How to read file line by line using ifstream in C++?

To read a file line by line in C++ using std::ifstream, follow these steps with detailed examples:

1. Include Required Headers

#include <iostream>
#include <fstream>
#include <string> // For std::string and std::getline

2. Open the File and Check for Success

Use std::ifstream to create a file input stream and check if the file opened successfully:

std::ifstream file("example.txt");
if (!file.is_open()) {
    std::cerr << "Failed to open the file!" << std::endl;
    return 1; // Exit with error code
}

3. Read Lines Using `std::getline`

Loop through the file line by line using std::getline, which reads until the newline (\n):

std::string line;
while (std::getline(file, line)) {
    // Process each line (e.g., print it)
    std::cout << line << '\n';
}

4. Close the File (Optional)

The file is automatically closed when the std::ifstream object goes out of scope, but you can explicitly close it:

file.close();

Complete Example

#include <iostream>
#include <fstream>
#include <string>

int main() {
    // Step 1: Open the file
    std::ifstream file("example.txt");
    if (!file.is_open()) {
        std::cerr << "Error: Could not open file!" << std::endl;
        return 1;
    }

    // Step 2: Read line by line
    std::string line;
    while (std::getline(file, line)) {
        std::cout << "Line: " << line << '\n';
    }

    // Optional: Check for errors (e.g., premature EOF)
    if (file.bad()) {
        std::cerr << "I/O error occurred!" << std::endl;
    } else if (!file.eof()) {
        std::cerr << "Unexpected end of file!" << std::endl;
    }

    // Step 3: Close the file (automatic if omitted)
    file.close();
    return 0;
}

Key Notes

Handling Empty Lines: std::getline will include empty lines (e.g., line.empty() == true).
Error Checking: Use file.eof() to check for end-of-file and file.bad() for I/O errors.
Line Endings: Works with \n (Unix), \r\n (Windows), and other formats.
Large Files: Efficiently handles large files since it reads line by line without loading the entire file into memory.

Advanced Use Cases

Example 1: Store Lines in a Vector

#include <vector>

std::vector<std::string> lines;
std::string line;
while (std::getline(file, line)) {
    lines.push_back(line);
}

Example 2: Skip Empty Lines

while (std::getline(file, line)) {
    if (!line.empty()) {
        std::cout << line << '\n';
    }
}

Example 3: Process Lines Conditionally

int line_number = 1;
while (std::getline(file, line)) {
    if (line.find("error") != std::string::npos) {
        std::cout << "Error found on line " << line_number << ": " << line << '\n';
    }
    line_number++;
}

Common Pitfalls

File Not Found: Always check is_open() before reading.
Unclosed File: Let the destructor handle closing unless explicitly required.
Incorrect Paths: Use absolute paths (e.g., "/path/to/file.txt") if the file isn’t in the working directory.

Summary

Use std::ifstream and std::getline for line-by-line reading.
Check for file open errors with is_open().
Handle edge cases like empty lines or read errors.
Prefer modern C++ practices (RAII) for resource management.