Modify record in a data file - c++

Question

I am creating a program to modify the records in a binary file. But i always end up creating two similar records.
I copy all the data to another file. if I find the record to be modified, I read the modified data and copy it to the new file. When all the records are copied, The remporary file is renamed
The class:
class student
{
char name[200];
int rno;
float marks;
char grad;
void grade()
{
if(marks < 33)
grad = 'F';
else if(marks < 50)
grad = 'D';
else if(marks < 60)
grad = 'C';
else if(marks < 80)
grad = 'B';
else
grad = 'A';
}
public:
void get()
{
fflush(stdin);
cout << "Name: ";
gets(name);
cout << "Roll number: ";
cin >> rno;
cout << "Marks:" ;
cin >> marks;
grade();
}
///////////////////////////////////////////////////////////
void showname()
{
cout << "Name: " << name << endl;
}
///////////////////////////////////////////////////////////
void show()
{
cout << "Name: " << name << endl;
cout << "Roll number: " << rno << endl;
cout << "Marks: " << marks << endl;
cout << "Grade: " << grad << endl;
}
///////////////////////////////////////////////////////////
int get_rno()
{
return rno;
}
////////////////////////////////////////////////////////////
float get_marks()
{
return marks;
}
////////////////////////////////////////////////////////////
char get_grade()
{
return grad;
}
////////////////////////////////////////////////////////////
char* getname()
{
return name;
}
};
The modify function:
void modify_rec()
{
int rno;
ifstream fin("stu.dat",ios::binary);
ofstream fout("modif.dat",ios::binary);
student s;
cout << "Enter the roll number whose details you want to edit:";
cin >> rno;
while(!fin.eof())
{
if(fin.eof())
break;
fin.read((char*)&s, sizeof(s));
if(s.get_rno() == rno)
{
s.get();
fout.write((char*)&s, sizeof(s));
rno = -1;
}
else
{
fout.write((char*)&s, sizeof(s));
}
}
fin.close();
fout.close();
remove("stu.dat");
rename("modif.dat","stu.dat");
}
Main function:
int main()
{
char op;
int ch;
do
{
system("CLS");
cout << "Select an option:" << endl;
cout << "\t1.)Add record." << endl;
cout << "\t2.)Display all." << endl;
cout << "\t3.)Search by roll number." << endl;
cout << "\t4.)Search by name" << endl;
cout << "\t5.)Modify record." << endl;
cout << "\t6.)Delete record." << endl;
cout << "\t7.)Display list of distinction holders." << endl;
cout << "\t8.)Create list of failures." << endl;
cout << "\t9.)Display report card." << endl;
cin >> ch;
switch(ch)
{
case 1: add_rec(); break;
case 2: disp_all(); break;
case 3: search_rno(); break;
case 4: search_name(); break;
case 5: modify_rec(); break;
case 6: delete_rec(); break;
case 7: disp_distinction(); break;
case 8: create_failure(); break;
case 9: disp_repcrd(); break;
}
cout << "Do you want to continue?" << endl;
cin >> op;
}while(tolower(op) == 'y');
}