fpos Class

 

The latest version of this topic can be found at fpos Class.

The template class describes an object that can store all the information needed to restore an arbitrary file-position indicator within any stream. An object of class fpos< St> effectively stores at least two member objects:

• A byte offset, of type streamoff.

• A conversion state, for use by an object of class basic_filebuf, of type St, typically mbstate_t.

It can also store an arbitrary file position, for use by an object of class basic_filebuf, of type fpos_t. For an environment with limited file size, however, streamoff and fpos_t may sometimes be used interchangeably. For an environment with no streams that have a state-dependent encoding, mbstate_t may actually be unused. Therefore, the number of member objects stored may vary.

Syntax

template <class Statetype>  
class fpos  

Parameters

Statetype
State information.

Constructors

fpos	Create an object that contains information about a position (offset) in a stream.

Member Functions

seekpos	Used internally by the Standard C++ Library only. Do not call this method from your code.
state	Sets or returns the conversion state.

Operators

operator!=	Tests file-position indicators for inequality.
operator+	Increments a file-position indicator.
operator+=	Increments a file-position indicator.
operator-	Decrements a file-position indicator.
operator-=	Decrements a file-position indicator.
operator==	Tests file-position indicators for equality.
operator streamoff	Casts object of type fpos to object of type streamoff.

Requirements

Header: <ios>

Namespace: std

fpos::fpos

Create an object that contains information about a position (offset) in a stream.

fpos(streamoff _Off =0);

fpos(Statetype _State, fpos_t _Filepos);

Parameters

_Off
The offset into the stream.

_State
The starting state of the fpos object.

_Filepos
The offset into the stream.

Remarks

The first constructor stores the offset _Off, relative to the beginning of file and in the initial conversion state (if that matters). If _Off is -1, the resulting object represents an invalid stream position.

The second constructor stores a zero offset and the object _State.

fpos::operator!=

Tests file-position indicators for inequality.

bool operator!=(const fpos<Statetype>& right) const;

Parameters

right
The file-position indicator against which to compare.

Return Value

true if the file-position indicators are not equal, otherwise false.

Remarks

The member function returns !( *this == right).

Example

// fpos_op_neq.cpp  
// compile with: /EHsc  
#include <fstream>  
#include <iostream>  
  
int main( )  
{  
   using namespace std;  
  
   fpos<int> pos1, pos2;  
   ifstream file;  
   char c;  
  
   // Compare two fpos object  
   if ( pos1 != pos2 )  
      cout << "File position pos1 and pos2 are not equal" << endl;  
   else  
      cout << "File position pos1 and pos2 are equal" << endl;  
  
   file.open( "fpos_op_neq.txt" );  
   file.seekg( 0 );   // Goes to a zero-based position in the file  
   pos1 = file.tellg( );  
   file.get( c);  
   cout << c << endl;  
  
   // Increment pos1  
   pos1 += 1;  
   file.get( c );  
   cout << c << endl;  
  
   pos1 = file.tellg( ) - fpos<int>( 2);  
   file.seekg( pos1 );  
   file.get( c );  
   cout << c << endl;  
  
   // Increment pos1  
   pos1 = pos1 + fpos<int>( 1 );  
   file.get(c);  
   cout << c << endl;  
  
   pos1 -= fpos<int>( 2 );  
   file.seekg( pos1 );  
   file.get( c );  
   cout << c << endl;  
  
   file.close( );  
}  

fpos::operator+

Increments a file-position indicator.

fpos<Statetype> operator+(streamoff _Off) const;

Parameters

_Off
The offset by which you want to increment the file-position indicator.

Return Value

The position in the file.

Remarks

The member function returns fpos(*this) += _Off.

Example

See operator!= for a sample of using operator+.

fpos::operator+=

Increments a file-position indicator.

fpos<Statetype>& operator+=(streamoff _Off);

Parameters

_Off
The offset by which you want to increment the file-position indicator.

Return Value

The position in the file.

Remarks

The member function adds _Off to the stored offset member object and then returns *this. For positioning within a file, the result is generally valid only for binary streams that do not have a state-dependent encoding.

Example

See operator!= for a sample of using operator+=.

fpos::operator-

Decrements a file-position indicator.

streamoff operator-(const fpos<Statetype>& right) const;
fpos<Statetype> operator-(streamoff _Off) const;

Parameters

right
File position.

_Off
Stream offset.

Return Value

The first member function returns (streamoff)*this - (streamoff)right. The second member function returns fpos(*this) -= _Off.

Example

See operator!= for a sample of using operator-.

fpos::operator-=

Decrements a file-position indicator.

fpos<Statetype>& operator-=(streamoff _Off);

Parameters

_Off
Stream offset.

Return Value

The member function returns fpos(*this) -= _Off.

Remarks

For positioning within a file, the result is generally valid only for binary streams that do not have a state-dependent encoding.

Example

See operator!= for a sample of using operator-=.

fpos::operator==

Tests file-position indicators for equality.

bool operator==(const fpos<Statetype>& right) const;

Parameters

right
The file-position indicator against which to compare.

Return Value

true if the file-position indicators are equal; otherwise false.

Remarks

The member function returns (streamoff)*this == (streamoff)right.

Example

See operator!= for a sample of using operator+=.

fpos::operator streamoff

Cast object of type fpos to object of type streamoff.

operator streamoff() const;

Remarks

The member function returns the stored offset member object and any additional offset stored as part of the fpos_t member object.

Example

// fpos_op_streampos.cpp  
// compile with: /EHsc  
#include <ios>  
#include <iostream>  
#include <fstream>  
  
int main( )   
{  
   using namespace std;  
   streamoff s;  
   ofstream file( "rdbuf.txt");  
  
   fpos<mbstate_t> f = file.tellp( );  
   // Is equivalent to ..  
   // streampos f = file.tellp( );  
   s = f;  
   cout << s << endl;  
}  

0  

fpos::seekpos

This method is used internally by the Standard C++ Library only. Do not call this method from your code.

fpos_t seekpos() const;

fpos::state

Sets or returns the conversion state.

Statetype state() const;

void state(Statetype _State);

Parameters

_State
The new conversion state.

Return Value

The conversion state.

Remarks

The first member function returns the value stored in the St member object. The second member function stores _State in the St member object.

Example

// fpos_state.cpp  
// compile with: /EHsc  
#include <ios>  
#include <iostream>  
#include <fstream>  
  
int main() {  
   using namespace std;  
   streamoff s;  
   ifstream file( "fpos_state.txt" );  
  
   fpos<mbstate_t> f = file.tellg( );  
   char ch;  
   while ( !file.eof( ) )  
      file.get( ch );  
   s = f;  
   cout << f.state( ) << endl;  
   f.state( 9 );  
   cout << f.state( ) << endl;  
}  

See Also

Thread Safety in the C++ Standard Library
iostream Programming
iostreams Conventions