In this article, we’ll take a look at using the function std::getline() in C++. This is a very handy function if you want to read characters from an input stream.
Let’s find out how we can use this properly, using some illustrative examples.
Basic Syntax of std::getline() in C++
This function reads characters from an input stream and puts them onto a string.
We need to import the header file <string>, since getline() is a part of this file.
While this takes template arguments, we’ll focus on string inputs (characters) , since the output is written to a string.
istream& getline(istream& input_stream, string& output, char delim);
What this says is that getline() takes an input stream, and writes it to output. Delimiters can be optionally specified using delim.
This also returns a reference to the same input stream, but for most cases, we don’t need this handle.
Using std::getline() to read from input streams
Now that we know the basic syntax, let’s get input from std::cin (standard input stream) to a string.
#include <iostream>
#include <string>
int main() {
// Define a name (String)
std::string name;
std::cout << "Enter the name: ";
// Get the input from std::cin and store into name
std::getline(std::cin, name);
std::cout << "Hello " << name << "!\n";
return 0;
}
Output
Enter the name: JournalDev
Hello JournalDev!
Indeed, we were able to get the input from std::cin without any problems!
Let’s now take another example, where we have a file input.txt containing the following content:
$ cat input.txt
Hello from JournalDev
Second Line of file
Last line
Let’s now read the file line by line and store them into a vector of strings!
The core logic will be to keep reading using std::getline(file) until the input stream reaches EOF.
We can easily write this using this format:
std::ifstream infile("input.txt");
// Temporary buffer
std::string temp;
// Get the input from the input file until EOF
while (std::getline(infile, temp)) {
// Add to the list of output strings
outputs.push_back(temp);
}
The complete code is shown below:
#include <iostream>
#include <string>
#include <vector> // For std::vector
#include <fstream> // For std::ifstream and std::ofstream
int main() {
// Store the contents into a vector of strings
std::vector<std::string> outputs;
std::cout << "Reading from input.txt....\n";
// Create the file object (input)
std::ifstream infile("input.txt");
// Temporary buffer
std::string temp;
// Get the input from the input file until EOF
while (std::getline(infile, temp)) {
// Add to the list of output strings
outputs.push_back(temp);
}
// Use a range-based for loop to iterate through the output vector
for (const auto& i : outputs)
std::cout << i << std::endl;
return 0;
}
Output
Reading from input.txt....
Hello from JournalDev
Second Line of file
Last line
Using std::getline() in C++ to split the input using delimiters
We can also use the delim argument to make the getline function split the input in terms of a delimiter character.
By default, the delimiter is \n (newline). We can change this to make getline() split the input based on other characters too!
Let’s set the delim character to a space ‘ ‘ character to the above example and see what happens!
#include <iostream>
#include <string>
#include <vector> // For std::vector
#include <fstream> // For std::ifstream and std::ofstream
int main() {
// Store the contents into a vector of strings
std::vector<std::string> outputs;
std::cout << "Reading from input.txt....\n";
// Create the file object (input)
std::ifstream infile("input.txt");
// Temporary buffer
std::string temp;
// Get the input from the input file until EOF
while (std::getline(infile, temp, ' ')) {
// Add to the list of output strings
outputs.push_back(temp);
}
// Use a range-based for loop to iterate through the output vector
for (const auto& i : outputs)
std::cout << i << std::endl;
return 0;
}
Output
Reading from input.txt....
Hello
from
JournalDev
Second
Line
of
file
Last
line
Indeed, we have our space separated string now!
Potential Issues with using std::getline()
While std::getline() is a very useful function, there could be some problems that you may face when using it along with some input streams such as std::cin.
std::getline()does not ignore any leading white-space / newline characters.
Because of this, if you call std::cin >> var; just before getline(), there will be a newline still remaining in the input stream, after reading the input variable.
So, if you call getline() immediately after cin, you will get a newline instead, since it is the first character in the input stream!
To avoid this, simply add a dummy std::getline() to consume this new-line character!
The below program shows an issue with using cin just before getline().
#include <iostream>
#include <string>
int main() {
// Define a name (String)
std::string name;
int id;
std::cout << "Enter the id: ";
std::cin >> id;
std::cout << "Enter the Name: ";
// Notice std::cin was the last input call!
std::getline(std::cin, name);
std::cout << "Id: " << id << std::endl;
std::cout << "Name: " << name << "\n";
return 0;
}
Output
Enter the id: 10
Enter the Name: Id: 10
Name:
Notice that I wasn’t able to enter the name at all! Since a trailing newline was there in the input stream, it simply took that, and since it is a delimiter, it stopped reading!
Now let’s add a dummy std::getline() call just before our actual std::getline().
#include <iostream>
#include <string>
int main() {
// Define a name (String)
std::string name;
int id;
std::cout << "Enter the id: ";
std::cin >> id;
std::cout << "Enter the Name: ";
// Add a dummy getline() call
std::getline(std::cin, name);
// Notice std::cin was the last input call!
std::getline(std::cin, name);
std::cout << "Id: " << id << std::endl;
std::cout << "Name: " << name << "\n";
return 0;
}
Output
Enter the id: 10
Enter the Name: JournalDev
Id: 10
Name: JournalDev
We’ve finally fixed our bug! This hopefully makes you think a bit more before blindly using std::getline().
Unfortunately, there are no elegant methods to get input in C++, so we must make do with what we have!
Conclusion
In this article, we learned about using std::getline() in C++. We also look at some examples which illustrate the power, and pitfalls of this function.
References
- cppreference.com page std::getline()
- StackOverflow Question on using std::getline()
