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

(constructor)	constructs the object (protected member function)
(destructor) [virtual]	destructs the object (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 [static]	returns a program-wide unique integer that is safe to use as index to `pword()` and `iword()` (public static member function)
iword	resizes the private storage if necessary and access to the long element at the given index (public member function)
pword	resizes the private storage if necessary and access to the void* element at the given index (public member function)
Miscellaneous
register_callback	registers event callback function (public member function)
sync_with_stdio [static]	sets whether C++ and C I/O libraries are interoperable (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

openmode

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)

fmtflags

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)

iostate

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)

seekdir

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)

event

specifies event type
(enum)

event_callback

callback function type
(typedef)

Type	Explanation
Deprecated member types
`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

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

std::ios_base

Member functions

Formatting

Locales

Internal extensible array

Miscellaneous

Member classes

Member types and constants

Deprecated member types

Defect reports

See also