
12 
13    // 1. Write binary data to disk
14    {
15        std::ofstream binaryOut("courses.dat", std::ios::binary | std::ios::out);
16        if (binaryOut) {
17            binaryOut.write(reinterpret_cast<const char*>(&recordOut), sizeof(CourseRecord));
18            std::cout << "Wrote " << sizeof(CourseRecord) << " raw bytes to courses.dat\n";
19        }
20    }
21 
22    // 2. Read binary data back into memory
23    CourseRecord recordIn{};
24    {
25        std::ifstream binaryIn("courses.dat", std::ios::binary | std::ios::in);
26        if (binaryIn) {
27            binaryIn.read(reinterpret_cast<char*>(&recordIn), sizeof(CourseRecord));
28            std::cout << "Read back -> ID: " << recordIn.courseId 
29                      << " | Fee: ₹" << recordIn.fee 
30                      << " | Duration: " << recordIn.durationWeeks << " weeks\n";
31        }
32    }
33 
34    return 0;
35}

⚠️ Important Constraint

Next Lesson

String Streams with std::stringstream for Parsing

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous Lesson	Next Lesson
File I/O with std::ifstream and std::ofstream	String Streams with std::stringstream for Parsing

Practice Quiz

Test your understanding of this lesson with 1 questions. Each question has one correct answer.

Previous Next