std::fpos

template< class State >
class fpos;

Specializations of the class template std::fpos identify absolute positions in a stream or in a file. Each object of type fpos holds the byte position in the stream (typically as a private member of type std::streamoff) and the current shift state, a value of type State (typically std::mbstate_t).

The following specializations of std::fpos are provided:

Member functions

In addition, the following member functions and operators must be provided, although it's unspecified if they are members or non-member.

• A constructor that accepts an argument of type int.
• A constructor that accepts an argument of type std::streamoff. This constructor must also accept the special value std::streamoff(-1): the std::fpos constructed in this manner is returned by some stream operations to indicate errors.
• A conversion operator that converts fpos to std::streamoff with the value equal to the offset from std::fpos(0).
• operator== that compares two objects of type std::fpos and returns a value of type convertible to bool
• operator!= that compares two objects of type std::fpos and returns a value of type convertible to bool
• operator+ and operator+= which can add std::streamoff to std::fpos
• operator- and operator-= which can subtract std::streamoff from an std::fpos
• operator- which can subtract two objects of type std::fpos producing an std::streamoff

Notes

std::streampos and std::wstreampos are required to be the same type because std::char_traits<char>::state_type and std::char_traits<wchar_t>::state_type are required to both be std::mbstate_t. C++98 had a self-contradictory statement that they may be different if the implementation supports no shift encoding in narrow-oriented iostreams but supports one or more shift encodings in wide-oriented streams, but that was corrected in C++03.

Some of the I/O streams member functions return and manipulate objects of member typedef pos_type. For streams, these member typedefs are provided by the template parameter Traits, which defaults to std::char_traits, which define their pos_types to be specializations of std::fpos. The behavior of the I/O streams library is implementation-defined when Traits::pos_type is not std::fpos<std::mbstate_t> (aka std::streampos or std::wstreampos).

See also