std:: ios_base

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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() .

Member functions

constructs the object
(protected member function)
[virtual]
destructs the object
(virtual public member function)
assigns to the stream
(public member function)
Formatting
manages format flags
(public member function)
sets specific format flag
(public member function)
clears specific format flag
(public member function)
manages decimal precision of floating point operations
(public member function)
manages field width
(public member function)
Locales
sets locale
(public member function)
returns current locale
(public member function)
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)
resizes the private storage if necessary and access to the long element at the given index
(public member function)
resizes the private storage if necessary and access to the void * element at the given index
(public member function)
Miscellaneous
registers event callback function
(public member function)
[static]
sets whether C++ and C I/O libraries are interoperable
(public static member function)

Member classes

stream exception
(public member class)
initializes standard stream objects
(public member class)

Member types and constants

Type Explanation
stream open mode type

The following constants are also defined:

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

(typedef)
formatting flags type

The following constants are also defined:

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

(typedef)
state of the stream type

The following constants are also defined:

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

(typedef)
seeking direction type

The following constants are also defined:

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

(typedef)
specifies event type
(enum)
callback function type
(typedef)
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)

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.

See also

manages an arbitrary stream buffer
(class template)
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