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

From cppreference.com
< cpp‎ | io
m (Text replace - "{{cpp|" to "{{c|")
m (→‎Notes: +FTM, fmt, trim.)
 
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{{cpp/title|basic_fstream}}
 
{{cpp/title|basic_fstream}}
−
{{cpp/io/basic_fstream/sidebar}}
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{{cpp/io/basic_fstream/navbar}}
−
{{ddcl | header=fstream |
+
{{ddcl|header=fstream|1=
−
template<  
+
template<
−
     class CharT,  
+
     class CharT,
−
     class Traits {{=}} std::char_traits<CharT>
+
     class Traits = std::char_traits<CharT>
 
> class basic_fstream : public std::basic_iostream<CharT, Traits>
 
> 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.png|map=
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{{inheritance diagram/std-basic_fstream}}
−
rect 5 33 107 66 [[cpp/io/ios_base]]
+
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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]]
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rect 452 31 573 69 [[cpp/io/basic_iostream]]
+
−
}}
+
  
−
Two specializations for common character types are also defined:
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{{cpp/basic char typedefs|fstream}}
  
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{{tdcl list begin}}
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{{cpp/io/member types|fstream}}
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{{tdcl list header | fstream}}
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{{tdcl list hitem | Type | Definition}}
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{{tdcl list item | {{tt|fstream}} | {{c|basic_fstream<char>}}}}
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{{tdcl list item | {{tt|wfstream}} | {{c|basic_fstream<wchar_t>}}}}
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{{tdcl list end}}
+
−
 
+
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===Member types===
+
−
{{tdcl list begin}}
+
−
{{tdcl list hitem | Member type | Definition}}
+
−
{{tdcl list template | cpp/io/tdcl list char_type}}
+
−
{{tdcl list template | cpp/io/tdcl list traits_type}}
+
−
{{tdcl list template | cpp/io/tdcl list int_type}}
+
−
{{tdcl list template | cpp/io/tdcl list pos_type}}
+
−
{{tdcl list template | cpp/io/tdcl list off_type}}
+
−
{{tdcl list end}}
+
  
 
===Member functions===
 
===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}}
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{{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 }}
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{{dsc h2|File operations}}
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{{dcl list template | cpp/io/basic_fstream/dcl list is_open | basic_fstream}}
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{{dsc inc|cpp/io/basic_fstream/dsc is_open|basic_fstream}}
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{{dcl list template | cpp/io/basic_fstream/dcl list open | basic_fstream}}
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{{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}}
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{{dsc end}}
  
 
===Non-member functions===
 
===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}}
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{{dcl list end}}
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{{dsc end}}
  
 
{{cpp/io/basic_istream/inherit}}
 
{{cpp/io/basic_istream/inherit}}
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{{cpp/io/basic_ios/inherit}}
 
{{cpp/io/basic_ios/inherit}}
 
{{cpp/io/ios_base/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}}

Latest revision as of 05:15, 4 August 2023

 
C++
 
Input/output library
 
std::basic_fstream
 
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>.

cpp/io/ios basecpp/io/basic ioscpp/io/basic istreamcpp/io/basic ostreamcpp/io/basic iostreamstd-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>

Contents

[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

 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]
(C++11)
 moves the file stream
(public member function) [edit]
(C++11)
 swaps two file streams
(public member function) [edit]
returns the underlying raw file device object
(public member function) [edit]
(C++26)
 returns the underlying implementation-defined handle
(public member function) [edit]
File operations
checks if the stream has an associated file
(public member function) [edit]
opens a file and associates it with the stream
(public member function) [edit]
closes the associated file
(public member function) [edit]

[edit] Non-member functions

(C++11)
 specializes the std::swap algorithm
(function template) [edit]

Inherited from std::basic_istream

Member functions

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

Member classes

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

Member classes

 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
checks if no error has occurred i.e. I/O operations are available
(public member function of std::basic_ios<CharT,Traits>) [edit]
checks if end-of-file has been reached
(public member function of std::basic_ios<CharT,Traits>) [edit]
checks if an error has occurred
(public member function of std::basic_ios<CharT,Traits>) [edit]
checks if a non-recoverable error has occurred
(public member function of std::basic_ios<CharT,Traits>) [edit]
checks if an error has occurred (synonym of fail())
(public member function of std::basic_ios<CharT,Traits>) [edit]
checks if no error has occurred (synonym of !fail())
(public member function of std::basic_ios<CharT,Traits>) [edit]
returns state flags
(public member function of std::basic_ios<CharT,Traits>) [edit]
sets state flags
(public member function of std::basic_ios<CharT,Traits>) [edit]
modifies state flags
(public member function of std::basic_ios<CharT,Traits>) [edit]
Formatting
copies formatting information
(public member function of std::basic_ios<CharT,Traits>) [edit]
manages the fill character
(public member function of std::basic_ios<CharT,Traits>) [edit]
Miscellaneous
manages exception mask
(public member function of std::basic_ios<CharT,Traits>) [edit]
sets the locale
(public member function of std::basic_ios<CharT,Traits>) [edit]
manages associated stream buffer
(public member function of std::basic_ios<CharT,Traits>) [edit]
manages tied stream
(public member function of std::basic_ios<CharT,Traits>) [edit]
narrows characters
(public member function of std::basic_ios<CharT,Traits>) [edit]
widens characters
(public member function of std::basic_ios<CharT,Traits>) [edit]

Inherited from std::ios_base

Member functions

Formatting
manages format flags
(public member function of std::ios_base) [edit]
sets specific format flag
(public member function of std::ios_base) [edit]
clears specific format flag
(public member function of std::ios_base) [edit]
manages decimal precision of floating point operations
(public member function of std::ios_base) [edit]
manages field width
(public member function of std::ios_base) [edit]
Locales
sets locale
(public member function of std::ios_base) [edit]
returns current locale
(public member function of std::ios_base) [edit]
Internal extensible array
[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]
resizes the private storage if necessary and access to the long element at the given index
(public member function of std::ios_base) [edit]
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
registers event callback function
(public member function of std::ios_base) [edit]
[static]
 sets whether C++ and C I/O libraries are interoperable
(public static member function of std::ios_base) [edit]
Member classes
stream exception
(public member class of std::ios_base) [edit]
initializes standard stream objects
(public member class of std::ios_base) [edit]

Member types and constants

Type Explanation
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]
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]
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]
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]
specifies event type
(enum) [edit]
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

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