std::fpos
Defined in header <ios>
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template< class State > class fpos; |
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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
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std::fpos<std::char_traits<char>::state_type> |
wstreampos
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std::fpos<std::char_traits<wchar_t>::state_type> |
u8streampos (since C++20)
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std::fpos<std::char_traits<char8_t>::state_type> |
u16streampos (since C++11)
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std::fpos<std::char_traits<char16_t>::state_type> |
u32streampos (since C++11)
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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.
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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) 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 objecto
of type (possibly const) std::streamoff
- p + o has type
fpos<State>
and stores an offset that is the result of addingo
to the offset ofp
- 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 subtractingo
from the offset ofp
- p + o has type
- 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 objectsp
andq
, 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 |
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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
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made clear |
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) |