Bob Hairgrove <invalid@bigfoot.com> wrote in message news:<oi7mp0t19kcbmd71mhh15s90o1p4n6cuji@4ax.com>. ..[color=blue]
> Surely this is a no-brainer, but sometimes I think I must have no
> brain (no comments, please<g>). Anyway, I have the following struct
> containing bit fields:
>
> struct ExtPenStyle
> {
> unsigned long style : 4;
> unsigned long : 4; // unused
> unsigned long endcap : 2;
> unsigned long : 2; // unused
> unsigned long join : 2;
> unsigned long : 2; // unused
> unsigned long type : 1;
> unsigned long : 15; // unused
> };
>
> I tried casting from/to unsigned long using static_cast,
> reinterpret_cast and C-style cast. The compiler refuses to do it, but
> I need to call API code which expects an unsigned long with bitmapped
> values.[/color]
The compile is right, and the API couldn't care whether you
actually used bitfiels.
E.g.
template< int pos, int length >
struct field {
static unsigned long mask = (1UL<<length-1);
long get( unsigned long src ) { return (src>>pos) & mask }
void set( unsigned long& dest, long val ) {
val &= mask;
dest &= ~(mask<<pos);
dest |= val << pos;
}
};
struct ExtPenStyle
{
unsigned long bits;
field<0, 4> style_pos;
unsigned long get_style_pos() { return style_pos.get(bits); }
void set_style_pos(unsigned long v) { style_pos.set(bits, v); }
//etcetera
};
No casts needed. The compiler will do pretty much the same,
but this is more reliable (there are at least two ways to stuff
bitfields in a long, MSB or LSB first. Here YOU choose.)
HTH,
Michiel Salters
