save expanded macro result before it is undefined - c

Question

With GCC, I am using X-macros for a collection of variables.
#define TEST_MAP \
X(0, 1, 2) \
X(0, 2, 2) \
X(0, 3, 5)
Let's suppose I need to sum up all the 2nd fields
#define X(a,b,c) (+b) /* sum all 2nd elements of TEST_MAP */
uint16_t d = (TEST_MAP);
#undef X
That works fine, because TEST_MAP is expanded as
uint16_t d = ((+1) (+2) (+3));
However, I'd prefer to use a compile-time constant, but if I do
#define X(a,b,c) (+b) /* sum all 2nd elements of TEST_MAP */
#define D (TEST_MAP)
#undef X
uint16_t d = D;
of course it won't work because at the time D is expanded X is not defined anymore, so I have:
uint16_t d = (X(0, 1, 2) X(0, 2, 2) X(0, 3, 5));
I have checked the use of # and ## but I haven't found yet how to solve this. How can I force #define D to expand to ((+1) (+2) (+3)) ?

Answer 1

Use enumeration:
#define X(a,b,c) (+b) /* sum all 2nd elements of TEST_MAP */
enum { D = (TEST_MAP) };
#undef X

Answer 2

You could rewrite your X macro so that X is a parameter to your macro:
#define TEST_MAP(X) \
X(0, 1, 2) \
X(0, 2, 2) \
X(0, 3, 5)
You can then pass the appropriate macro (which must take three arguments) without having to repeatedly #define and #undef X:
#define COL1(a, b, c) + (a)
#define COL2(a, b, c) + (b)
#define COL3(a, b, c) + (c)
#define SUM(a, b, c) + (a) + (b) + (c)
#define D1 (TEST_MAP(COL1));
#define D2 (TEST_MAP(COL2));
#define D3 (TEST_MAP(COL3));
#define S (TEST_MAP(SUM));

Answer 3

You are not able to expand the macro as you want because you undefine the the X macro before the point where you apply it:
#define X(a,b,c) (+b) /* sum all 2nd elements of TEST_MAP */
#define D (TEST_MAP)
#undef X //You undefine X here before usage
uint16_t d = D; //Here X doesn't exist anymore and can't expand.
Consider that the preprocessor performs expansion when encounters the macro, at that time all submacros have to be defined.
To use the macros the way you requested you have simply to undefine macro X after usage.
See the sample:
#include <stdio.h>
#define TEST_MAP \
X(0, 1, 2) \
X(0, 2, 2) \
X(0, 3, 5)
#define D (TEST_MAP)
#define X(a,b,c) +(b) /* sum all 2nd elements of TEST_MAP */
int d = (TEST_MAP);
#undef X
int main(void)
{
#define X(a,b,c) +(b) /* sum all 2nd elements of TEST_MAP */
printf("Sum of 2nd -> %d - %d\n", d, D);
#undef X
#define X(a,b,c) +(c) /* sum all 3rd elements of TEST_MAP */
printf("Sum of 3rd -> %d\n", D);
#undef X
return 0;
}
Or you can simply revert the order of preprocessor commands:
#undef X
#define X(a,b,c) +(b) /* sum all 2nd elements of TEST_MAP */
In this case the last assignement is on up to a new undefine/redefine...