初学者的请教

#include <fstream>
#include <iostream>
#include <sstream>
#include <vector>

using namespace std;

int main()
{
    double d;
    string line;
    ifstream in("input.txt");
    ofstream out("output.txt");

    if (!in)                       //文件打开失败
    {
        cerr << "opening file failed!" << endl;
        return 1;
    }
     
    while (!in.eof())
    {   
        vector<double> vec;
        double sum = 0;
        getline(in,line);
        if (!line.empty())
        {
            istringstream stream(line);

            while (stream >> d)
            {
                vec.push_back(d);
            }

            for (vector<double> :: iterator iter=vec.begin(); iter!=vec.end(); ++iter)
            {
                sum += *iter;
            }
            
            out << sum << endl;
        }

    }

    return 0;
}