NAME¶

std::basic_fstream - std::basic_fstream

Synopsis¶

Defined in header <fstream>
template<

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

> class basic_fstream : public std::basic_iostream<CharT, Traits>

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

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

std-basic fstream-inheritance.svg

Inheritance diagram

Several typedefs for common character types are provided:

Defined in header <fstream>
Type Definition
std::fstream std::basic_fstream<char>
std::wfstream std::basic_fstream<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
native_handle_type(C++26) implementation-defined type that is TriviallyCopyable and
semiregular

Member functions¶

constructor constructs the file stream
(public member function)
destructor destructs the basic_fstream and the associated
[virtual] (implicitly declared) buffer, closes the file
(virtual public member function)
operator= moves the file stream
(C++11) (public member function)
swap swaps two file streams
(C++11) (public member function)
rdbuf returns the underlying raw file device object
(public member function)
native_handle returns the underlying implementation-defined handle
(C++26) (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_fstream) specializes the std::swap algorithm
(C++11) (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 a 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>)
returns number of characters extracted by last unformatted input
gcount 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_ostream

Member functions¶

Formatted output¶

operator<< inserts formatted data
(public member function of std::basic_ostream<CharT,Traits>)

Unformatted output¶

put inserts a character
(public member function of std::basic_ostream<CharT,Traits>)
write inserts blocks of characters
(public member function of std::basic_ostream<CharT,Traits>)

Positioning¶

tellp returns the output position indicator
(public member function of std::basic_ostream<CharT,Traits>)
seekp sets the output position indicator
(public member function of std::basic_ostream<CharT,Traits>)

Miscellaneous¶

flush synchronizes with the underlying storage device
(public member function of std::basic_ostream<CharT,Traits>)

Member classes¶

sentry implements basic logic for preparation of the stream for output operations
(public member class of std::basic_ostream<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 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)

Notes¶

Feature-test macro Value Std Feature
__cpp_lib_fstream_native_handle 202306L (C++26) native handles support

Example¶

// Run this code

#include <fstream>
#include <iostream>
#include <string>

int main()
{
std::string filename{"test.bin"};
std::fstream s{filename, s.binary | s.trunc | s.in | s.out};

if (!s.is_open())
std::cout << "failed to open " << filename << '\n';
else
{
// write
double d{3.14};
s.write(reinterpret_cast<char*>(&d), sizeof d); // binary output
s << 123 << "abc"; // text output

// for fstream, this moves the file position pointer (both put and get)
s.seekp(0);

// read
d = 2.71828;
s.read(reinterpret_cast<char*>(&d), sizeof d); // binary input
int n;
std::string str;
if (s >> n >> str) // text input
std::cout << "read back from file: " << d << ' ' << n << ' ' << str << '\n';
}
}

Output:¶

read back from file: 3.14 123 abc

See also¶

getline read data from an I/O stream into a string
(function template)