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

Michael DOUBEZ <>
Thu, 15 May 2008 09:14:24 +0200
Victor Bazarov a ?crit :

Damfino wrote:

Hi all,
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.

class DataPacket {
    // somehow control the alignment and make it 1 byte
    // to avoid padding between members of this class
    char syncByte; // not sure you need this to be kept
    struct BitStuff {
        unsigned XML_type:2;
        unsigned XML_subtype:2;
        unsigned record_value:3;
        unsigned payout:1;
    } fields;
    char restOfPacket[38];
    // member functions go here

Why do you use an intermediary structure, I would have simply defined
(without LSB/MSB logic):
struct XML_Data
  char syncByte;//8 bits
  unsigned char XML_type:2; // 2 bits
  unsigned char XML_subtype:2; //+2 bits
  unsigned char record_value:3; //+3 bits
  unsigned char payout:1; //+1 bits = 8 bits => aligned
  char restOfPacket[38];


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