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< cpp‎ | io
m (1 revision: import content)
(Added LWG issue #1357 DR.)
 
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{{cpp/title|ios_base}}
 
{{cpp/title|ios_base}}
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{{cpp/io/ios_base/sidebar}}
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{{cpp/io/ios_base/navbar}}
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{{ddcl | header=ios |
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{{ddcl|header=ios|
 
class ios_base;
 
class ios_base;
 
}}
 
}}
  
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The class {{tt|ios_base}} provides facilities for inspecting error state of the stream object. Character type independent formatting flags are stored here as well.
+
The class {{tt|ios_base}} is a multipurpose class that serves as the base class for all I/O stream classes. It maintains several kinds of data:
  
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{{dcl list begin}}
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@1@ state information: stream status flags.
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{{dcl list h1 | Member functions}}
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@2@ control information: flags that control formatting of both input and output sequences and the imbued locale.
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{{dcl list template | cpp/io/ios_base/dcl list constructor}}
+
@3@ private storage: indexed extensible data structure that allows both {{c/core|long}} and {{c/core|void*}} members, which may be implemented as two arbitrary-length arrays or a single array of two-element structs or another container.
−
{{dcl list template | cpp/io/ios_base/dcl list destructor}}
+
@4@ callbacks: arbitrary number of user-defined functions to be called from {{ltf|cpp/io/ios_base/imbue}}, {{lc|std::basic_ios::copyfmt()}}, and {{ltf|cpp/io/ios_base/~ios_base}}.
  
−
{{dcl list h2 | Formatting}}
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Typical implementation holds member constants corresponding to all values of {{ltt|cpp/io/ios_base/fmtflags}}, {{ltt|cpp/io/ios_base/iostate}}, {{ltt|cpp/io/ios_base/openmode}}, and {{ltt|cpp/io/ios_base/seekdir}} shown below, member variables to maintain current precision, width, and formatting flags, the exception mask, the buffer error state, a resizeable container holding the callbacks, the currently imbued locale, the private storage, and a static integer variable for {{ltf|cpp/io/ios_base/xalloc}}.
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{{dcl list template | cpp/io/ios_base/dcl list flags}}
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{{dcl list template | cpp/io/ios_base/dcl list setf}}
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{{dcl list template | cpp/io/ios_base/dcl list unsetf}}
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{{dcl list template | cpp/io/ios_base/dcl list precision}}
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{{dcl list template | cpp/io/ios_base/dcl list width}}
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{{dcl list h2 | Locales}}
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===Member functions===
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{{dcl list template | cpp/io/ios_base/dcl list imbue}}
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{{dsc begin}}
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{{dcl list template | cpp/io/ios_base/dcl list getloc}}
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{{dsc inc|cpp/io/ios_base/dsc constructor}}
 +
{{dsc inc|cpp/io/ios_base/dsc destructor}}
 +
{{dsc inc|cpp/io/ios_base/dsc operator{{=}}}}
  
−
{{dcl list h2 | Internal extensible array}}
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{{dsc h2|Formatting}}
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{{dcl list template | cpp/io/ios_base/dcl list xalloc}}
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{{dsc inc|cpp/io/ios_base/dsc flags}}
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{{dcl list template | cpp/io/ios_base/dcl list iword}}
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{{dsc inc|cpp/io/ios_base/dsc setf}}
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{{dcl list template | cpp/io/ios_base/dcl list pword}}
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{{dsc inc|cpp/io/ios_base/dsc unsetf}}
 +
{{dsc inc|cpp/io/ios_base/dsc precision}}
 +
{{dsc inc|cpp/io/ios_base/dsc width}}
  
−
{{dcl list h2 | Miscellaneous}}
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{{dsc h2|Locales}}
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{{dcl list template | cpp/io/ios_base/dcl list register_callback}}
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{{dsc inc|cpp/io/ios_base/dsc imbue}}
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{{dcl list template | cpp/io/ios_base/dcl list sync_with_stdio}}
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{{dsc inc|cpp/io/ios_base/dsc getloc}}
  
−
{{dcl list h1 | Member classes}}
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{{dsc h2|Internal extensible array}}
−
{{dcl list template | cpp/io/ios_base/dcl list failure}}
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{{dsc inc|cpp/io/ios_base/dsc xalloc}}
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{{dcl list template | cpp/io/ios_base/dcl list Init}}
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{{dsc inc|cpp/io/ios_base/dsc iword}}
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{{dcl list end}}
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{{dsc inc|cpp/io/ios_base/dsc pword}}
  
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{{tdcl list begin}}
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{{dsc h2|Miscellaneous}}
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{{tdcl list h1 | Member types and constants}}
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{{dsc inc|cpp/io/ios_base/dsc register_callback}}
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{{tdcl list hitem | Type | Explanation}}
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{{dsc inc|cpp/io/ios_base/dsc sync_with_stdio}}
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{{tdcl list template | cpp/io/ios_base/tdcl list openmode}}
+
 
−
{{tdcl list template | cpp/io/ios_base/tdcl list fmtflags}}
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{{dsc h1|Member classes}}
−
{{tdcl list template | cpp/io/ios_base/tdcl list iostate}}
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{{dsc inc|cpp/io/ios_base/dsc failure}}
−
{{tdcl list template | cpp/io/ios_base/tdcl list seekdir}}
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{{dsc inc|cpp/io/ios_base/dsc Init}}
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{{tdcl list template | cpp/io/ios_base/tdcl list event}}
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{{dsc end}}
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{{tdcl list template | cpp/io/ios_base/tdcl list event_callback}}
+
 
−
{{tdcl list end}}
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{{dsc begin}}
 +
{{dsc h1|Member types and constants}}
 +
{{dsc hitem|Type|Explanation}}
 +
{{dsc inc|cpp/io/ios_base/dsc openmode}}
 +
{{dsc inc|cpp/io/ios_base/dsc fmtflags}}
 +
{{dsc inc|cpp/io/ios_base/dsc iostate}}
 +
{{dsc inc|cpp/io/ios_base/dsc seekdir}}
 +
{{dsc inc|cpp/io/ios_base/dsc event}}
 +
{{dsc inc|cpp/io/ios_base/dsc event_callback}}
 +
{{dsc end}}
 +
{{rev begin}}
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{{rev|until=c++17|
 +
{{dsc begin}}
 +
{{dsc h2|Deprecated member types}}
 +
{{dsc hitem|Type|Explanation}}
 +
{{dsc|{{tt|io_state}} {{mark deprecated}}|integer type that may be used like {{tt|iostate}}}}
 +
{{dsc|{{tt|open_mode}} {{mark deprecated}}|integer type that may be used like {{tt|openmode}}}}
 +
{{dsc|{{tt|seek_dir}} {{mark deprecated}}|integer type that may be used like {{tt|seekdir}}}}
 +
{{dsc|{{tt|streamoff}} {{mark deprecated}}|unspecified type that may be used like {{tt|off_type}}, not necessarily {{lc|std::streamoff}}}}
 +
{{dsc|{{tt|streampos}} {{mark deprecated}}|unspecified type that may be used like {{tt|pos_type}}, not necessarily {{lc|std::streampos}}}}
 +
{{dsc end}}
 +
}}
 +
{{rev end}}
 +
 
 +
===Defect reports===
 +
{{dr list begin}}
 +
{{dr list item|wg=lwg|dr=1357|paper=N3110|std=C++98|before={{tt|std::ios_base}} defined {{c/core|operator~}}, {{c/core|operator&}}<br>and {{c/core|operator{{!}}}} for types {{tt|openmode}}, {{tt|fmtflags}} and<br>{{tt|iostate}}, violating the requirements of {{named req|BitmaskType}}<ref>A {{named req|BitmaskType}} needs to support bitwise opertaions on its own. The bitwise operation support should not be provided externally.</ref>|after=removed these definitions}}
 +
{{dr list end}}
 +
<references/>
 +
 
 +
===See also===
 +
{{dsc begin}}
 +
{{dsc inc|cpp/io/dsc basic_ios}}
 +
{{dsc end}}
 +
 
 +
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

Latest revision as of 21:38, 3 August 2023

 
C++
 
Input/output library
 
std::ios_base
 
Defined in header <ios>
class ios_base;

The class ios_base is a multipurpose class that serves as the base class for all I/O stream classes. It maintains several kinds of data:

1) state information: stream status flags.
2) control information: flags that control formatting of both input and output sequences and the imbued locale.
3) private storage: indexed extensible data structure that allows both long and void* members, which may be implemented as two arbitrary-length arrays or a single array of two-element structs or another container.
4) callbacks: arbitrary number of user-defined functions to be called from imbue(), std::basic_ios::copyfmt(), and ~ios_base().

Typical implementation holds member constants corresponding to all values of fmtflags, iostate, openmode, and seekdir shown below, member variables to maintain current precision, width, and formatting flags, the exception mask, the buffer error state, a resizeable container holding the callbacks, the currently imbued locale, the private storage, and a static integer variable for xalloc().

Contents

[edit] Member functions

 constructs the object
(protected member function) [edit]
[virtual]
 destructs the object
(virtual public member function) [edit]
assigns to the stream
(public member function) [edit]
Formatting
manages format flags
(public member function) [edit]
sets specific format flag
(public member function) [edit]
clears specific format flag
(public member function) [edit]
manages decimal precision of floating point operations
(public member function) [edit]
manages field width
(public member function) [edit]
Locales
sets locale
(public member function) [edit]
returns current locale
(public member function) [edit]
Internal extensible array
[static]
 returns a program-wide unique integer that is safe to use as index to pword() and iword()
(public static member function) [edit]
resizes the private storage if necessary and access to the long element at the given index
(public member function) [edit]
resizes the private storage if necessary and access to the void* element at the given index
(public member function) [edit]
Miscellaneous
registers event callback function
(public member function) [edit]
[static]
 sets whether C++ and C I/O libraries are interoperable
(public static member function) [edit]

Member classes

 stream exception
(public member class) [edit]
initializes standard stream objects
(public member class) [edit]

Member types and constants

Type Explanation
stream open mode type

The following constants are also defined:

Constant Explanation[edit]
app seek to the end of stream before each write[edit]
binary open in binary mode[edit]
in open for reading[edit]
out open for writing[edit]
trunc discard the contents of the stream when opening[edit]
ate seek to the end of stream immediately after open[edit]
noreplace (C++23) open in exclusive mode[edit]

(typedef) [edit]
formatting flags type

The following constants are also defined:

Constant Explanation[edit]
dec use decimal base for integer I/O: see std::dec[edit]
oct use octal base for integer I/O: see std::oct[edit]
hex use hexadecimal base for integer I/O: see std::hex[edit]
basefield dec | oct | hex. Useful for masking operations[edit]
left left adjustment (adds fill characters to the right): see std::left[edit]
right right adjustment (adds fill characters to the left): see std::right[edit]
internal internal adjustment (adds fill characters to the internal designated point): see std::internal[edit]
adjustfield left | right | internal. Useful for masking operations[edit]
scientific generate floating point types using scientific notation, or hex notation if combined with fixed: see std::scientific[edit]
fixed generate floating point types using fixed notation, or hex notation if combined with scientific: see std::fixed[edit]
floatfield scientific | fixed. Useful for masking operations[edit]
boolalpha insert and extract bool type in alphanumeric format: see std::boolalpha[edit]
showbase generate a prefix indicating the numeric base for integer output, require the currency indicator in monetary I/O: see std::showbase[edit]
showpoint generate a decimal-point character unconditionally for floating-point number output: see std::showpoint[edit]
showpos generate a + character for non-negative numeric output: see std::showpos[edit]
skipws skip leading whitespace before certain input operations: see std::skipws[edit]
unitbuf flush the output after each output operation: see std::unitbuf[edit]
uppercase replace certain lowercase letters with their uppercase
equivalents in certain output operations: see std::uppercase[edit]

(typedef) [edit]
state of the stream type

The following constants are also defined:

Constant Explanation[edit]
goodbit no error[edit]
badbit irrecoverable stream error[edit]
failbit input/output operation failed (formatting or extraction error)[edit]
eofbit associated input sequence has reached end-of-file[edit]

(typedef) [edit]
seeking direction type

The following constants are also defined:

Constant Explanation[edit]
beg the beginning of a stream[edit]
end the ending of a stream[edit]
cur the current position of stream position indicator[edit]

(typedef) [edit]
specifies event type
(enum) [edit]
callback function type
(typedef) [edit]
Deprecated member types
Type Explanation
io_state (deprecated) integer type that may be used like iostate
open_mode (deprecated) integer type that may be used like openmode
seek_dir (deprecated) integer type that may be used like seekdir
streamoff (deprecated) unspecified type that may be used like off_type, not necessarily std::streamoff
streampos (deprecated) unspecified type that may be used like pos_type, not necessarily std::streampos
(until C++17)

[edit] 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 1357
(N3110) 		C++98 std::ios_base defined operator~, operator&
and operator| for types openmode, fmtflags and
iostate, violating the requirements of BitmaskType[1] 		removed these definitions
  1. ↑ A BitmaskType needs to support bitwise opertaions on its own. The bitwise operation support should not be provided externally.

[edit] See also

 manages an arbitrary stream buffer
(class template) [edit]
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