Reading and writing files

f78 Reading and writing files

Reading and writing files

These methods are declared in fstream.h. You can read data from class ifstream with any operation from class istream. There are also a few specialized facilities, as in the following methods.

ifstream::ifstream () 
Constructor

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)

ifstream::ifstream (int fd) 
Constructor

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, although it is not part of the ANSI working paper.)

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

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 al-ready existed.
ios::binary
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).

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

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.

You can write data to class ofstream with any operation from class ostream. The following documentation discusses a few specialized facilities.

ofstream::ofstream () 
Constructor

Make an ofstream associated with a new file for output.

ofstream::ofstream (int fd) 
Constructor

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

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

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).

ofstream::~ofstream () 
Destructor

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

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

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.

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

The class fstreambase underlies both ifstream and ofstream. They both inherit the following additional method.

void fstreambase::close () 
Method

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