How do I create a class object in Lua-C API 5.2?
Here's how I would satisfy both your criteria as well as j_schultz's
#include <lua.h>
#include <lauxlib.h>
#include <stdlib.h>
#include <stdio.h>
#define LUA_FOO "Foo"
typedef struct {
int x;
} Foo;
static int foo_gc(lua_State *L) {
printf("## __gc\n");
Foo *foo = *(Foo**)luaL_checkudata(L, 1, LUA_FOO);
free(foo);
return 0;
}
static int foo_doSomething(lua_State *L) {
printf("## doSomething\n");
Foo *foo = *(Foo**)luaL_checkudata(L, 1, LUA_FOO);
lua_pushinteger(L, foo->x);
return 1;
}
static int foo_new(lua_State* L) {
printf("## new\n");
Foo *foo = malloc(sizeof(Foo));
int i = 1 + lua_istable(L, 1);
foo->x = !lua_isnoneornil(L, i) ? luaL_checkinteger(L, i) : 0;
*(Foo**)lua_newuserdata(L, sizeof(Foo*)) = foo;
luaL_setmetatable(L, LUA_FOO);
return 1;
}
static int foo_index(lua_State *L) {
printf("## index\n");
int i = luaL_checkinteger(L, 2);
lua_pushinteger(L, i);
return 1;
}
int luaopen_foo(lua_State *L) {
// instance functions
static const luaL_Reg meta[] =
{ { "__gc" ,foo_gc },
{ NULL ,NULL } };
static const luaL_Reg meth[] =
{ { "doSomething" ,foo_doSomething },
{ NULL ,NULL } };
luaL_newmetatable(L, LUA_FOO);
luaL_setfuncs (L, meta, 0);
luaL_newlib (L, meth);
lua_setfield (L, -2, "__index");
lua_pop (L, 1);
// static functions
static const luaL_Reg static_meta[] =
{ { "__index" ,foo_index },
{ "__call" ,foo_new },
{ NULL ,NULL } };
static const luaL_Reg static_meth[] =
{ { "new" ,foo_new },
{ NULL ,NULL } };
luaL_newlib (L, static_meth);
luaL_newlib (L, static_meta);
lua_setmetatable (L, -2);
return 1;
}
Lua code:
local Foo = require('foo')
local foo = Foo.new(12)
local bar = Foo(24)
print(Foo[13])
print(foo:doSomething())
print(bar:doSomething())
Lua output:
## new
## new
## index
13
## doSomething
12
## doSomething
24
## __gc
## __gc
I tried replying to your solution but apparently I don't have the reputation to do so yet, so here goes a separate answer.
Your solution is pretty nice, but it does not allow for something that I'd like to do: Have both "array-like" access to an object and still have functions on it. Have a look at this Lua code:
Foo = {}
mt = {
__index = function(table, key)
print("Accessing array index ", tostring(key), "\n")
return 42
end
}
setmetatable(Foo, mt)
Foo.bar = function()
return 43
end
print(tostring(Foo[13]), "\n")
print(tostring(Foo.bar()), "\n")
--[[
Output:
Accessing array index 13
42
43
]]--
Registering a class using your solution does not seem to allow for this, as the __index
entry is overwritten.
It might not make sense to have both array access and function access on a class, but for the sake of simplicity (offering one C function for registering both types of classes) I'd like to use the same code everywhere. Does anyone have an idea how this restriction could be circumvented, so that I can create a class from C which has both a function Foo.bar() but also Foo[13]?
Ok, got it working. I had to add __index
and __metatable
to Foo
's new metatable, as shown below:
void register_foo_class(lua_State* L) {
int lib_id, meta_id;
/* newclass = {} */
lua_createtable(L, 0, 0);
lib_id = lua_gettop(L);
/* metatable = {} */
luaL_newmetatable(L, "Foo");
meta_id = lua_gettop(L);
luaL_setfuncs(L, _meta, 0);
/* metatable.__index = _methods */
luaL_newlib(L, _methods);
lua_setfield(L, meta_id, "__index");
/* metatable.__metatable = _meta */
luaL_newlib(L, _meta);
lua_setfield(L, meta_id, "__metatable");
/* class.__metatable = metatable */
lua_setmetatable(L, lib_id);
/* _G["Foo"] = newclass */
lua_setglobal(L, "Foo");
}