fstream

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fstream.h is a standard C++ library that handles reading and writing to files either in text or in binary formats. It is an object oriented alternative to C's FILE from the C standard library. fstream is the result of a multiple inheritance with ifstream and ofstream, which both inherit from ios.

Text Files in fstream

For text files, a common method includes using the << and >> syntax that is followed with cout in the iostream library. The following example creates a file called 'file.txt' and puts the text 'Hello World' into it.

#include <fstream>
 
int main()
{
    std::ofstream file;
    file.open("file.txt");
    file << "Hello world!\n";
    file.close();
}

Binary Files in fstream

Reading from and writing to binary files uses the following functions:

ostream& fstream::write(const char *s, streamsize n );
istream& fstream::read(char *s, streamsize n );

where s is the buffer written to or read from, and size is the number of bytes to read or write (streamsize being an implementation-defined typedef; likely int or long). www.cplusplus.com explains that these functions are necessary as the use of "the extraction and insertion operators (<< and >>) and functions like getline is not efficient, since we do not need to format any data, and data may not use the separation codes used by text files to separate elements (like space, newline, etc...)."

The following example creates a file called 'file.bin', creates a 100 element buffer, and writes 20 bytes from the buffer into the file.

char buffer[100];
 
std::ofstream outFile;
outFile.open( "output.bin", ios::out | ios::binary );    //opens the file for binary output
outFile.write( buffer, 20 );                             //this writes 20 bytes from buffer to file.bin
outFile.close(); // Manually close the file.
 
std::ifstream inFile( "input.bin", ios::in | ios::binary ); 
inFile.read( buffer, 20 );
// The file will close itself when its destructor is called.

The sizeof operator can help properly read or write the correct number of bytes.

file.write(buffer, sizeof(int)*5); // Write five ints to file.

Initializing files

When initializing a file, there are several modes you can choose, depending on what operation or function you are using the file for. The different modes you can use are as follows:

The following is a snippet of code that shows the function of these modes.

#include <fstream>
 
int main()
{
        std::ofstream ofile;
        ofile.open("ex.txt", ios::out); //opens file as output
        int x=1;           
        ofile<<x;
        ofile.close();
        //ex.txt is now "1" 
        std::ifstream ifile;
        ifile.open("ex.txt",ios::in); //opens file as input
        int a;
        ifile>>a;//a now equals 1
        a=2;
        ifile.close();
        ofile.open("ex.txt", ios::out|ios::trunc); //opens file as output
        //ex.txt is now empty due to ios::trunc
        ofile<<x;//write 1 to file
        //ex.txt is now "1"
        ofile.close();
        ofile.open("ex.txt", ios::out|ios::app); //open file as output
        ofile<<a;//write 2 to file
        //ex.txt is now "12"
        ofile.close();
        ofile.open("ex.txt", ios::in|ios::out|ios::ate); //open file as output
        ofile<<x;//write 1 to file
        //ex.txt is now "121"        
        ofile.close();                         
}

• NOTE: for ios::ate, simply using this line of code: "ofile.open("ex.txt",ios::out|ios::ate);" will result in the file being overwritten as if ios::ate was not included in the function. Adding ios::in will enable this to function as described in the above table.

Member functions

What follows is an incomplete list of functions in iostream library:

See also ifstream, which is the C++ standard library class that enables input of data from files.

External References

• fstream reference
• http://www.cplusplus.com/doc/tutorial/files.html
• http://www.functionx.com/cpp/articles/filestreaming.htm

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