__index overloading only for non-existing keys?
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- Subject: __index overloading only for non-existing keys?
- From: Leroy van Engelen <leroy@...>
- Date: 2003年7月10日 14:10:24 +0200
Hi all,
I am trying to learn embedding Lua in C++ for a couple of days now and
I've entered the world of operator overloading using metatables.
In the following C++ code I try to overload the __index operator of a
table 'variables' with a method printing a message. However, the message
only appears if the key doesn't match any in the 'variables' table: Why
isn't the indexHandler method called for every indexing operation?
Am I doing something wrong here? Have I misunderstood something? I've
tried to understand the code at
http://lua-users.org/wiki/BindingWithMembersAndMethods but it's still a
bit too advanced for me...
Oh, and another question: what is the difference between the globals
table and the registry table? And which should I use when?
Thanks,
-Leroy
extern "C" {
#include <lua.h>
#include <lualib.h>
#include <lauxlib.h>
} /* extern "C" */
#include <iostream>
#include <string>
static int indexHandler(lua_State * l)
{
std::cout << "indexHandler" << std::endl;
lua_pushnumber(l, 10.20);
return 1;
}
int main(int argc, char *argv[])
{
lua_State *l;
l = lua_open();
luaopen_base(l);
luaopen_io(l);
// create 'variables' table
lua_pushstring(l, "variables");
lua_newtable(l);
// add myvar to variables
lua_pushstring(l, "myvar");
lua_pushnumber(l, 123.45);
lua_rawset(l, -3);
// create metatable
lua_newtable(l);
// add __index method to metatable
lua_pushstring(l, "__index");
lua_pushcfunction(l, indexHandler);
lua_rawset(l, -3);
lua_setmetatable(l, -2);
lua_settable(l, LUA_GLOBALSINDEX);
// myvar is a key in the variables table
// indexHandler isn't called here, why?
char cmd1[] = "print(variables.myvar)";
luaL_loadbuffer(l, cmd1, sizeof(cmd1) - 1, "xxx");
lua_pcall(l, 0, 0, 0);
// foo is not a key in the variables table
// indexHandler is called here, as I would expect
char cmd2[] = "print(variables.foo)";
luaL_loadbuffer(l, cmd2, sizeof(cmd2) - 1, "xxx");
lua_pcall(l, 0, 0, 0);
lua_close(l);
return 0;
}