istream_iterator输入流结束问题

请看如下来自书《The C++ Standard Libray》代码：
//从标准输入读取所有输入文字\排序\将它们打印于屏幕
//stl/ioiter1.cpp

#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <iterator>
using namespace std;

int main()
{
vector<string> coll;

/*read all words from the standard input
*- source: all strings until end-of-file(or error)
*- destination: coll (inserting)
*/
copy (istream_iterator<string>(cin), //start of source
istream_iterator<string>(), //end of source
back_inserter(coll)); //destination

//sort elements
sort (coll.begin(), coll.end());

/*print all elements without duplicates
*- source: coll
*- destination: standard output (with newline between elements)
*/
unique_copy(coll.begin(), coll.end(), //source
ostream_iterator<string>(cout, "\n")); //destination
system("pause");
}

为什么要用两次ctrl Z再加回车来结束输入流?
当我连续输入两个相同的字符，比如'x'，'x'时，再按回车，为什么就直接结束程序了，后面的sort()等都没有执行？