std::fpos

 

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streamoff

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fpos

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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 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.

Template parameter

State	-	the type representing the shift state
Type requirements
- State must meet the requirements of Destructible, CopyAssignable, CopyConstructible and DefaultConstructible.

Member functions

state

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) 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).

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.).

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

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<CharT,Traits>)
seekp	sets the output position indicator (public member function of std::basic_ostream<CharT,Traits>)
fgetpos	gets the file position indicator (function)