How does while (std::cin >> value) work?

Question

In an example given in C++ Primer,

#include <iostream>
using namespace std;

int main() {
    int sum = 0, value = 0;  
    while (std::cin >> value) {       
        sum += value; // equivalent to sum = sum + value
    }    
    std::cout << "Sum is: " << sum << std::endl;    
    return 0; 

}

How does (std::cin >> value) return true? And what is an "End Of File"? It seems that I must understand that term in order to understand my primary question.

Thanks!

Answer 1

The overloaded operator>> function returns a reference to the stream itself, and the stream have an overloaded operator that allows it to be used in a boolean condition to see if the last operation went okay or not. Part of the "okay or not" includes end of file reached, or other errors.