std::basic_fstream<CharT,Traits>::open

 

C++

Compiler support

Freestanding and hosted

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Standard library

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)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

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_fstream

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)

 

void open( const char* filename,            std::ios_base::openmode mode                = std::ios_base::in | std::ios_base::out );	(1)
void open( const std::filesystem::path::value_type* filename,            std::ios_base::openmode mode                = std::ios_base::in | std::ios_base::out );	(2)	(since C++17)
void open( const std::string& filename,            std::ios_base::openmode mode                = std::ios_base::in | std::ios_base::out );	(3)	(since C++11)
void open( const std::filesystem::path& filename,            std::ios_base::openmode mode                = std::ios_base::in | std::ios_base::out );	(4)	(since C++17)

Opens and associates the file with name filename with the file stream.

Calls clear() on success. Calls setstate(failbit) on failure.

1,2) Effectively calls rdbuf()->open(filename, mode) (see std::basic_filebuf::open for the details on the effects of that call). Overload (2) is only provided if std::filesystem::path::value_type is not char.(since C++17)

3,4) Effectively calls (1,2) as if by open(filename.c_str(), mode).

Parameters

filename	-	the name of the file to be opened
mode	-	specifies stream open mode. It is a BitmaskType, the following constants are 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

Return value

(none)

Example

Run this code

#include <fstream>
#include <iostream>
#include <string>
 
int main()
{
    std::string filename = "example.123";
 
    std::fstream fs;
    fs.open(filename);
 
    if (!fs.is_open())
    {
        fs.clear();
        fs.open(filename, std::ios::out); // create file
        fs.close();
        fs.open(filename);
    }
 
    std::cout << std::boolalpha;
    std::cout << "fs.is_open() = " << fs.is_open() << '\n';
    std::cout << "fs.good() = " << fs.good() << '\n';
}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 22	C++98	it was unclear how error state changes upon a successful open	the error state is unchanged
LWG 409	C++98	the error state was unchanged upon a successful open	it is cleared
LWG 460	C++98	the default argument of mode in overload (1) was missing (it is present in the synopsis)	added

See also

is_open	checks if the stream has an associated file (public member function)
close	closes the associated file (public member function)
open	opens a file and configures it as the associated character sequence (public member function of std::basic_filebuf<CharT,Traits>)