16.5 Binary I/O

Binary I/O is accomplished through two member functions: read and write. The syntax for read is:

in_file.read(data_ptr, size); 

data_ptr

Pointer to a place to put the data.

size

Number of bytes to be read.

The member function gcount returns the number of bytes gotten by the last read. This may be less than the number of bytes requested. For example, the read might encounter an end-of-file or error:

struct { 
    int     width; 
    int     height; 
} rectangle; 

in_file.read(static_cast<char *>(&rectangle), sizeof(rectangle));
if (in_file.bad(  )) {
    cerr << "Unable to read rectangle\n"; 
    exit (8); 
} 
if (in_file.gcount(  ) != sizeof(rectangle)) {
    cerr << "Error: Unable to read full rectangle\n";
    cerr << "I/O error of EOF encountered\n";
}

In this example you are reading in the structure rectangle. The & operator makes rectangle into a pointer. The cast static_cast<char *> is needed since read wants a character array. The sizeof operator is used to determine how many bytes to read as well as to check that read was successful.

The member function write has a calling sequence similar to read:

out_file.write(data_ptr, size);