Spake Jonathon Conte on Sunday, May 28, 2006 at 01:39AM -0400:
> I'm writing a little project in C and have hit a wall. Let's say I have
> Functions A, B, C, & D. How can I make A dynamically call B, C or D based
> on a keyword in a text file *without* A knowing the exact names of
> functions B, C or D beforehand?
>
> Is there some trick I can do with #define, enum and function pointers? Or
> perhaps some other way to go about it? The goal is to make it easy to add
> new functions to the program without having to continually modify all the
> functions that might call them.
>
> Perhaps I am overly complicating things with abstraction but I am still
> curious to see if it can be done.
>
If you need this level of expressivity, C is not the ideal language.
The best you can get in C would be a hashtable of function pointers, and
this is not very pretty. Nevertheless...
A hashtable of function pointers will work.
I would recommend using the "glib" (Note: Note glibc).
http://developer.gnome.org/doc/API/2.0/glib/index.html
Specifically, we can use its GHashTable data structure and functions.
http://developer.gnome.org/doc/API/2.0/glib/glib-Hash-Tables.html
Here's an example program that you should be able to modify to your
liking:
--- funhash.c ---
#include <stdio.h>
#include <glib.h>
/* compile with something like:
* gcc -o funhash `pkg-config glib-2.0 --libs --cflags` funhash.c
*/
/*
* gboolean is the same as bool, except it exists on all platforms glib support.
* gpointer is the same as void *
*/
typedef gboolean (*Callback)(gpointer);
void defun(GHashTable *hash, char *name, Callback callback)
{
g_assert(hash != NULL);
g_assert(name != NULL);
g_assert(callback != NULL);
g_hash_table_insert(hash, (gpointer) name, (gpointer) callback);
}
gboolean apply(GHashTable *hash, char *name, char *arg)
{
Callback callback;
g_assert(hash != NULL);
g_assert(name != NULL);
callback = (Callback) g_hash_table_lookup(hash, (gpointer) name);
return callback(arg);
}
gboolean fun_a(char *msg)
{
printf("A: %s\n", msg);
}
gboolean fun_b(char *msg)
{
printf("B: %s\n", msg);
}
gboolean fun_c(char *msg)
{
printf("C: %s\n", msg);
}
int main(char *argv, int argc)
{
GHashTable *funs = g_hash_table_new(g_str_hash, g_str_equal);
defun(funs, "A", (Callback) fun_a);
defun(funs, "B", (Callback) fun_b);
defun(funs, "C", (Callback) fun_c);
apply(funs, "C", "foo");
apply(funs, "B", "bar");
apply(funs, "A", "baz");
return 0;
}
/* EOF */
--
Entropy requires no maintenance.
-- Markoff Chaney
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