🚀 go-pugleaf

Ian Dall (dall@hfrd015.dsto.gov.au) wrote: 
: I have a problem where I wish to construct a table to access structure
: members. Eg:

:     struct state {
:       int dummy0;
:       int r0, r2;
:       int dummy1;
:       int r1, r3;
:     };

:     int read_reg(struct state *t, int regno)
:     {
:     #define offset(n) (char *) &t->r ## n - (char *) &t->r0
:       static const reg_offsets[] = { offset(0), offset(1), offset(2),
:                                      offset(3)};
:     #undef offset
:       return *(int *)((char *) &t->r0 + reg_offsets[regno]);
:     }

[ Question about gcc's rules on evaluating constants deleted ]

: Is there a better way of doing what I want? The struct state is not
: mutable. I'd prefer not to have a stack of hand calculated constants
: around.

Is there some reason why you don't use:

	int read_reg(struct state* t, int regno)
	{
		switch(regno)
		{
		case 0: return t->r0;
		case 1: return t->r1;
		case 2: return t->r2;
		case 3: return t->r3;
		default: return 666; /* or whatever you want to do
					given a duff regno! */
		}
	}

Happy Halloween