Having problem understanding following C++ code. (seekp)

Question

#include<stdio.h>
#include<fstream.h>
class Test
{
    char name[10];
    int data;
    public:
        void getData()
        {
            cin>>name;
            cin>>data;
        }
        void display()
        {
            cout<<name<<data;
        }
        void modify()
        {
            cin>>name;
            cin>>data;
        }
};
int main()
{
    Test t1,t2,t3,t4;
//  remove("FileIO.dat");
    t1.getData();
    t2.getData();
    t3.getData();
    t4.getData();
    fstream fp1("FileIO.dat",ios::out|ios::app);
    fp1.write((char*)&t1,sizeof(t1));
    fp1.write((char*)&t2,sizeof(t2));
    fp1.write((char*)&t3,sizeof(t3));
    fp1.write((char*)&t4,sizeof(t4));
    fp1.close();
    fstream fp2("FileIO.dat",ios::in|ios::out);
    fp2.read((char*)&t1,sizeof(t1));
    fp2.read((char*)&t2,sizeof(t2));
    int pos=-1*sizeof(t2); // ****** not understanding this line
    cout<<pos;
    fp2.seekp(pos,ios::cur);
    t2.modify();
    fp2.write((char*)&t2,sizeof(t2));
    fp2.read((char*)&t3,sizeof(t3));
    fp2.read((char*)&t4,sizeof(t4));
    t1.display();
    t2.display();
    t3.display();
    t4.display();
    fp2.close();
    return 0;
}

The program is written in turbo C++ and it deals with writing objects into the files and reading them back as well as updating the object that have been written into the file.

In the above code i am not understanding why -1 is multiplied by sizeof object to get the position. Anyone please explain.

score 7 · Accepted Answer · answered Oct 19 '11 at 22:48

Its because your program is reading the data from t1, reading the data from t2, modifying t2, and then writing over the contents of t2.

When the fp2 is first opened, the file looks like this (^ represents the current position of the file pointer):

+-----------+-----------+-----------+-----------+
|  t1 data  |  t2 data  |  t3 data  |  t4 data  |
+-----------+-----------+-----------+-----------+
^
|

After reading t1 and t2, the pointer is now going to be pointing to the beginning of t3:

+-----------+-----------+-----------+-----------+
|  t1 data  |  t2 data  |  t3 data  |  t4 data  |
+-----------+-----------+-----------+-----------+
                        ^
                        |

Now, in order to write over t2's data, we need to move the file pointer back to the start of t2. How far back is that? -1 * sizeof(t2):

+-----------+-----------+-----------+-----------+
|  t1 data  |  t2 data  |  t3 data  |  t4 data  |
+-----------+-----------+-----------+-----------+
                        ^
                        |
            <-----------+
                  |
                  This distance == sizeof(t2)

From there, your program runs fp2.seekp(pos,ios::cur);. Since pos is negative, it moves the file pointer backwards, and your file is left in this state:

+-----------+-----------+-----------+-----------+
|  t1 data  |  t2 data  |  t3 data  |  t4 data  |
+-----------+-----------+-----------+-----------+
            ^
            |

And now you can overwrite t2's data.

Thanks a lot for making myself clear.Clearly understood the concept... :-) — ATR, Oct 19 '11 at 22:53
@ankur.trapasiya, ASCII art is the solution to all of the worlds problems :) — riwalk, Oct 19 '11 at 22:54

Having problem understanding following C++ code. (seekp)

1 Answers1