NAME¶

std::basic_istream - std::basic_istream

Synopsis¶

Defined in header <istream>
template<

class CharT,
class Traits = std::char_traits<CharT>

> class basic_istream : virtual public std::basic_ios<CharT, Traits>

The class template basic_istream provides support for high level input operations on
character streams. The supported operations include formatted input (e.g. integer
values or whitespace-separated characters and characters strings) and unformatted
input (e.g. raw characters and character arrays). This functionality is implemented
in terms of the interface provided by the underlying basic_streambuf class, accessed
through the basic_ios base class. The only non-inherited data member of
basic_istream, in most implementations, is the value returned by
basic_istream::gcount().

std-basic istream-inheritance.svg

Inheritance diagram

Several typedefs for common character types are provided:

Defined in header <istream>
Type Definition
std::istream std::basic_istream<char>
std::wistream std::basic_istream<wchar_t>

Global objects¶

Two global basic_istream objects are provided by the standard library.

Defined in header <iostream>
cin reads from the standard C input stream stdin
wcin (global object)

Member types¶

Member type Definition
char_type CharT
traits_type Traits; the program is ill-formed if Traits::char_type is not
CharT.
int_type Traits::int_type
pos_type Traits::pos_type
off_type Traits::off_type

Member functions¶

constructor constructs the object
(public member function)
destructor destructs the object
[virtual] (virtual public member function)
operator= move-assigns from another basic_istream
(C++11) (protected member function)

Formatted input¶

operator>> extracts formatted data
(public member function)

Unformatted input¶

get extracts characters
(public member function)
peek reads the next character without extracting it
(public member function)
unget unextracts a character
(public member function)
putback puts a character into input stream
(public member function)
getline extracts characters until the given character is found
(public member function)
ignore extracts and discards characters until the given character is found
(public member function)
read extracts blocks of characters
(public member function)
readsome extracts already available blocks of characters
(public member function)
returns number of characters extracted by last unformatted input
gcount operation
(public member function)

Positioning¶

tellg returns the input position indicator
(public member function)
seekg sets the input position indicator
(public member function)

Miscellaneous¶

sync synchronizes with the underlying storage device
(public member function)
swap swaps stream objects, except for the associated buffer
(C++11) (protected member function)

Member classes¶

sentry implements basic logic for preparation of the stream for input operations
(public member class)

Non-member functions¶

operator>>(std::basic_istream) extracts characters and character arrays
(function template)

Inherited from std::basic_ios

Member types¶

Member type Definition
char_type CharT
traits_type Traits
int_type Traits::int_type
pos_type Traits::pos_type
off_type Traits::off_type

Member functions¶

State functions¶

good checks if no error has occurred i.e. I/O operations are available
(public member function of std::basic_ios<CharT,Traits>)
eof checks if end-of-file has been reached
(public member function of std::basic_ios<CharT,Traits>)
fail checks if an error has occurred
(public member function of std::basic_ios<CharT,Traits>)
bad checks if a non-recoverable error has occurred
(public member function of std::basic_ios<CharT,Traits>)
operator! checks if an error has occurred (synonym of fail())
(public member function of std::basic_ios<CharT,Traits>)
operator bool checks if no error has occurred (synonym of !fail())
(public member function of std::basic_ios<CharT,Traits>)
rdstate returns state flags
(public member function of std::basic_ios<CharT,Traits>)
setstate sets state flags
(public member function of std::basic_ios<CharT,Traits>)
clear modifies state flags
(public member function of std::basic_ios<CharT,Traits>)

Formatting¶

copyfmt copies formatting information
(public member function of std::basic_ios<CharT,Traits>)
fill manages the fill character
(public member function of std::basic_ios<CharT,Traits>)

Miscellaneous¶

exceptions manages exception mask
(public member function of std::basic_ios<CharT,Traits>)
imbue sets the locale
(public member function of std::basic_ios<CharT,Traits>)
rdbuf manages associated stream buffer
(public member function of std::basic_ios<CharT,Traits>)
tie manages tied stream
(public member function of std::basic_ios<CharT,Traits>)
narrow narrows characters
(public member function of std::basic_ios<CharT,Traits>)
widen widens characters
(public member function of std::basic_ios<CharT,Traits>)

Inherited from std::ios_base

Member functions¶

Formatting¶

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

Locales¶

imbue sets locale
(public member function of std::ios_base)
getloc returns current locale
(public member function of std::ios_base)

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 of std::ios_base)
resizes the private storage if necessary and access to the long
iword element at the given index
(public member function of std::ios_base)
resizes the private storage if necessary and access to the void*
pword element at the given index
(public member function of std::ios_base)

Miscellaneous¶

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

Member classes¶

failure stream exception
(public member class of std::ios_base)
Init initializes standard stream objects
(public member class of std::ios_base)

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)