C++ In a Nutshell

Name

fpos class template — Represents a file position

Synopsis

template <typename stateT>
class fpos{
public:
  stateT state(  ) const;
  void state(stateT);
   
  // The following functionality is required, although not necessarily as member
  // functions.
  fpos(int i);
  fpos(streamoff offset);
  operator streamoff(  ) const;
  bool operator==(const fpos& rhs) const;
  bool operator!=(const fpos& rhs) const;
  fpos operator+(streamoff offset) const;
  fpos operator-(streamoff offset) const;
  fpos& operator+=(streamoff offset);
  fpos& operator-=(streamoff offset);
  streamoff operator-(const fpos& rhs) const;
};

The fpos class template represents a position in a stream. The stateT template parameter is a multibyte shift state, such as mbstate_t. Objects of type fpos can be compared for equality or inequality, they can be subtracted to yield a stream offset, or a stream offset can be added to an fpos position to produce a new fpos. Also, stream offsets can be converted to and from fpos values. Although the declaration in this section shows these functions as member functions, they might be global functions or be provided in some other fashion.

The Shift State Problem

The C++ standard requires a library implementation to implement fpos<> and the streamoff types so that you can convert an object whose type is any fpos<> instance into a streamoff object and back again, recovering the original value.

Most library implementations do not work this way.

Instead, streamoff is usually implemented as an integral type, such as ...