Re: [Dev-C++] Shift operators portability

[<or...@vp...>]

Hi!

As I see a lot of things are told about this shifting stuff, so I think 
I might also add something.
Bit shifting/or-ing/and-ing and so on, are VERY VERY useful things. 
There might be times, when you need to use all 8 bits of a byte. For 
example, when you have a big-big table (2D), and all you want to know, 
whether one cell is alive or not - 1 byte to store each cell is too much 
waste. Use 1 byte to store 8 cells, and use bit-masking to get the bits. 
But this is only one example, there are hundreds.

And I also want to add something for the endiannes-stuff. Lets consider, 
that you wan't to save a 4 byte integer in binary form into a file 
(binary - save the int in 4 bytes to a file). You will probably begin 
pondering about endianness, and write specific loading code for big- and 
little-endian machines. However, there is a tricky solution to evade the 
endiannes problem, and saving an integer in binary form and loading it 
back will work on both big and little endian machines with the exact 
same code. Here it is (let's consider, that int is 4 bytes, but you may 
rewrite it to uint32_t to be sure):

void write_int(FILE *outfile, int i)
{
   putc( i&0x000000ff,outfile);      /* Write the last byte */
   putc((i&0x0000ff00)>>8,outfile);  /* etc. */
   putc((i&0x00ff0000)>>16,outfile);
   putc((i&0xff000000)>>24,outfile);
}

int read_int(FILE *infile)
{
   int i=0;

   i |= ( (int)getc(infile));      /* Read the last byte: 0x000000xx */
   i |= (((int)getc(infile))<<8);  /* Next: 0x000000xx<<8 = 0x0000xx00 */
   i |= (((int)getc(infile))<<16); /* etc. */
   i |= (((int)getc(infile))<<24);
   return i;
}

The code above doesn't use any explicit conversion from int to char*, 
which is - lets say it - ugly. It works, because bit-shifting is 
"endian-free" i.e. it doesn't depend on endiannes. It works on both 
types of machines. As you can see, it uses the so-blessed shifting and 
bit-masking operators to ahcieve this goal.

I hope I could add something useful.
--
Greetings,
   Balázs