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Guest

Hi,

Here is my attempt at a typesafe vector 'object' for C. There are
certainly some tricky aspects of the code, but I think these are
confined to the header file and (I hope) wont create problems for users.

I am re-inventing the (bad) wheel here?

Do you have any comments or advice for me regarding possible problems
and improvements?

Thanks,

Doug Eleveld

/* Vector interface and implementation */
#include <stdlib.h>

typedef struct {
void* data; /* This MUST be first */
unsigned len;
} GENERICVECTOR;

#define VECTOR(a) \
union { \
a* data; \
GENERICVECTOR vector; \
}
void extendvector5(GENERICVECTOR* v, unsigned newlen, size_t datasize)
{
/* For now just pretend realloc never fails */
if (v->len < newlen)
v->data = realloc(v->data, newlen * datasize);
}

#define EXTENDVECTOR5(v,newlen) extendvector5(&v->vector, newlen,
sizeof(v->data[0]))

/* Vector demo code */
#include <stdio.h>
#include <string.h>

typedef struct {
int x;
} DATA;

typedef VECTOR(DATA) VECT;

int main(void)
{
int i, j;
VECT* ptr;
VECT vector[2];

memset(&vector, 0, sizeof(vector));

ptr = vector;
for (j=0; j<2; j++)
EXTENDVECTOR5(ptr++, 20); /*This looks dangerous but is safe */

for (j=0; j<2; j++) {
for (i=0; i<vector[j].len; i++) {
vector[j].data[i].x = i;
printf(“%i “, vector.data[i].x);
}
}

return EXIT_SUCCESS;
}

But wait, you say! EXTENDVECTOR5 is not a safe macro because it
evaluates its first argument twice, once for the first argument and
again for the second argument. From the previous example:

EXTENDVECTOR5(ptr++, 20);

expands to:

extendvector5(&(ptr++)->vector, (20), sizeof((ptr++)->data[0])))

It is very counterintuitive, but ptr is only incremented once here!
This is because everything inside sizeof() is evaluated at
compile-time, not at run-time. During compilation sizeof() code gets
replaced with a constant and the run-time code never sees the second
ptr++. In a sense, the second expansion ptr++ in the second argument
gets calculated and then discarded before the program even starts.
Because the ptr++ only happens at compile time and not at run time it
does not actually change the ptr variable. So the EXTENDVECTOR5 macro
is actually safe to use in this way.
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Douglas A. Gwyn · Guest

Hans-Bernhard Bröker wrote: