Reading Files in C++

Our first example demonstrates reading files in C++. Reading and writing files are basic tasks needed for many C++ programs. Let’s look at some examples of reading files.

#include <iostream>
#include <fstream>
#include <vector>
#include <stdexcept>

// Helper function to check for errors
void check(bool condition, const std::string& message) {
    if (!condition) {
        throw std::runtime_error(message);
    }
}

int main() {
    // Perhaps the most basic file reading task is
    // slurping a file's entire contents into memory.
    std::ifstream file("/tmp/dat");
    check(file.is_open(), "Failed to open file");
    
    std::string content((std::istreambuf_iterator<char>(file)),
                         std::istreambuf_iterator<char>());
    std::cout << content;

    // You'll often want more control over how and what
    // parts of a file are read. For these tasks, we can use
    // various methods of std::ifstream.
    file.clear();
    file.seekg(0, std::ios::beg);

    // Read some bytes from the beginning of the file.
    // Allow up to 5 to be read but also note how many
    // actually were read.
    std::vector<char> b1(5);
    file.read(b1.data(), 5);
    int n1 = file.gcount();
    std::cout << n1 << " bytes: " << std::string(b1.begin(), b1.begin() + n1) << "\n";

    // You can also seek to a known location in the file
    // and read from there.
    file.seekg(6, std::ios::beg);
    std::vector<char> b2(2);
    file.read(b2.data(), 2);
    int n2 = file.gcount();
    std::cout << n2 << " bytes @ 6: " << std::string(b2.begin(), b2.begin() + n2) << "\n";

    // Other methods of seeking are relative to the
    // current position or the end of the file.
    file.seekg(4, std::ios::cur);
    file.seekg(-10, std::ios::end);

    // The C++ standard library provides some functions that may
    // be helpful for file reading. For example, we can use
    // std::getline to read a line from the file.
    file.seekg(6, std::ios::beg);
    std::string line;
    std::getline(file, line);
    std::cout << line.length() << " bytes @ 6: " << line << "\n";

    // There is no built-in rewind, but
    // seekg(0, std::ios::beg) accomplishes this.
    file.seekg(0, std::ios::beg);

    // The C++ standard library also provides a buffered reader
    // in the form of std::istream. It's already being used by
    // std::ifstream, so we don't need to create a separate one.
    char b4[5];
    file.read(b4, 5);
    std::cout << "5 bytes: " << std::string(b4, 5) << "\n";

    // Close the file when you're done (this is done automatically
    // when the std::ifstream object goes out of scope).
    file.close();

    return 0;
}

To run this program, you would first need to create a file at /tmp/dat with some content. Then compile and run the C++ program:

$ echo "hello" > /tmp/dat
$ echo "cpp" >>   /tmp/dat
$ g++ reading-files.cpp -o reading-files
$ ./reading-files
hello
cpp
5 bytes: hello
2 bytes @ 6: cp
2 bytes @ 6: cpp
5 bytes: hello

This example demonstrates various ways to read files in C++, including reading the entire file at once, reading specific portions, seeking to different positions, and using buffered reading. The C++ standard library provides powerful tools for file I/O through the <fstream> header.

Next, we’ll look at writing files in C++.

Base64 Encoding in C++

Embed Directive in C++

Learn X By Example

Title here

Reading Files in C++