ifstream();
ifstream( const char* szName, int nMode = ios::in, int nProt = filebuf::openprot );
ifstream( filedesc fd );
ifstream( 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::in | The file is opened for input (default). |
| ios::nocreate | If the file does not already exist, the function fails. |
| ios::binary | Opens the file in binary mode (the default is text mode). |
Note that the ios::nocreate flag is necessary if you intend to test for the file's existence (the usual case).
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 ifstream constructors are described as follows:
| Constructor | Description |
| ifstream() | Constructs an ifstream object without opening a file. |
| ifstream( const char*, int, int ) | Contructs an ifstream object, opening the specified file. |
| ifstream( filedesc ) | Constructs an ifstream object that is attached to an open file. |
| ifstream( filedesc, char*, int ) | Constructs an ifstream object that is associated with a filebuf object. The filebuf object is attached to an open file and to a specified reserve area. |
All ifstream constructors construct a filebuf object. The first three use an internally allocated reserve area, but the fourth uses a user-allocated area.