很诡异的现象----const引用问题

模块设计的原则是高内聚低耦合....看的好像有点乱..你可以调整下结构

#include<vector>
#include<string>
#include<fstream>
#include<iostream>
using namespace std;
namespace demo{
class  Information
{
private :
   vector<string> vc;
   class student
    {
        public:
        string num;
        string name;
        float  score[3];
    };
public:
      void save_file();
      string repeat(const string &str);
};
void Information::save_file()
{
    ofstream out;
    out.open("stu.txt",ios::out | ios::app);
    student stu;
    while(cin)
    {
        cout<<"\n请输入8位学号:";
        cin>>stu.num;
   
        if(!cin)
            break;
        string str=repeat(stu.num);
        vc.push_back(str);
        out<<str<<"\t";
   }
}

string Information::repeat(const string &str)
{
    ifstream in;
    in.open("stu.txt",ios::in);
    while(in)
    {
        student stu;
        in>>stu.num>>stu.name>>stu.score[0]>>stu.score[1]>>stu.score[2];
        vc.push_back(stu.num);
    }
   

    string m_str=str;
   
    for(vector<string>::size_type i=0;i!=vc.size()-1;++i)
        {
            if(m_str==vc[i])
            {
                i=vc.size()-1;
                cout<<"\n重复输入:";
                cout<<"\n请输入8位学号:";
                cin>>m_str;
                repeat(m_str);
            }
        }
   
    return m_str;
}
}
int main()
{
    demo::Information  inf;
    inf.save_file();
    return 0;
}