C++ : Deleting records from binary file

Question

I have the following class declaration:

class student
{
    int roll,marks;
    public:
    void input()
      {
        cout<<"Roll:";cin>>roll;
         cout<<"Marks:";cin>>marks;
      }
      void display()
      {
        cout<<"Roll:"<<roll<<endl;
         cout<<"Marks:"<<marks<<endl;
      }
      int retroll()
      {
        return roll;
      }
};

I am storing 3 records of the type student in a binary file

void writerec()
{
   student a;
   ofstream fout ("student.dat",ios::binary);
   for (int x = 1 ; x<=3 ; x++)
    {
      a.input();
      fout.write((char*)&a,sizeof(a));
   }
}

This is my function to read the records from the file

void readrec()
{
    student a;
    ifstream fin ("student.dat",ios::binary);
    for (int x = 1 ; x<=3 ; x++)
    {
       fin.read((char*)&a,sizeof(a));
       a.display();
    }
}

Here is my function to delete the records from the file

void deleterec(int r) //takes roll number that needs to be deleted 
{
   student a;
   ofstream fout ("temp.dat",ios::binary);
   ifstream fin ("student.dat",ios::binary);
   while(fin.read((char*)&a,sizeof(a)))
     if(a.retroll()!=r)
        fout.write((char*)&a,sizeof(a));
   fin.close();
   fout.close();
   remove("student.dat");
   rename("temp.dat","student.dat");
}

Here is my main function:

void main()
{
   student a;
   writerec();
   deleterec(2);
   cout<<"Record Deleted\n";
   readrec();
}

So I am deleting all the records with roll number 2.

Here is my sample input:

Roll:2

Marks:20

Roll:2

Marks:20

Roll:2

Marks:20

Here is my output:

Record Deleted

Roll:0

Marks:1

Roll:0

Marks:1

Roll:0

Marks:1

So my question is after deleting the 3 records with roll number 2 , how does the binary file still retain values . Also how come the value of marks is 1?

Thanks for the help :)

Answer 1

There is a problem in your readrec() function

You are forcefully reading three records from a file (even if they dont exist)

So, a.display() displays the initial values of variables (since nothing can be read from an empty file)

I think you need to modify readrec() as :

 while(fin.read((char*)&a,sizeof(a)))
     a.display();