std::basic_ofstream

 

C++

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Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

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Filesystem library (C++17)

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Technical specifications

Symbols index

External libraries

 

Input/output library

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

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

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)

 

std::basic_ofstream

Member functions

basic_ofstream::basic_ofstream

basic_ofstream::operator= (C++11)

basic_ofstream::swap (C++11)

basic_ofstream::rdbuf

basic_ofstream::native_handle (C++26)

File operations

basic_ofstream::is_open

basic_ofstream::open

basic_ofstream::close

Non-member functions

swap(std::basic_ofstream) (C++11)

 

Defined in header <fstream>
template<     class CharT,     class Traits = std::char_traits<CharT> > class basic_ofstream : public std::basic_ostream<CharT, Traits>

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

A typical implementation of std::basic_ofstream 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
ofstream	basic_ofstream<char>
wofstream	basic_ofstream<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_ofstream 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_ofstream) (C++11)

specializes the std::swap algorithm (function template)

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 [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)

Example

Run this code

#include <iostream>
#include <fstream>
#include <limits>
#include <string>
 
struct Birthday { long day, month, year; };
 
class Entry {
    std::string name;
    Birthday bday;
public:
    Entry(std::string name_, long day_, long month_, long year_) : 
        name(name_), bday({day_, month_, year_}) {}
    void printToStdOut() { 
        std::cout << name << ' ' << bday.day << '/' << bday.month << '/'
                  << bday.year << '\n';
    }
    //name and bday are private so we must make << and >> friends of the class or
    // otherwise write getter and setter methods for name and bday
    friend std::ostream &operator<<(std::ostream &, Entry &);
    friend std::istream &operator>>(std::istream &, Entry &);
};
 
std::ostream &operator<<(std::ostream &strm, Entry &e) {
    //Since the length of e.name is not necessarily known at compile time, we
    // can't simply use: strm.write(reinterpret_cast<char*>(&e), sizeof e);
    unsigned int length = e.name.length();
    strm.write(reinterpret_cast<char*>(&length), sizeof length);
    strm.write(&e.name[0], length);
    strm.write(reinterpret_cast<char*>(&e.bday), sizeof e.bday);
    return strm;
}
 
std::istream &operator>>(std::istream &strm, Entry &e) {
    unsigned int length;
    strm.read(reinterpret_cast<char*>(&length), sizeof length);
    e.name.resize(length);
    strm.read(&e.name[0], length);
    strm.read(reinterpret_cast<char*>(&e.bday), sizeof e.bday);
    return strm;
}
 
int main() {
    Entry e("Alpha", 1, 1, -13820000000);
    e.printToStdOut();
 
    std::string filename = "Test.b";
    std::ofstream ostrm;
    //Open the file or else create it
    ostrm.open(filename, std::fstream::binary);
    if(!ostrm.is_open()) {
        ostrm.clear();
        ostrm.open(filename, std::fstream::out);  //Create the file
        ostrm.close();
        ostrm.open(filename, std::fstream::binary);
        if(!ostrm.good())
            return 1;
    }
 
    //write the Entry e to the file
    ostrm << e;
    ostrm.close();
 
    Entry e_recovered("Omega", 31, 12, std::numeric_limits<long>::max());
    e_recovered.printToStdOut();
 
    //Open the file for reading
    std::ifstream istrm;
    istrm.open(filename, std::fstream::binary);
    if(!istrm.good())
        return 1;
 
    //overwrite e_recovered with the values from istrm
    istrm >> e_recovered;
    istrm.close();
 
    e_recovered.printToStdOut();
    return 0;
}

Output:

Alpha 1/1/-13820000000
Omega 31/12/9223372036854775807
Alpha 1/1/-13820000000