continuing an old thread...
I don't remember if anybody suggested solution for Chris question that was
stable on (hypothetical) memory compacting GC and at the same time allowed
for easy pointer comparison; but this is what i've just tried:
/*--------- string equality helper ------------*/
static int myStatKey;
static void regString (lua_State *L, const char *s) {
lua_pushlightuserdata (L, myStatKey);
lua_gettable (L, LUA_REGISTRYINDEX);
if (lua_isnil (L, -1)) {
lua_pop (L, 1);
lua_newtable (L);
lua_pushlightuserdata (L, myStatKey);
lua_pushvalue (L, -2);
lua_settable (L, LUA_REGISTRYINDEX);
}
lua_pushstring (L, s);
lua_pushlightuserdata (L, s);
lua_settable (L, -3);
}
static const char *toStringReg (lua_State *L, int n) {
const char *r = NULL;
lua_pushlightuserdata (L, myStatKey);
lua_gettable (L, LUA_REGISTRYINDEX);
if (lua_isnil (L, -1)) {
lua_pop (L, 1);
return NULL;
}
lua_pushvalue (L, n);
lua_gettable (L, -2);
r = lua_touserdata (L, -1);
lua_pop (L, 1);
return r;
}
/*------ end --------*/
the static int variable is used as a key in the registry, to avoid collisions.
to use it, just register any constant string you'd like to use (at module
initialization time):
regString (L, "foo");
regString (L, "bar");
and, in your __index() metamethod (or anywhere you'd like to test a Lua
string against several C strings), first convert it to the registered
pointer:
const char *k = *toStringReg (L, n); /* n is a stack position */
if (k == "foo") {
do_something ();
} else if (k == "bar") {
do_someelse ():
} else {
not_found ();
}
of course, this depends on the compiler optimizing equal constant strings to
the same location in memory.
a little variation would be to store small integers (from an enum type)
instead of the const char pointers. this would make possible to use
switch(getreg(L,n)) {.....}
--
Javier
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