NAME¶

std::ios_base - std::ios_base

Synopsis¶

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¶

constructor constructs the object
(protected member function)
destructor destructs the object
[virtual] (virtual public member function)
operator= assigns to the stream
(public member function)

Formatting¶

flags manages format flags
(public member function)
setf sets specific format flag
(public member function)
unsetf clears specific format flag
(public member function)
precision manages decimal precision of floating point operations
(public member function)
width manages field width
(public member function)

Locales¶

imbue sets locale
(public member function)
getloc returns current locale
(public member function)

Internal extensible array¶

xalloc returns a program-wide unique integer that is safe to use as index
[static] to pword() and iword()
(public static member function)
resizes the private storage if necessary and access to the long
iword element at the given index
(public member function)
resizes the private storage if necessary and access to the void*
pword element at the given index
(public member function)

Miscellaneous¶

register_callback registers event callback function
(public member function)
sync_with_stdio sets whether C++ and C I/O libraries are interoperable
[static] (public static member function)

Member classes¶

failure stream exception
(public member class)
Init 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
openmode 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
generate floating point types using scientific notation,
scientific or hex notation if combined with fixed: see
std::scientific
fmtflags generate floating point types using fixed notation, or
fixed 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
generate a prefix indicating the numeric base for integer
showbase 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
replace certain lowercase letters with their uppercase
uppercase equivalents in certain output operations: see
std::uppercase

(typedef)
state of the stream type

The following constants are also defined:

Constant Explanation
iostate 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:

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

(typedef)
event specifies event type
(enum)
event_callback 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 (until C++17)
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

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
std::ios_base defined operator~, operator&
LWG 1357 C++98 and operator| for types openmode, fmtflags and removed these
(N3110) iostate, violating the requirements of definitions
BitmaskType^[1]

1. ↑ A BitmaskType needs to support bitwise opertaions on its own. The bitwise
operation support should not be provided externally.

See also¶

basic_ios manages an arbitrary stream buffer
(class template)