# std::basic_istream

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< cpp‎ | io

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()`.

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 wcin | reads from the standard C input stream stdin (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) [virtual] | destructs the object (virtual public member function)
operator= (C++11) | move-assigns from another `basic_istream` (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)
gcount | returns number of characters extracted by last unformatted input 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 (C++11) | swaps stream objects, except for the associated buffer (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 [static] | returns a program-wide unique integer that is safe to use as index to `pword()` and `iword()` (public static member function of `std::ios_base`)
iword | resizes the private storage if necessary and access to the long element at the given index (public member function of `std::ios_base`)
pword | resizes the private storage if necessary and access to the void* 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 [static] | sets whether C++ and C I/O libraries are interoperable (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
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)

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