# std::fpos

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< cpp‎ | io

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

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