> Hello all,
>
> Another Question on C/C++, Please be patient just learning.
>
> Example from Palm OS Bible librarian sample.
>
> librarianDB.c
>
> // Utility macros for handling bit fields
>
> #define BitAtPosition(pos) ((UInt16)1 << (pos))
>
> #define GetBitMacro(bitfield, index) ((bitfield) & BitAtPosition(index))
>
> #define SetBitMacro(bitfield, index) ((bitfield) |= BitAtPosition(index))
>
> #define RemoveBitMacro(bitfield, index) ((bitfield) &=
> ~BitAtPosition(index))
>
> Here's the question in my newfound C speak;
>
> if ((#define BitAtPosition(pos) ((UInt16)1 << (pos))) == ("Sets a 16 bit
> bitmask with (pos)")) {
>
> " Why not declare bitmap constants?";
>
> "Is there a performance issue?";
>
> a memory space issue?;
>
> "Or just to avoid constants for the many flags in this app?";
>
> }
>
> else {
>
> "Did I miss the whole point of the Macro?";
>
> "I hope not given the time it took to come up with the interpretation";
>
> }
>
Whether to use macros like this or use bitmap constants depends on
what you're wanting to do in your program. I'd use this kind of thing
to set up a bit mask where I had, say, five things I wanted to
reference in a bitmask, and wanted to iterate over them, so the code
would be something like:
for (k=0; k<5; k++)
if (GetBitMacro(bitfield, k))
{
//Do something
}
On the other hand, if I wanted to set up a bit mask where I had five
dissimilar items, and there would never be a need to iterate over
them, I'd probably use bitmap constants. An example of this would
probably be option flags passed in to a program or function, where
each option would require different handling.
Dean
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