fstream();
fstream( const char* szName, int nMode, int nProt = filebuf::openprot );
fstream( filedesc fd );
fstream( filedesc fd, char* pch, int nLength );
szName
The name of the file to be opened during construction.
nMode
An integer that contains mode bits defined as ios enumerators that can be combined with the bitwise-OR (|) operator:
| Value | Meaning |
| ios::app | The function performs a seek to the end of file. When new bytes are written to the file, they are always appended to the end, even if the position is moved with the ostream::seekp function. |
| ios::ate | The function performs a seek to the end of file. When the first new byte is written to the file, it is appended to the end, but when subsequent bytes are written, they are written to the current position. |
| ios::in | The file is opened for input. The original file (if it exists), will not be truncated. |
| ios::out | The file is opened for output. |
| ios::trunc | If the file already exists, its contents are discarded. This mode is implied if ios::out is specified and ios::ate, ios::app, and ios:in are not specified. |
| ios::nocreate | If the file does not already exist, the function fails. |
| ios::noreplace | If the file already exists, the function fails. |
| ios::binary | Opens the file in binary mode (the default is text mode). |
Note that there is no ios::in or ios::out default mode for fstream objects. You must specify both modes if your fstream object must both read and write files.
nProt
The file protection specification; defaults to the static integer filebuf::openprot that is equivalent to filebuf::sh_compat. The possible nProt values are as follows:
| Value | Meaning |
| filebuf::sh_compat | Compatibility share mode. |
| filebuf::sh_none | Exclusive mode—no sharing. |
| filebuf::sh_read | Read sharing allowed. |
| filebuf::sh_write | Write sharing allowed. |
The filebuf::sh_read and filebuf::sh_write modes can be combined with the logical OR (|) operator.
fd
A file descriptor as returned by a call to the run-time function _open or _sopen. filedesc is a typedef equivalent to int.
pch
Pointer to a previously allocated reserve area of length nLength. A NULL value (or nLength = 0) indicates that the stream will be unbuffered.
nLength
The length (in bytes) of the reserve area (0 = unbuffered).
The four fstream constructors are described as follows:
| Constructor | Description |
| fstream() | Constructs an fstream object without opening a file. |
| fstream( const char*, int, int ) | Contructs an fstream object, opening the specified file. |
| fstream( filedesc ) | Constructs an fstream object that is attached to an open file. |
| fstream( filedesc, char*, int ) | Constructs an fstream object that is associated with a filebuf object. The filebuf object is attached to an open file and to a specified reserve area. |
All fstream constructors construct a filebuf object. The first three use an internally allocated reserve area, but the fourth uses a user-allocated area. The user-allocated area is not automatically released during destruction.