ifstream in("test.txt");
 if (!in.is_open()){
     cout << "Error opening file";
     exit (1);
 }
 string str;
 while( getline(in,str)){
     cout << str << endl;
 }

 char buffer[256];
 while((!in.eof()){
     in.getline (buffer,100);  // 读取最多99个字符('\0')，保存在buffer中，读到换行符或者终止符。
     // in.getline (buffer,100, 'c');  // 相比于上面，多了个若读到字符c，也结束。
     cout << buffer << endl;
 }

 ofstream out("test.txt");
 if (!out.is_open()){
     cout << "Error opening file";
     exit (1);
 }
 int i = 0;
 while(i < 100){
     cout<<"i = " << i << endl;
     i++;
 }