std::basic_fstream::open

C++

Language

Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

Input/output library

I/O manipulators

C-style I/O

Buffers

basic_streambuf

basic_filebuf

basic_stringbuf

strstreambuf (deprecated)

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

istrstream (deprecated)

ostrstream (deprecated)

strstream (deprecated)

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

File operations

basic_fstream::is_open

basic_fstream::open

basic_fstream::close

Non-member functions

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

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

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

Calls setstate(failbit) on failure.

Calls clear() on success.

(since C++11)

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 bitmask type, 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

Return value

(none)

Example

Run this code

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

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';
}

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)