std:: fpos
|
Defined in header
<ios>
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||
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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):
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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
|
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::fposconstructed in this manner is returned by some stream operations to indicate errors.
-
Explicit conversion from (possibly const)
fposto std::streamoff . The result is the stored offset.
-
operator
==
and
operator
!
=
that compare two objects of type (possibly const)
std::fposand 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 .
-
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::fposproducing 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_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
,
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
|
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) |