How to read/write a binary file in C++？

In C++, reading and writing binary files primarily rely on the input/output stream library (<fstream>), which includes ifstream for reading files and ofstream for writing files.

Writing Binary Files

To write to a binary file, use ofstream and specify the ios::binary flag when opening the file, indicating binary mode. Here is an example code snippet demonstrating how to write binary data:

cpp
#include <fstream>
#include <iostream>

int main() {
    // Create an ofstream object for writing to the file in binary mode
    std::ofstream outFile("example.bin", std::ios::binary);
    if (!outFile) {
        std::cerr << "Unable to open file for writing" << std::endl;
        return 1;
    }

    int data = 12345;
    // Write data
    outFile.write(reinterpret_cast<const char*>(&data), sizeof(data));

    // Close the file
    outFile.close();

    return 0;
}

In this example, we first include the <fstream> and <iostream> header files. The ofstream object outFile creates a file named example.bin. The write function writes the integer data in binary format to the file. The first parameter is a pointer to the data (here, reinterpret_cast converts the integer's address to const char*), and the second parameter specifies the number of bytes to write.

Reading Binary Files

Reading binary files follows a similar process but uses ifstream. Here is an example code snippet demonstrating how to read binary data:

cpp
#include <fstream>
#include <iostream>

int main() {
    // Create an ifstream object for reading the file in binary mode
    std::ifstream inFile("example.bin", std::ios::binary);
    if (!inFile) {
        std::cerr << "Unable to open file for reading" << std::endl;
        return 1;
    }

    int data;
    // Read data
    inFile.read(reinterpret_cast<char*>(&data), sizeof(data));

    // Output the read data
    std::cout << "Read data: " << data << std::endl;

    // Close the file
    inFile.close();

    return 0;
}

In this example, we use ifstream to open the file and the read function to read binary data from the file. The parameters for read are similar to write, requiring the pointer to the data and the size of the data.

Summary

Reading and writing binary files in C++ is straightforward, with the key being to use the correct file mode (std::ios::binary) and the appropriate methods (write and read). This approach is very useful for handling binary data such as image files, custom data records, etc.