vector的简单函数

请问下我明明写的是double型的怎么结果却是int的本人用的是VC
#include<iostream>
#include<vector>
using namespace std;
int main()
{
    vector<double> dou;
    double result;
    double element;
    while(cin>>element)
        dou.push_back(element);
    double sum(vector<double>::iterator,vector<double>::iterator);
    result=sum(dou.begin(),dou.end());
    cout<<"result="<<result<<endl;
    return 0;
}
double sum(vector<double>::iterator beg,vector<double>::iterator end)
{
    int sum=0;
    for(;beg!=end;++beg)
        sum+=*beg;
    return sum;
}

int sum=0;           //是不是这个定义搞的鬼？
    for(;beg!=end;++beg)
        sum+=*beg;
    return sum;

对就是这个原因