istream_iterator with for loop

[pdk5]

Hello,
I have added the following function, just for learning about stls . Not professional code, so please bear with me

#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>

#include <iostream>
#include "string"
#include <vector>
#include "sstream"
#include "iterator"
#include "algorithm"
using namespace std;
// Write your Student class here
class Student
{
public:
	vector<int> scores;
	int sum;
public:
	Student() :scores(0), sum(0) {}

	int calculateTotalScore() { return sum; }

	void input()
	{
		string str;

		std::getline(cin, str);

		if (str.empty())
			std::getline(cin, str);

		std::stringstream ss(str);
		copy(istream_iterator<int>(ss), istream_iterator<int>(), back_inserter(scores));
		for_each(scores.begin(), scores.end(), [this](int p) {sum += p; });
		//for_each(istream_iterator<int>(ss), istream_iterator<int>(), [this](int p) {sum += p; });
	}
};
int main() {
	int n; // number of students
	cin >> n;

	//cin.ignore();
	Student *s = new Student[n]; // an array of n students

	for (int i = 0; i < n; i++) {
		s[i].input();
	}

	// calculate kristen's score
	int kristen_score = s[0].calculateTotalScore();

	// determine how many students scored higher than kristen
	int count = 0;
	for (int i = 1; i < n; i++) {
 		int total = s[i].calculateTotalScore();
		if (total > kristen_score) {
			count++;
		}
	}

	// print result
	cout << count;

	return 0;
}

Here i have a doubt why the red commented line doesnot work !!!.
Also, my doubt about cin and getline ignoring newline..I am bit confused about the input delimiters

like lets say:

line1\nline2\nline3\n.

How the getline and cin work ? I guess entire input is read as buffer and assigned to each getline/cin ?

thanks a lot
pdk