Input/Output with files
C++ has support both for input and output with files through the following classes:
ofstream: File class for writing operations (derived from ostream)
ifstream: File class for reading operations (derived from istream)
fstream: File class for both reading and writing operations (derived from iostream)
Open a file
The first operation generally done on an object of one of these classes is to associate it to a real file, that is to say, to open a file. The open file is represented within the program by a stream object (an instantiation of one of these classes) and any input or output performed on this stream object will be applied to the physical file.
In order to open a file with a stream object we use its member function open():
void open (const char * filename, openmode mode);
where filename is a string of characters representing the name of the file to be opened and mode is a combination of the following flags:
These flags can be combined using 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 function-member open:
ofstream file;
file.open ("example.bin", ios::out | ios::app | ios::binary);
All of the member functions open of classes ofstream, ifstream and fstream include a default mode when opening files that varies from one to the other:
The default value is only applied if the function is called without specifying a mode parameter. If the function is called with any value in that parameter the default mode is stepped on, not combined.
Since the first task that is performed on an object of classes ofstream, ifstream and fstream is frequently to open a file, these three classes include a constructor that directly calls the open member function and has the same parameters as this. This way, we could also have declared the previous object and conducted the same opening operation just by writing: ofstream file ("example.bin", ios::out | ios::app | ios::binary);
Both forms to open a file are valid. You can check if a file has been correctly opened by calling the member function is_open():
bool is_open();
that returns a bool type value indicating true in case that indeed the object has been correctly associated with an open file or false otherwise.
Closing a File
When reading, writing or consulting operations on a file are complete we must close it so that it becomes available again. In order to do that we shall call the member function close(), that is in charge of flushing the buffers and closing the file. Its form is quite simple:
void 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.
In case that an object is destructed while still associated with an open file, the destructor automatically calls the member function close.
Text Mode Files
Classes ofstream, ifstream and fstream are derived from ostream, istream and iostream respectively. That's why fstream objects can use the members of these parent classes to access data.
Generally, when using text files we shall use the same members of these classes that we used in communication with the console (cin and cout). As in the following example, where we use the overloaded insertion operator <<:
// writing on a text file Output(file example.txt) #include <fstream.h> This is a line. int main () { This is another line. ofstream examplefile ("example.txt"); if (examplefile.is_open()) { examplefile << "This is a line.\n"; examplefile << "This is another line.\n"; examplefile.close(); } return 0; }
Data input from file can also be performed in the same way that we did with cin:
// reading a text file Output #include <iostream.h> This is a line. #include <fstream.h> This is another line. #include <stdlib.h>
int main () { char buffer[256]; ifstream examplefile ("example.txt"); if (! examplefile.is_open()) { cout << "Error opening file"; exit (1); } while (! examplefile.eof() ) { examplefile.getline (buffer,100); cout << buffer << endl; } return 0; } Binary Files
In binary files inputting and outputting data with operators like << and >> and functions like getline,
does not make too much sense, although they are perfectly valid. File streams include two member
functions specially designed for input and output of data sequentially: write and read. The first one
(write) is a member function of ostream, also inherited by ofstream. And read is member function of
istream and it is inherited by ifstream. Objects of class fstream have both.
Their prototypes are:
write ( char * buffer, streamsize size );
read ( char * buffer, streamsize size );
Where buffer is the address of a memory block where the read data are stored or from where the data
to be written are taken. The size parameter is an integer value that specifies the number of characters
to be read/written from/to the buffer.
// reading binary file Output #include <iostream.h> the complete file is in a buffer #include <fstream.h> const char * filename = "example.txt"; int main () { char * buffer; long size; ifstream file (filename, ios::in|ios::binary|ios::ate); size = file.tellg(); file.seekg (0, ios::beg); buffer = new char [size]; file.read (buffer, size); file.close(); cout << "the complete file is in a buffer"; delete[] buffer; return 0; }