std::getline() and std::cin in C++

getline() 

istream& getline (istream& is, string& str, char delim);

It reads the input from the sequence of characters entered by user until a new line character(\n) is found. It then replaces the str by the newly extracted sequence. 

Getting an Input from user

#include <iostream>

int main()
{
    std::string word = "";
    
    getline(std::cin, word, ' ');
    std::cout<<"User Entered word is " << word << std::endl;

    return 0;
}

Output:

shirley hello User Entered word is shirley hello

Let's see what is happening in the below program.

#include <iostream>

int main()
{
    std::string userInput;
    std::cout << "Enter User Input: \n";
    std::cin >> userInput;
    
    std::string userLament;
    std::cout << "Enter User Lamentations: \n";
    std::getline(std::cin, userLament);
    
    std::cout << "\nuserInput " << userInput 
              << " userLament " << userLament << std::endl;

    return 0;
}

Output:

Enter User Input: Shirley Enter User Lamentations: userInput Shirley userLament

Click here to debug. 

My program first reads a string with cin >> userInput, and then a line with getline(cin, userLament). 

The user types a string, followed by the ENTER key. The cin >> userInput will read the string, but the ENTER key, interpreted as a newline character, will be left in the input buffer.

When your program then goes on to read a complete line with getline(cin, userLament), it will read the very short line that consists of just that left-over newline character. This looks like the program "skips to the next read".

Reference

getline and cin difference