From cppreference.com
Defined in header
template<classState>classfpos;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
) and the current shift state, a value of type State (typically
).
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
Type Definition std::streamposstd::fpos<std::char_traits<char>::state_type>std::wstreamposstd::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
,
,
,
, and
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.
Template parameter
Member functions
gets/sets the value of the shift state
(public member function)In addition, member and non-member functions are provided to support the following operations:
A default constructor that stores an offset of zero and value-initializes the state object.
A non-explicit constructor that accepts an argument of type (possibly const)
, 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
. 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)
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)
and adds/subtracts it from the stored offset, respectively.
operator- which can subtract two objects of type (possibly const) std::fpos producing an
, such that for two such objects p and q, p==q+(p-q).
Notes
Some of the
return and manipulate objects of member typedef pos_type. For streams, these member typedefs are provided by the template parameter Traits, which defaults to
, 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.).
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
C++98 streampos and wstreampos were contradictionally
allowed to be different while required to be the same clarified to
be the same
C++98 specification was unclear and incomplete cleaned up
C++11 unclear what and in which header the
definitions u16streampos and u32streampos are made clear
(
) C++98 non-bool return types of equality comparisons were allowed disallowed See also
represents relative file/stream position (offset from fpos), sufficient to represent any file size
(typedef)
returns the output position indicator
(public member function of std::basic_ostream<CharT,Traits>)
sets the output position indicator
(public member function of std::basic_ostream<CharT,Traits>)
gets the file position indicator
(function)