这个正确的程序为啥运行后输入没反应呢

就是输入“a b ee gg”后按回车了啊，小写变大写的程序
#include <iostream>
#include <vector>
#include <string>

using std::cin;
using std::cout;
using std::endl;
using std::vector;
using std::string;

int main()
{
    vector<string> vec;
    for (string word; cin >> word; vec.push_back(word));
    for (auto &str : vec) for (auto &c : str) c = toupper(c);

    for (int i = 0; i != vec.size(); ++i)
    {
        if (i != 0 && i % 8 == 0) cout << endl;
        cout << vec[i] << " ";
    }
    cout << endl;

    return 0;
}