Line reading
- C++ course
- Intermediate
- Lesson 92/116
- 5 min
Learn to read line by line with getline.
Reading lines with getline
std::getline reads from a file up to the end of a line, not to whitespace. This way you get lines that contain spaces in full.
Example: std::string line; std::getline(in, line);. This reads one line from the in stream and stores it in line. >> would stop at a space; getline stops at a newline and discards it. To process all lines in a file, while (std::getline(in, line)) is common: it loops once per successful read and stops at the end. This is the standard way to process text files line by line.
The getline loop
To process all lines in a file use while (std::getline(in, line)). The loop runs while reading succeeds and stops at the end of the file.
Example: while (std::getline(in, line)) { std::cout << line << "\n"; }. This prints every line in the file one below the other. The loop variable line is refilled on each iteration. Empty lines are also read by getline; in that case line is an empty string. Calling in.close(); after the loop to close the file is a good habit.
Practice Read the first line and print it
The starting code writes two lines to data.txt. Open the file with std::ifstream, read the first line with std::getline and print it with std::cout.
Starter code
#include <iostream>
#include <fstream>
#include <string>
int main() {
// Prepare the file
std::ofstream out("data.txt");
out << "First line\nSecond line";
out.close();
// Read the first line and print it
return 0;
}Sample solution
#include <iostream>
#include <fstream>
#include <string>
int main() {
std::ofstream out("data.txt");
out << "First line\nSecond line";
out.close();
std::ifstream in("data.txt");
std::string line;
std::getline(in, line);
std::cout << line;
return 0;
}Expected output
First line
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does std::getline(in, line) do?
std::string line;
std::getline(in, line);- Reads up to the end of the line Correct answer
- Reads one number
- Reads one character
- Closes the file
Explanation: getline reads up to the end of the line.
2 Match each piece to its role
| Term | Meaning |
|---|---|
std::getline |
Line reading |
>> |
Word reading |
in.close() |
Closing the file |
while (getline(...)) |
Line loop |
3 Complete the getline call
std::____(in, line);std::getline(in, line);Explanation: std::getline reads one line.