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

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< cpp‎ | io‎ | basic fstream

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<CharT,Traits>`)

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