关于fstream的用法，求解

royal夜枫

#include <fstream>
#include <string>
#include <iostream>
using namespace std;
string readstr(string s)
{
    int i=0;
    string a,b;
    const char* str=s.data();
    ifstream fin( str );
    while(fin>>a[i])
    {
        cout<<a[i];//这里就可以输出字符
        i++;
    }
    cout<<a;//这里就不能输出a了
    return a;//返回值会为空
    fin.close();
}

main()
{
    string filename1,sequence1;
    cout<<"please input the filename about string:"<<endl;
    cin>>filename1;
    filename1=filename1+".txt";
    sequence1=readstr(filename1);
    cout<<sequence1<<endl;
    return 0;
}