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fpos Class

  • Artikkeli

The class template 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

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

Member functions

Member function Description
seekpos Used internally by the C++ Standard Library only. Don't call this method from your code.
state Sets or returns the conversion state.

Operators

Operator Description
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 _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 aren't 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. When working with files, the result is valid only for binary streams that don't 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

When working with files, the result is valid only for binary streams that don't 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 C++ Standard Library only. Don't 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