Re: Newbie question: How to define a class that will work on bits from a binary file?

James Kanze <>
Thu, 15 May 2008 02:35:44 -0700 (PDT)
On May 15, 9:14 am, Michael DOUBEZ <> wrote:

Victor Bazarov a =E9crit :

Damfino wrote:

Newbie question here wrt defining a class that will work on
bits read from a binary file. How would you go about doing
it? As an example please look at the structure of my data
given below. The data comes in 40 byte packets via stdin or
a binary file.
 syncByte (8bits)
 XML_type (2bits)
 XML_subtype (2bits)
 record_value (3bits)
 playout_flag (1bit)
 if (playout_flag=='1') {
    playout_length (8bits)
    for (i=0; i< playout_length; i++){
 payload to fill the rest of the 40 bytes
How would this be defined as a class?

// assuming that 'char' is 8 bits

And assuming the machine is in little endian. In big endian,
you would have to invert the bit fields.

Or maybe it wouldn't work at all. How the compiler lays out bit
fields is implementation dependent, and varies greatly. Bit
fields cannot be used for mapping external data formats.

It's possible to write a stream which reads arbitrary bit
lengths; I did it once in the past (for use with a compression
algorithm). It's rarely necessary, however, since in practice,
files aren't defined as streams of bits, but as streams of
bytes. So his actual file format is probably something more

    byte 1 | sync |
    byte 2 | type |subtype| rec. value| F |


He streams in bytes (as unsigned char), and processes them one
after the other.

Internally, it's up to him (and his design) whether he maintains
the information in byte 2 in separate variables, or in a single
unsigned char, extracting the individual fields on an as needed
basis. At any rate, the functions to get the field from the
byte would look something like:

    getType( unsigned char byte2 )
        return static_cast< XMLTypeId >( (byte2 >> 6) & 0x03 ) ;

(except, of course, that one would use named constants instead
of the magic numbers). Depending on the application, he might
be able to define XMLTypeId as something like:

    enum XMLTypeId
        type1 = 0x00,
        type2 = 0x40,
        type3 = 0x80,
        type4 = 0xC0,
        typeMask = type1 | type2 | type3 | type 4
    } ;

and skip the shift, i.e.:

    getType( unsigned char byte2 )
        return static_cast< XMLTypeId >( byte2 & typeMask ) ;

There's no portable way to use bit fields to map to an external
representation. (You could use bit fields for the internal
representation, in order to save memory.)

And one last point, while I'm at it. In the original posting,
it was mentionned that he might be reading from standard in.
There is no way to read binary data from std::cin (nor from
stdin in C).

James Kanze (GABI Software)
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