std::fpos

 

C++

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Technical Specifications

 

Input/output library

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basic_streambuf

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Array I/O

istrstream (deprecated)

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Types

streamoff

streamsize

fpos

Error category interface

iostream_category (C++11)

io_errc (C++11)

 

Defined in header <ios>
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:

Type	Definition
streampos	std::fpos<std::char_traits<char>::state_type>
wstreampos	std::fpos<std::char_traits<wchar_t>::state_type>

In addition, std::u16streampos and std::u32streampos are provided as implementation-defined types that satisfy the requirements of pos_type

(since C++11)

Member functions

state

gets/sets the value of the shift state (public member function)

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

streamoff	represents relative file/stream position (offset from fpos), sufficient to represent any file size (typedef)
tellp	returns the output position indicator (public member function of std::basic_ostream)
seekp	sets the output position indicator (public member function of std::basic_ostream)
fgetpos	gets the file position indicator (function)