# std::basic_filebuf::open

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< cpp‎ | io‎ | basic filebuf

std::basic_filebuf<CharT, Traits>* open( const char* s, std::ios_base::openmode mode ) | (1) |
std::basic_filebuf<CharT, Traits>* open( const std::string& str, std::ios_base::openmode mode ) | (2) | (since C++11)
std::basic_filebuf<CharT, Traits>* open( const std::filesystem::path& p, std::ios_base::openmode mode ) | (3) | (since C++17)
std::basic_filebuf<CharT, Traits>* open( const std::filesystem::path::value_type* s, std::ios_base::openmode mode ) | (4) | (since C++17)

Opens the file with the given name (s, p.c_str() (since C++17) or str.c_str(), depending on the overload).

Overload (4) is only provided if std::filesystem::path::value_type is not char. | (since C++17)

The file is opened as if by calling std::fopen(s, modestring), where `modestring` is determined as follows:

modestring | openmode & ~ate | Action if file already exists | Action if file does not exist
--- | --- | --- | ---
"r" | in | Read from start | Failure to open
"w" | out, out|trunc | Destroy contents | Create new
"a" | app, out|app | Append to file | Create new
"r+" | out|in | Read from start | Error
"w+" | out|in|trunc | Destroy contents | Create new
"a+" | out|in|app, in|app | Write to end | Create new
"rb" | binary|in | Read from start | Failure to open
"wb" | binary|out, binary|out|trunc | Destroy contents | Create new
"ab" | binary|app, binary|out|app | Write to end | Create new
"r+b" | binary|out|in | Read from start | Error
"w+b" | binary|out|in|trunc | Destroy contents | Create new
"a+b" | binary|out|in|app, binary|in|app | Write to end | Create new

If `openmode` is not one of the modes listed, the `open()` fails.

If the open operation succeeds and openmode & std::ios_base::ate != 0 (the `ate` bit is set), repositions the file position to the end of file, as if by calling std::fseek(file, 0, SEEK_END). If the repositioning fails, calls close() and returns a null pointer to indicate failure.

If the associated file was already open, returns a null pointer right away.

### Parameters

s, str, p | - | the file name to open
openmode | - | the file opening mode, a binary OR of the std::ios_base modes

### Return value

this on success, a null pointer on failure.

### Notes

`open()` is typically called through the constructor or the `open()` member function of std::basic_fstream.

### Example

| This section is incomplete Reason: no example

### See also

is_open | checks if the associated file is open (public member function)
close | flushes the put area buffer and closes the associated file (public member function)

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