File Pointers

The C++ input and output system manages two integer values associates with a file.
These are:

• get pointer – specifies the location in a file where the next read operation will occur.
• put pointer – specifies the location in a file where the next write operation will occur.
In other words, these pointers indicate the current positions for read and write operations, respectively. Each time an input or an output operation takes place, the pointers are automatically advances sequentially.

The term pointers should not be confused with normal C++ pointers used as address variables.

Often you may want to start reading an existing file from the beginning and continue sequentially until the end. When writing, you ma want to start from the beginning, deleting any existing contents, or appending new records (in which case you can open the file with the ios:app mode specifier). These are default actions, so no manipulation of the pointers is necessary.

Sometimes you may have to manipulate file pointers to read from and write to a particular location in a file. The seekg() and tellg() functions allow you to set and examine the get pointer, and the seekp() and tellp() functions perform these same actions on the put pointer. In other words, these four functions allow you to access the file in a non-sequential or random mode.

All the iostream clas objects can be repositioned by using either the seekg() or the seekp() member function. These functions move the get and put pointers respectively to an absolute address within the file or toa certain number of bytes from a particular position.

The tellg() and tellp() functions can be used to find out the current position of the get and put file pointers respectively in a file.

The seekg() member function takes two arguments:
• Number of bytes to move.
• Reference in the file from which the pointer has to be repositioned.
For example:
If stream iFi1;
iFi1.seekg(10,ios:beg);
means, “position the get pointer 10 bytes from the beginning of the file”

The first argument is an integer that specifies the number of bytes positions(also called offset). The second argument is the reference point. There are three reference points defined in the ios class:
• ios:beg – the beginning of the file.
• ios:cur – the current position of the file pointer
• ios:end – the end of the file

A negative value can also be specified for the first argument. For example, the following statement moves the file pointer 20 bytes backward from the end of the file.
iFi1.seekg(-20,ios:end);

If the seekg() function is used with only one argument, the ios:beg reference point is assumed. For example in the statement:
iFi1.seekg(16);

ios:beg will be the reference point and hence the get pointer will be positioned 16 bytes from the beginning of the file.

The tellg() member function does not have any arguments. It returns the current byte position of the get pointer relative to the beginning of the file. For example the statement:
intiPosition = iFi1.tellg();
will result in the variable iPosition having the value of the current position of the get pointer.

The seekp() and tellp() member functions are identical to the above two functions, but they are identified with the put pointer. The seekg() and tellg() member functions are defined in the istream class. The seekp() and tellp() member functions are defined in the istream class. The seekp() and tellp() member functions are defined in the ostream class.

Program 20.1 finds out the number of records in the file billfile.dat by using the seekg() and tellg() functions.

#include<fstream.h>
#include<iostream.h>
class bill
{
private:
intiBill_no;
floatfBill_amt;
public:
void getdata()
{
cout<<”Enter Bill number”;
cin>>iBill_no;
cout<<”Enter Bill amount”;
cin>>fBill_amt;
}
void showdata()
{
cout<<”Bill number ”<<iBill_no<<endl;
cout<<”Bill amount ”<<fBill_amt<<endl;
}
};
void main()
{
fstream Fi1(“billfile.dat”,ios::in);
Fi1.seekg(0,ios::end);
ini iEnd;
iEnd=Fi1.tellg();
cout<<”The size of the file is “<<iEnd<<endl;
cout<<”Size of one record is ”<<sizeof(bill)<<endl’
ini iNorec=iEnd/sizeof(bill);
cout<<”There are “<<iNored<<”records in the file”<<endl;
}

In the given program, the statement:
Fi1.seekg(0,ios::end);
is used to reach the end-of-file. The byte position is found by the following statement:
iEnd=Fi1.tellg();

The value of iEnd is divided by the size of one record to find out the number of records in the file.