std::basic_ifstream

Defined in header `<fstream>`
template< class CharT, class Traits = std::char_traits<CharT> > class basic_ifstream : public std::basic_istream<CharT, Traits>

The class template basic_ifstream implements high-level input operations on file based streams. It interfaces a file-based streambuffer (std::basic_filebuf) with the high-level interface of (std::basic_istream).

A typical implementation of std::basic_ifstream holds only one non-derived data member: an instance of std::basic_filebuf<CharT, Traits>.

Inheritance diagram

Two specializations for common character types are also defined:

Defined in header `<fstream>`
Type	Definition
`ifstream`	basic_ifstream<char>
`wifstream`	basic_ifstream<wchar_t>

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 file stream (public member function)
(destructor) [virtual] (implicitly declared)	destructs the basic_ifstream and the associated buffer, closes the file (virtual public member function)
operator= (C++11)	moves the file stream (public member function)
swap (C++11)	swaps two file streams (public member function)
rdbuf	returns the underlying raw file device object (public member function)
File operations
is_open	checks if the stream has an associated file (public member function)
open	opens a file and associates it with the stream (public member function)
close	closes the associated file (public member function)

Non-member functions

std::swap(std::basic_ifstream)

(C++11)

specializes the std::swap algorithm
(function template)

Inherited from std::basic_istream

Member functions

Formatted input
operator>>	extracts formatted data (public member function of `std::basic_istream<CharT,Traits>`)
Unformatted input
get	extracts characters (public member function of `std::basic_istream<CharT,Traits>`)
peek	reads the next character without extracting it (public member function of `std::basic_istream<CharT,Traits>`)
unget	unextracts a character (public member function of `std::basic_istream<CharT,Traits>`)
putback	puts character into input stream (public member function of `std::basic_istream<CharT,Traits>`)
getline	extracts characters until the given character is found (public member function of `std::basic_istream<CharT,Traits>`)
ignore	extracts and discards characters until the given character is found (public member function of `std::basic_istream<CharT,Traits>`)
read	extracts blocks of characters (public member function of `std::basic_istream<CharT,Traits>`)
readsome	extracts already available blocks of characters (public member function of `std::basic_istream<CharT,Traits>`)
gcount	returns number of characters extracted by last unformatted input operation (public member function of `std::basic_istream<CharT,Traits>`)
Positioning
tellg	returns the input position indicator (public member function of `std::basic_istream<CharT,Traits>`)
seekg	sets the input position indicator (public member function of `std::basic_istream<CharT,Traits>`)
Miscellaneous
sync	synchronizes with the underlying storage device (public member function of `std::basic_istream<CharT,Traits>`)

Member classes

sentry

implements basic logic for preparation of the stream for input operations
(public member class of std::basic_istream<CharT,Traits>)

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 void*operator bool (until C++11)(since C++11)	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 IO 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

(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)

Example

Run this code

#include <iostream>
#include <fstream>
#include <string>
 
int main() {
  std::string filename = "Test.b";
  // prepare a file to read
  double d = 3.14;
  std::ofstream(filename, std::ios::binary).write(reinterpret_cast<char*>(&d), sizeof d)
     << 123 << "abc";
  // open file for reading
  std::ifstream istrm(filename, std::ios::binary);
  if (!istrm.is_open()) {
    std::cout << "failed to open " << filename << '\n';
  } else {
    double d;
    istrm.read(reinterpret_cast<char*>(&d), sizeof d); // binary input
    int n;
    std::string s;
    if (istrm >> n >> s)                               // text input
      std::cout << "read back from file: " << d << ' ' << n << ' ' << s << '\n';
  }
}

Output:

read back from file: 3.14 123 abc

Language
Standard Library Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

I/O manipulators
C-style I/O
Buffers
basic_streambuf
basic_filebuf
basic_stringbuf
strstreambuf (deprecated in C++98)
basic_syncbuf (C++20)
Streams
Abstractions
ios_base
basic_ios
basic_istream
basic_ostream
basic_iostream
File I/O
basic_ifstream
basic_ofstream
basic_fstream
String I/O
basic_istringstream
basic_ostringstream
basic_stringstream
Array I/O
istrstream (deprecated in C++98)
ostrstream (deprecated in C++98)
strstream (deprecated in C++98)
Synchronized Output
basic_osyncstream (C++20)
Types
streamoff
streamsize
fpos
Error category interface
iostream_category (C++11)
io_errc (C++11)

Member functions
basic_ifstream::basic_ifstream
basic_ifstream::operator= (C++11)
basic_ifstream::swap (C++11)
basic_ifstream::rdbuf
File operations
basic_ifstream::is_open
basic_ifstream::open
basic_ifstream::close
Non-member functions
basic_ifstream::swap(std::basic_ifstream) (C++11)

std::basic_ifstream

Member types

Member functions

File operations

Non-member functions

Inherited from std::basic_istream

Member functions

Formatted input

Unformatted input

Positioning

Miscellaneous

Member classes

Inherited from std::basic_ios

Member types

Member functions

State functions

Formatting

Miscellaneous

Inherited from std::ios_base

Member functions

Formatting

Locales

Internal extensible array

Miscellaneous

Member classes

Member types and constants

Example