文件读取姓名操作出现问题

string a;
getline(cin, a);

为什么要输入2次回车，才结束。怎样才能输入一次回车结束，避免用户不知道这情况！

// Note:Your choice is C++ IDE
#include <iostream>
#include <vector>
#include <string>
#include <fstream>
using namespace std;

struct cmd
{
    string a;
    string b;
    friend ostream& operator << (ostream &os, const cmd& t);
    friend istream& operator >> (istream &is, cmd& t);
};

ostream& operator << (ostream &os, const cmd& t)
{
    //cin.sync();
    os<<t.a<<'\t'<<t.b;
    return os;
}

istream& operator >> (istream &is, cmd& t)
{
    //is>>t.a>>t.b>>t.c;
    //cin.sync();
    getline(is, t.a);
    getchar();
    getline(is, t.b);
    return is;
}

int main()
{
    cmd a[2];
    cmd b;
    vector<cmd> obj;
   
    ofstream outfile("g:\\ct.txt", ios_base::app | ios_base::out);
    for(int i = 0 ; i != 2; ++i)
    {
        cin>>a[i];
        outfile<<a[i]<<endl;
    }
   
    outfile.close();
    outfile.clear();
   
    ifstream infile("g:\\ct.txt");
    while(!infile.eof())
    {
        infile>>b;
        obj.push_back(b);
        cout<<b<<endl;
    }
   
    for(vector<cmd>::iterator iter = obj.begin(); iter != obj.end(); ++iter)
    {
        cout<<*iter<<endl;
    }
   
    infile.close();
      
    return 0;
}