Understanding the problem after 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 C ++ turbo and is engaged in writing objects to files and reading them, as well as updating the object that was written to the file.

In the above code, I don't understand why -1 is multiplied by the sizeof of the object to get the position. Someone please explain.

Answer 1

Because your program reads data from t1, reading data from t2, changing t2, and then writing the contents of t2.

When fp2 is opened first, 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 will now point to the beginning of t3:

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

Now, to write t2 data, we need to move the file pointer back to the beginning of t2. How far is it? -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 back, and your file remains in this state:

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

And now you can overwrite t2 data.