Page 286 - DCAP404 _Object Oriented Programming

Unit 13: Working with Files

myfile.seekg(10) //Take the pointer to 10 characters after the current location Notes
myfile.seekg(-10) //Take the pointer to 10 characters before the current location
Moreover, the function seekg()is overloaded to take two parameters. Thus, another form of the
function is,
myfile.seekg(-4, ios::end) //Take the pointer 4 characters before the end of file
Most of the time it is necessary to check whether a specified file exists or not while opening it.
Opening of file may also fail due to other reasons. Remember that if the file opening fails the
open() function return a non-zero value. By checking the return type of the function one can
ensure whether the file was opened successfully or not as demonstrated in the following program
snippet.
fstream myfile(“data.dat”);

if (!myfile)
{
cout << “Error : File could not be opened\n”;
exit(1);
}
The ofstream class also provides a fail() function which return true if the opening of file operation
failed as shown in the following code snippet
ofstream myfile(“data.dat”, ios::nocreate);
if(myfile.fail())
{
cout << “Error : File could not be opened\n”;

exit(1);
}
binary files are unformatted and uninterpreted file of binary digits. It simply contains binary
numbers whose meaning is provided by the program that manipulates the file contents. The
functions that give you the possibility to write/read unformatted files are get() and put(). To
read a byte, you can use get() and to write a byte, use put(). Both get() and put() functions take
one parameter - a char variable or character.
If you want to read/write whole blocks of data, then you can use the read() and write() functions.
Their prototypes are:

istream &read(char *buf, streamsize num);
ostream &write(const char *buf, streamsize num);
For the read() function, but should be an array of chars, where the read block of data will be put.
For the write() function, buf is an array of chars, where is the data you want to save in the file. For
the both functions, num is a number, that defines the amount of data (in symbols) to be read/
written.
Another function that provides the number of symbols read so far - gcount(). It simply function
returns the number of read symbols for the last unformatted input operation. You can specify
that the file is going to be operated on in binary mode in the open() function. The required mode
is ios::binary.

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