What is best solution for initialize struct array? - c

Question

First solution:
struct str {
char *name;
int flag;
};
enum {
HELP,
OUTPUT,
OTHER
};
typedef struct str table;
table arr[] = {
{ "help", HELP },
{ "output", OUTPUT },
{ NULL, OTHER },
};
int main(int argc, char **argv) {
table *opt = arr;
printf("%s\n", (opt+HELP)->name);
printf("%s\n", (opt+OUTPUT)->name);
return 0;
}
Second solution:
struct str {
char *name;
int flag;
};
enum {
HELP,
OUTPUT,
OTHER
};
typedef struct str table;
table arr[OTHER];
void start_table() {
arr[HELP] = (struct str) { "help", HELP };
arr[OUTPUT] = (struct str) { "output", OUTPUT };
arr[OTHER] = (struct str) { NULL, OTHER };
}
int main(int argc, char **argv) {
start_table();
table *opt = arr;
printf("%s\n", (opt+HELP)->name);
printf("%s\n", (opt+OUTPUT)->name);
return 0;
}
What are the best? Second solution change automatically if I add or change any element of the array, but is efficient? Is the best enumerate or using the #define preprocessor directive?

Answer 1

It depends!
When initialization can be used, use it — so the first solution is often better, especially if the code never changes the structure (array) contents.
If your code can change the contents but might need to be able to reset to the original state, then the initialization function becomes more appropriate and using direct initialization is not appropriate.
There is no one 'best' solution; what is best depends on what you need to do with the array.

Answer 2

Using a C99 specific intializer syntax, you can get the advantages of the second approach in a static initializer:
#include <stdio.h>
struct str {
const char *name;
int flag;
};
enum {
HELP,
OUTPUT,
OTHER,
};
typedef struct str table;
table arr[] = {
[HELP] = { "help", HELP },
[OUTPUT] = { "output", OUTPUT },
[OTHER] = { NULL, OTHER },
};
int main(int argc, char **argv) {
table *opt = arr;
printf("%s\n", opt[HELP].name);
printf("%s\n", opt[OUTPUT].name);
return 0;
}
The advantage of this technique is it makes no assumption on the order of the enum values, nor on their actual values as long as they are positive, distinct and reasonably small.