13.5. Unformatted I/O Using read, write and gcount

Unformatted input/output is performed using the read and write member functions of istream and ostream, respectively. Member function read inputs bytes to a built-in array of chars in memory; member function write outputs bytes from a built-in array of chars. These bytes are not formatted in any way. They’re input or output as raw bytes. For example, the call

char buffer[] = "HAPPY BIRTHDAY";
cout.write( buffer, 10 );

outputs the first 10 bytes of buffer (including null characters, if any, that would cause output with cout and << to terminate). The call

cout.write( "ABCDEFGHIJKLMNOPQRSTUVWXYZ", 10 );

displays the first 10 characters of the alphabet.

The read member function inputs a designated number of characters into a built-in array of chars. If fewer than the designated number of characters are read, failbit is set. Section 13.8 shows how to determine whether failbit has been set. Member function gcount reports the number of characters read by the last input operation.

Figure 13.7 demonstrates istream member functions read and gcount, and ostream member function write. The program inputs 20 characters (from a longer input sequence) into the array buffer with read (line 13), determines the number of characters input with gcount (line 17) and outputs the characters in buffer with write (line 17).

Click here to view code image

 1   // Fig. 13.7: fig13_07.cpp
 2   // Unformatted I/O using read, gcount and write.
 3   #include <iostream>
 4   using namespace std;
 5
 6   int main()
 7   {
 8      const int SIZE = 80;
 9      char buffer[ SIZE ]; // create array of 80 characters
10
11      // use function read to input characters into buffer
12      cout << "Enter a sentence:" << endl;
13      cin.read( buffer, 20 );
14
15      // use functions write and gcount to display buffer characters
16      cout << endl << "The sentence entered was:" << endl;
17      cout.write( buffer, cin.gcount() );
18      cout << endl;
19   } // end main

Enter a sentence:
Using the read, write, and gcount member functions
The sentence entered was:
Using the read, writ

Fig. 13.7. Unformatted I/O using read, gcount and write.

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