std::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 typedef names for std::fpos<std::mbstate_t > are provided (although they are spelled differently in the standard, they denote the same type):

Defined in header <iosfwd>

Type Definition

std::streampos std::fpos<std::char_traits <char>::state_type>

std::wstreampos std::fpos<std::char_traits <wchar_t>::state_type>

std::u8streampos(C++20) std::fpos<std::char_traits <char8_t>::state_type>

std::u16streampos (C++11) std::fpos<std::char_traits <char16_t>::state_type>

std::u32streampos (C++11) std::fpos<std::char_traits <char32_t>::state_type>

All specializations of fpos meet the DefaultConstructible, CopyConstructible, CopyAssignable, Destructible, and EqualityComparable requirements.

If State is trivially copy constructible, fpos has a trivial copy constructor. If State is trivially copy assignable, fpos has a trivial copy assignment operator. If State is trivially destructible, fpos has a trivial destructor.

A non-explicit constructor that accepts an argument of type (possibly const) std::streamoff , which stores that offset and value-initializes the state object. 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.

Explicit conversion from (possibly const) fpos to std::streamoff . The result is the stored offset.

operator== and operator!= that compare two objects of type (possibly const) std::fpos and return a bool prvalue. p != q is equivalent to !(p == q).

operator+ and operator- such that, for an object p of type (possibly const) fpos<State> and an object o of type (possibly const) std::streamoff

p + o has type fpos<State> and stores an offset that is the result of adding o to the offset of p.
o + p has a type convertible to fpos<State> and the result of the conversion is equal to p + o.
p - o has type fpos<State> and stores an offset that is the result of subtracting o from the offset of p.

operator+= and operator-= which can accept a (possibly const) std::streamoff and adds/subtracts it from the stored offset, respectively.

operator- which can subtract two objects of type (possibly const) std::fpos producing an std::streamoff , such that for two such objects p and q, p == q + (p - q).

[edit] Notes

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, std::wstreampos, etc.).

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 57	C++98	`streampos` and `wstreampos` were contradictionally allowed to be different while required to be the same	clarified to be the same
P0759R1	C++98	specification was unclear and incomplete	cleaned up
P1148R0	C++11	unclear what and in which header the definitions `u16streampos` and `u32streampos` are	made clear
LWG 2114 (P2167R3)	C++98	non-bool return types of equality comparisons were allowed	disallowed