Why Does std::getline() Skip Input After a Formatted Extraction?

In C++, the std::getline() function is a common method for reading a line of text from an input stream. However, when used after a formatted extraction operation, it may sometimes skip input. In this article, we will learn why does std::getline() skips input after a formatted extraction and how to prevent it.

Problem of std::getline() Skipping Input After a Formatted Extraction

The std::getline() function skips input when used after a formatted extraction operation. This issue arises due to the behavior of formatted extraction operations that are used to extract data in a predefined format from an input stream, such as std::cin >>, which leaves the newline character (‘\n’) in the input buffer. When std::getline() is called after that, it reads until it encounters a newline character, which is still in the buffer from the previous input. As a result, std::getline() immediately stops reading and returns an empty string, giving the impression that it has skipped input.

Example 1: Showing the Case where  std::getline() Skips the Input

C++

// C++ Program where getline() skips input after a formatted
// extraction
#include <iostream>
#include <string>
using namespace std;

int main()
{
    int num;
    string str;

    cout << "Enter a number: ";
    // read a number
    cin >> num;

    cout << "Enter a string: ";
    // try to read string
    getline(cin, str);
    cout << endl;

    cout << "Number: " << num << endl;
    cout << "String: " << str << endl;

    return 0;
}

Output

Enter a number: 5
Enter a string: 
Number: 5
String:

Explanation: In the above code, we entered a number and then a string, we noticed that the program doesn’t pause for the string input and str remains empty only.

Preventing std::getline() from Skipping Input

To resolve the above issue we can use the std::cin.ignore() function after the formatted extraction operation that extracts and discards characters from the input buffer until it encounters a newline character or reaches a specified limit.

C++ Program to Use getline() Without Skipping Input after a Formatted Extraction

The below example demonstrates how we can take input using gettline() without skipping input after a formatted extraction.

C++

// C++ Program to demonstrates how we can take input using
// gettline() without skipping input after a formatted
// extraction.
#include <iostream>
#include <string>
using namespace std;
int main()
{
    int number;
    string str;

    // Read an integer
    cout << "Enter a number: ";
    cin >> number;

    // Ignore the newline character left in the buffer
    cin.ignore();

    // Read a line of text
    cout << "Enter a string: ";
    getline(cin, str);

    // print the number and string
    cout << "Number: " << number << endl;
    cout << "String: " << str << endl;

    return 0;
}

Output

Enter a number: 5
Enter a string: Hello
Number: 5
String: Hello

Explanation: In the above code, std::cin.ignore() discards all characters up to the end including the next newline character. Now, the program correctly pauses for the string input after the number input.