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

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< cpp‎ | io‎ | basic ifstream

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

| This section is incomplete Reason: no example

### 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[1]

1. ↑ The resolution of LWG issue #22 is overriden.

### 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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