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.
Table of Contents
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()
