Difference between revisions of "cpp/io/basic fstream"

Latest revision as of 05:15, 4 August 2023

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>.

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>

[edit] Member types

Member type	Definition
`char_type`	`CharT`[edit]
`traits_type`	`Traits`; the program is ill-formed if `Traits::char_type` is not `CharT`.[edit]
`int_type`	`Traits::int_type`[edit]
`pos_type`	`Traits::pos_type`[edit]
`off_type`	`Traits::off_type`[edit]
`native_handle_type`(C++26)	implementation-defined type that is TriviallyCopyable and `semiregular`[edit]

[edit] Member functions

(constructor)	constructs the file stream (public member function) [edit]
(destructor) [virtual] (implicitly declared)	destructs the `basic_fstream` and the associated buffer, closes the file (virtual public member function) [edit]
operator= (C++11)	moves the file stream (public member function) [edit]
swap (C++11)	swaps two file streams (public member function) [edit]
rdbuf	returns the underlying raw file device object (public member function) [edit]
native_handle (C++26)	returns the underlying implementation-defined handle (public member function) [edit]
File operations
is_open	checks if the stream has an associated file (public member function) [edit]
open	opens a file and associates it with the stream (public member function) [edit]
close	closes the associated file (public member function) [edit]

[edit] Non-member functions

std::swap(std::basic_fstream)

(C++11)

specializes the std::swap algorithm
(function template) [edit]

Inherited from std::basic_istream

Member functions

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

Member classes

sentry

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

Inherited from std::basic_ostream

Member functions

Formatted output
operator<<	inserts formatted data (public member function of `std::basic_ostream<CharT,Traits>`) [edit]
Unformatted output
put	inserts a character (public member function of `std::basic_ostream<CharT,Traits>`) [edit]
write	inserts blocks of characters (public member function of `std::basic_ostream<CharT,Traits>`) [edit]
Positioning
tellp	returns the output position indicator (public member function of `std::basic_ostream<CharT,Traits>`) [edit]
seekp	sets the output position indicator (public member function of `std::basic_ostream<CharT,Traits>`) [edit]
Miscellaneous
flush	synchronizes with the underlying storage device (public member function of `std::basic_ostream<CharT,Traits>`) [edit]

Member classes

sentry

implements basic logic for preparation of the stream for output operations
(public member class of std::basic_ostream<CharT,Traits>) [edit]

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>`) [edit]
eof	checks if end-of-file has been reached (public member function of `std::basic_ios<CharT,Traits>`) [edit]
fail	checks if an error has occurred (public member function of `std::basic_ios<CharT,Traits>`) [edit]
bad	checks if a non-recoverable error has occurred (public member function of `std::basic_ios<CharT,Traits>`) [edit]
operator!	checks if an error has occurred (synonym of fail()) (public member function of `std::basic_ios<CharT,Traits>`) [edit]
operator bool	checks if no error has occurred (synonym of `!`fail()) (public member function of `std::basic_ios<CharT,Traits>`) [edit]
rdstate	returns state flags (public member function of `std::basic_ios<CharT,Traits>`) [edit]
setstate	sets state flags (public member function of `std::basic_ios<CharT,Traits>`) [edit]
clear	modifies state flags (public member function of `std::basic_ios<CharT,Traits>`) [edit]
Formatting
copyfmt	copies formatting information (public member function of `std::basic_ios<CharT,Traits>`) [edit]
fill	manages the fill character (public member function of `std::basic_ios<CharT,Traits>`) [edit]
Miscellaneous
exceptions	manages exception mask (public member function of `std::basic_ios<CharT,Traits>`) [edit]
imbue	sets the locale (public member function of `std::basic_ios<CharT,Traits>`) [edit]
rdbuf	manages associated stream buffer (public member function of `std::basic_ios<CharT,Traits>`) [edit]
tie	manages tied stream (public member function of `std::basic_ios<CharT,Traits>`) [edit]
narrow	narrows characters (public member function of `std::basic_ios<CharT,Traits>`) [edit]
widen	widens characters (public member function of `std::basic_ios<CharT,Traits>`) [edit]

Inherited from std::ios_base

Member functions

Formatting
flags	manages format flags (public member function of `std::ios_base`) [edit]
setf	sets specific format flag (public member function of `std::ios_base`) [edit]
unsetf	clears specific format flag (public member function of `std::ios_base`) [edit]
precision	manages decimal precision of floating point operations (public member function of `std::ios_base`) [edit]
width	manages field width (public member function of `std::ios_base`) [edit]
Locales
imbue	sets locale (public member function of `std::ios_base`) [edit]
getloc	returns current locale (public member function of `std::ios_base`) [edit]
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`) [edit]
iword	resizes the private storage if necessary and access to the long element at the given index (public member function of `std::ios_base`) [edit]
pword	resizes the private storage if necessary and access to the void* element at the given index (public member function of `std::ios_base`) [edit]
Miscellaneous
register_callback	registers event callback function (public member function of `std::ios_base`) [edit]
sync_with_stdio [static]	sets whether C++ and C I/O libraries are interoperable (public static member function of `std::ios_base`) [edit]
Member classes
failure	stream exception (public member class of `std::ios_base`) [edit]
Init	initializes standard stream objects (public member class of `std::ios_base`) [edit]

Member types and constants

Type Explanation

openmode

stream open mode type

The following constants are also defined:

Constant	Explanation[edit]
`app`	seek to the end of stream before each write[edit]
`binary`	open in binary mode[edit]
`in`	open for reading[edit]
`out`	open for writing[edit]
`trunc`	discard the contents of the stream when opening[edit]
`ate`	seek to the end of stream immediately after open[edit]
`noreplace` (C++23)	open in exclusive mode[edit]

(typedef) [edit]

fmtflags

formatting flags type

The following constants are also defined:

Constant	Explanation[edit]
`dec`	use decimal base for integer I/O: see std::dec[edit]
`oct`	use octal base for integer I/O: see std::oct[edit]
`hex`	use hexadecimal base for integer I/O: see std::hex[edit]
`basefield`	dec \| oct \| hex. Useful for masking operations[edit]
`left`	left adjustment (adds fill characters to the right): see std::left[edit]
`right`	right adjustment (adds fill characters to the left): see std::right[edit]
`internal`	internal adjustment (adds fill characters to the internal designated point): see std::internal[edit]
`adjustfield`	left \| right \| internal. Useful for masking operations[edit]
`scientific`	generate floating point types using scientific notation, or hex notation if combined with fixed: see std::scientific[edit]
`fixed`	generate floating point types using fixed notation, or hex notation if combined with scientific: see std::fixed[edit]
`floatfield`	scientific \| fixed. Useful for masking operations[edit]
`boolalpha`	insert and extract bool type in alphanumeric format: see std::boolalpha[edit]
`showbase`	generate a prefix indicating the numeric base for integer output, require the currency indicator in monetary I/O: see std::showbase[edit]
`showpoint`	generate a decimal-point character unconditionally for floating-point number output: see std::showpoint[edit]
`showpos`	generate a + character for non-negative numeric output: see std::showpos[edit]
`skipws`	skip leading whitespace before certain input operations: see std::skipws[edit]
`unitbuf`	flush the output after each output operation: see std::unitbuf[edit]
`uppercase`	replace certain lowercase letters with their uppercase equivalents in certain output operations: see std::uppercase[edit]

(typedef) [edit]

iostate

state of the stream type

The following constants are also defined:

Constant	Explanation[edit]
`goodbit`	no error[edit]
`badbit`	irrecoverable stream error[edit]
`failbit`	input/output operation failed (formatting or extraction error)[edit]
`eofbit`	associated input sequence has reached end-of-file[edit]

(typedef) [edit]

seekdir

seeking direction type

The following constants are also defined:

Constant	Explanation[edit]
`beg`	the beginning of a stream[edit]
`end`	the ending of a stream[edit]
`cur`	the current position of stream position indicator[edit]

(typedef) [edit]

event

specifies event type
(enum) [edit]

event_callback

callback function type
(typedef) [edit]

[edit] Notes

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

[edit] 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

[edit] See also

getline

read data from an I/O stream into a string
(function template) [edit]

@@ Line 1: / Line 1: @@
 {{cpp/title|basic_fstream}}
-{{cpp/io/basic_fstream/sidebar}}
+{{cpp/io/basic_fstream/navbar}}
-{{ddcl | header=fstream |
+{{ddcl|header=fstream|1=
 template<
      class CharT,
-     class Traits {{=}} std::char_traits<CharT>
+     class Traits = std::char_traits<CharT>
 > class basic_fstream : public std::basic_iostream<CharT, Traits>
 }}
-The class template {{tt|basic_fstream}} implements high-level input/output operations on file based streams. It interfaces a file-based streambuffer ({{c|std::basic_filebuf}}) with the high-level interface of ({{c|std::basic_iostream}}).
+The class template {{tt|basic_fstream}} implements high-level input/output operations on file based streams. It interfaces a file-based streambuffer ({{lc|std::basic_filebuf}}) with the high-level interface of ({{lc|std::basic_iostream}}).
-A typical implementation of {{tt|std::basic_fstream}} holds only one non-derived data member: an instance of {{c|std::basic_filebuf<CharT, Traits>}}.
+A typical implementation of {{tt|std::basic_fstream}} holds only one non-derived data member: an instance of {{c/core|std::basic_filebuf<CharT, Traits>}}.
-{{inheritance diagram|image=std-basic_fstream-inheritance.svg|map=
+{{inheritance diagram/std-basic_fstream}}
-rect 5 33 107 66 [[cpp/io/ios_base]]
-rect 142 31 263 69 [[cpp/io/basic_ios]]
-rect 297 4 418 42 [[cpp/io/basic_istream]]
-rect 297 58 418 96 [[cpp/io/basic_ostream]]
-rect 452 31 573 69 [[cpp/io/basic_iostream]]
-}}
-Two specializations for common character types are also defined:
+{{cpp/basic char typedefs|fstream}}
-{{dcl list begin}}
+{{cpp/io/member types|fstream}}
-{{dcl list header | fstream}}
-{{dcl list hitem | Type | Definition}}
-{{dcl list item | {{tt|fstream}} | {{c|basic_fstream<char>}}}}
-{{dcl list item | {{tt|wfstream}} | {{c|basic_fstream<wchar_t>}}}}
-{{dcl list end}}
-===Member types===
-{{dcl list begin}}
-{{dcl list hitem | Member type | Definition}}
-{{dcl list template | cpp/io/tdcl list char_type}}
-{{dcl list template | cpp/io/tdcl list traits_type}}
-{{dcl list template | cpp/io/tdcl list int_type}}
-{{dcl list template | cpp/io/tdcl list pos_type}}
-{{dcl list template | cpp/io/tdcl list off_type}}
-{{dcl list end}}
 ===Member functions===
-{{dcl list begin}}
+{{dsc begin}}
-{{dcl list template | cpp/io/basic_fstream/dcl list constructor | basic_fstream}}
+{{dsc inc|cpp/io/basic_fstream/dsc constructor|basic_fstream}}
-{{dcl list template | cpp/io/basic_fstream/dcl list destructor | basic_fstream}}
+{{dsc inc|cpp/io/basic_fstream/dsc destructor|basic_fstream}}
-{{dcl list template | cpp/io/basic_fstream/dcl list operator{{=}} | basic_fstream}}
+{{dsc inc|cpp/io/basic_fstream/dsc operator{{=}}|basic_fstream}}
-{{dcl list template | cpp/io/basic_fstream/dcl list swap | basic_fstream}}
+{{dsc inc|cpp/io/basic_fstream/dsc swap|basic_fstream}}
-{{dcl list template | cpp/io/basic_fstream/dcl list rdbuf | basic_fstream}}
+{{dsc inc|cpp/io/basic_fstream/dsc rdbuf|basic_fstream}}
+{{dsc inc|cpp/io/basic_fstream/dsc native_handle}}
-{{dcl list h2 | File operations }}
+{{dsc h2|File operations}}
-{{dcl list template | cpp/io/basic_fstream/dcl list is_open | basic_fstream}}
+{{dsc inc|cpp/io/basic_fstream/dsc is_open|basic_fstream}}
-{{dcl list template | cpp/io/basic_fstream/dcl list open | basic_fstream}}
+{{dsc inc|cpp/io/basic_fstream/dsc open|basic_fstream}}
-{{dcl list template | cpp/io/basic_fstream/dcl list close | basic_fstream}}
+{{dsc inc|cpp/io/basic_fstream/dsc close|basic_fstream}}
-{{dcl list end}}
+{{dsc end}}
 ===Non-member functions===
-{{dcl list begin}}
+{{dsc begin}}
-{{dcl list template | cpp/io/basic_fstream/dcl list swap2 | basic_fstream}}
+{{dsc inc|cpp/io/basic_fstream/dsc swap2|basic_fstream}}
-{{dcl list end}}
+{{dsc end}}
 {{cpp/io/basic_istream/inherit}}
@@ Line 62: / Line 42: @@
 {{cpp/io/basic_ios/inherit}}
 {{cpp/io/ios_base/inherit}}
+===Notes===
+{{feature test macro|__cpp_lib_fstream_native_handle|std=C++26|value=202306L|native handles support}}
+===Example===
+{{example
+|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===
+{{dsc begin}}
+{{dsc inc|cpp/string/basic_string/dsc getline}}
+{{dsc end}}
+{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

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I/O manipulators
Print functions (C++23)
C-style I/O
Buffers
basic_streambuf
basic_filebuf
basic_stringbuf
basic_spanbuf (C++23)
strstreambuf (C++98/26*)
basic_syncbuf (C++20)
Streams
Abstractions
ios_base
basic_ios
basic_istream
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File I/O
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basic_fstream
String I/O
basic_istringstream
basic_ostringstream
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Array I/O
basic_ispanstream (C++23)
basic_ospanstream (C++23)
basic_spanstream (C++23)
istrstream (C++98/26*)
ostrstream (C++98/26*)
strstream (C++98/26*)
Synchronized Output
basic_osyncstream (C++20)
Types
streamoff
streamsize
fpos
Error category interface
iostream_category (C++11)
io_errc (C++11)

Member functions
basic_fstream::basic_fstream
basic_fstream::operator= (C++11)
basic_fstream::swap (C++11)
basic_fstream::rdbuf
basic_fstream::native_handle (C++26)
File operations
basic_fstream::is_open
basic_fstream::open
basic_fstream::close
Non-member functions
swap(std::basic_fstream) (C++11)

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Difference between revisions of "cpp/io/basic fstream"

Latest revision as of 05:15, 4 August 2023

Contents

[edit] Member types

[edit] Member functions

File operations

[edit] Non-member functions

Inherited from std::basic_istream

Member functions

Formatted input

Unformatted input

Positioning

Miscellaneous

Member classes

Inherited from std::basic_ostream

Member functions

Formatted output

Unformatted output

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

[edit] Notes

[edit] Example

[edit] See also

Navigation

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