std::fpos

 

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Technical Specifications

 

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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>
u8streampos (since C++20)	std::fpos<std::char_traits<char8_t>::state_type>
u16streampos (since C++11)	std::fpos<std::char_traits<char16_t>::state_type>
u32streampos (since 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 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 returns a value of type convertible to bool. 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

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).

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
P0759R1	C++98	specification was unclear and incomplete	cleaned up
P1148R0	C++11	unclear what header defines u16streampos and u32streampos or what their definitions are	made clear

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)