[GRASS5] g51 Vect_set/get_header

[Eric G. Miller]

On Mon, Apr 29, 2002 at 06:02:05PM +0200, Radim Blazek wrote:
> Hi,
> 
> in g51 vector library, i would like to replace direct access to
> Map_info structure and replace fuctions like V2_num_lines()
> by one or few funcitons. I see two possibilities:
> 1. 
> int Vect_get_header( struct Map_info Map, int item, char *cval, 
>               int *ival, double *dval)
> 
>     - *val would be pointers to variables where the value should be saved
> 
> 2.
> char Vect_get_header_c( struct Map_info Map, int item)
> int   Vect_get_header_i( struct Map_info Map, int item)
> double Vect_get_header_d( struct Map_info Map, int item)
>     
>      - all return value (pointer to value)
> 
> item is code for header field (GV_HEAD_SCALE, GV_NUM_LINES, ...)
> 
> What should i use? Or do you know about anything better?

I personally don't like either.  They don't increase generality much,
and require you to look at the arguments to figure out what the function
is supposed to do.  Maybe a "property" structure containing
a union would be more general?

enum map_header_enum {GV_HEAD_SCALE, GV_NUM_LINES, ...);
enum header_field_type (HFT_CONST_CPTR, HFT_INT, ...);

struct map_header_prop {
    enum map_header_enum   header_field;
    enum header_field_type field_type;
    union {
       const char *cptr;
       int        i;
       double     d;
       /* other ? ... */
    } data;
};

int Vect_header_get_prop (struct Map_Info *Map, 
                          enum map_header_enum which,
			  struct map_header_prop *prop);

Though, I'm not sure what wrong with just having a bunch of functions
to get/set things and then hide the details of the Map_info struct
via a typedef.  Then:

/* Vect.h */
/* Error codes */
#define VERR_NONE    0x0000
#define VERR_EMEM    0x0001
#define VERR_EINVAL  0x0002
#define VERR_ERANGE  0x0004
/* ... */

typedef struct Map_info * Map_info_ptr;

int Vect_log_errors (FILE *, int, const char *);
int Vect_open_old (Map_info_ptr *, const char *, const char *);
int Vect_close (Map_info_ptr *);
int Vect_get_num_lines(Map_info_ptr);
const char * Vect_get_organization(Map_info_ptr);
int Vect_set_organization(Map_info_ptr, const char *);
...


/* Vect_P.h */
struct Map_info {
   /* ... */
};
void * dig_alloc_space (...);
...

-- 
Eric G. Miller <egm2 at jps.net>