The GNU C++ Iostream Library

4.1: Reading and writing files

These methods are declared in fstream.h.

4.1.1: File Input

You can read data from class ifstream with any operation from class istream. There are also a few specialized facilities:

Constructor:  ifstream::ifstream ()

Make an ifstream associated with a new file for input. (If you use this version of the constructor, you need to call ifstream::open before actually reading anything)

Constructor:  ifstream::ifstream (int fd)

Make an ifstream for reading from a file that was already open, using file descriptor fd. (This constructor is compatible with other versions of iostreams for POSIX systems, but is not part of the ANSI working paper.)

Constructor:  ifstream::ifstream (const char* fname [, int mode [, int prot]])

Open a file *fname for this ifstream object.

By default, the file is opened for input (with ios::in as mode). If you use this constructor, the file will be closed when the ifstream is destroyed.

You can use the optional argument mode to specify how to open the file, by combining these enumerated values (with `|' bitwise or). (These values are actually defined in class ios, so that all file-related streams may inherit them.) Only some of these modes are defined in the latest draft ANSI specification; if portability is important, you may wish to avoid the others.

ios::in

Open for input. (Included in ANSI draft.)

ios::out

Open for output. (Included in ANSI draft.)

ios::ate

Set the initial input (or output) position to the end of the file.

ios::app

Seek to end of file before each write. (Included in ANSI draft.)

ios::trunc

Guarantee a fresh file; discard any contents that were previously associated with it.

ios::nocreate

Guarantee an existing file; fail if the specified file did not already exist.

ios::noreplace

Guarantee a new file; fail if the specified file already existed.

ios::bin

Open as a binary file (on systems where binary and text files have different properties, typically how `\n' is mapped; included in ANSI draft).

The last optional argument prot is specific to Unix-like systems; it specifies the file protection (by default `644').

Method:  void ifstream::open (const char* fname [, int mode [, int prot]])

Open a file explicitly after the associated ifstream object already exists (for instance, after using the default constructor). The arguments, options and defaults all have the same meanings as in the fully specified ifstream constructor.

4.1.2: File Output

You can write data to class ofstream with any operation from class ostream. There are also a few specialized facilities:

Constructor:  ofstream::ofstream ()

Make an ofstream associated with a new file for output.

Constructor:  ofstream::ofstream (int fd)

Make an ofstream for writing to a file that was already open, using file descriptor fd.

Constructor:  ofstream::ofstream (const char* fname [, int mode [, int prot]])

Open a file *fname for this ofstream object.

By default, the file is opened for output (with ios::out as mode). You can use the optional argument mode to specify how to open the file, just as described for ifstream::ifstream.

The last optional argument prot specifies the file protection (by default `644').

Destructor:  ofstream::~ofstream ()

The files associated with ofstream objects are closed when the corresponding object is destroyed.

Method:  void ofstream::open (const char* fname [, int mode [, int prot]])

Open a file explicitly after the associated ofstream object already exists (for instance, after using the default constructor). The arguments, options and defaults all have the same meanings as in the fully specified ofstream constructor.

4.1.3: File Input/Output

The class fstream combines the facilities of ifstream and ofstream, just as iostream combines istream and ostream.

4.1.4: File Stream Base

The class fstreambase underlies both ifstream and ofstream. They both inherit this additional method:

Method:  void fstreambase::close ()

Close the file associated with this object, and set ios::fail in this object to mark the event.