Input/output with files C++

Input/output with files C++ provides the following classes to perform output and input of characters to/from files:

1. ofstream: Stream class to write on files

2. ifstream: Stream class to read from files

3. fstream: Stream class to both read and write from/to files.

These classes are derived directly or indirectly from the classes istream, and ostream. We have already used objects whose types were these classes: cin is an object of class istream and cout is an object of class ostream. Therefore, we have already been using classes that are related to our file streams. And in fact, we can use our file streams the same way we are already used to use cin and cout, with the only difference that we have to associate these streams with physical files.

Opening files using open():

The function open () can be used to open multiple files that use same stream object. 

Eg fstream inoutfile; 

inoutfile.open(“book.dat”); 

Where filename is a null-terminated character sequence of type const char * (the same type that string literals have) representing the name of the file to be opened, and mode is an optional parameter with a combination of the following flags:

All these flags can be combined using the bitwise operator OR (|). For example, if we want to open the file example.bin in binary mode to add data we could do it by the following call to member function open(): 

ofstream myfile; 

myfile.open ("example.bin", ios::out | ios::app | ios::binary);

For ifstream and ofstream classes, ios::in and ios::out are automatically and respectively assumed, even if a mode that does not include them is passed as the second argument to the open() member function. The default value is only applied if the function is called without specifying any value for the mode parameter. If the function is called with any value in that parameter the default mode is overridden, not combined. 

File streams opened in binary mode perform input and output operations independently of any format considerations. Non-binary files are known as text files, and some translations may occur due to the formatting of some special characters (like newline and carriage return characters). Since the first task that is performed on a file stream object is generally to open a file, these three classes include a constructor that automatically calls the open() member functions and has the exact same parameters as this member. Therefore, we could also have declared the previous myfile object and conducted the same opening operation in our previous example by writing: 

ofstream myfile ("example.bin", ios::out | ios::app | ios::binary);

Combining object construction and stream opening in a single statement. Both forms to open a file are valid and equivalent. To check if a file stream was successful opening a file, you can do it by calling to member is_open() with no arguments. This member function returns a bool value of true in the case that indeed the stream object is associated with an open file, or false otherwise: 

if (myfile.is_open()) { /* ok, proceed with output */ }

Closing a file: 

When we are finished with our input and output operations on a file we shall close it so that its resources become available again. In order to do that we have to call the stream's member function close(). This member function takes no parameters, and what it does is to flush the associated buffers and close the file: 

myfile.close();

Once this member function is called, the stream object can be used to open another file, and the file is available again to be opened by other processes. 

Checking state flags:

eof() Returns true if a file open for reading has reached the end. Functions for manipulation of file pointers: All the actions on the file pointers take place automatically by default. The file stream classes support following functions to manage file pointers manually. 

tellg() and tellp().

These two member functions have no parameters and return a value of the member type pos_type, which is an integer data type representing the current position of the get stream pointer (in the case of tellg) or the put stream pointer (in the case of tellp).

seekg() and seekp():

These functions allow us to change the position of the get and put stream pointers. Both functions are overloaded with two different prototypes. The first prototype is: 

seekg ( position ); 

seekp ( position ); 

Using this prototype the stream pointer is changed to the absolute position (counting from the beginning of the file). The type for this parameter is the same as the one returned by functions tellg and tellp: the member type pos_type, which is an integer value.