Lua: give custom userdata a tostring method

Question

I have a custom userdata in lua. When I execute tostring on an instance of the userdata (inside Lua), it always returns a string like "userdata: 0xaddress". I would like it to return the name of the userdata ("Point: 0xaddress"), meaning that I want to override the tostring to include a case for my userdata. Does anyone know if this can be done?

#define check_point(L) \
    (Point**)luaL_checkudata(L,1,"Point")

static int 
luaw_point_getx(lua_State* L)
{
    Point** point_ud = check_point(L);
    lua_pushinteger(L, (*point_ud)->x);
    return 1;
}

static int 
luaw_point_gety(lua_State* L)
{
    Point** point_ud = check_point(L);
    lua_pushinteger(L, (*point_ud)->y);
    return 1;
}

void 
luaw_point_push(lua_State* L, Point* point)
{
    Point** point_ud = (Point**)lua_newuserdata(L, sizeof(Point*));
    *point_ud = point;
    luaL_getmetatable(L, "Point");
    lua_setmetatable(L, -2);
}

static const struct luaL_Reg luaw_point_m [] = {
    {"x", luaw_point_getx},
    {"y", luaw_point_gety},
    {NULL, NULL}
};

int 
luaopen_wpoint(lua_State* L)
{
    luaL_newmetatable(L, "WEAVE.Point");
    lua_pushvalue(L, -1);
    lua_setfield(L, -2, "__index");
    luaL_register(L, NULL, luaw_point_m);
    return 1;
}

prapin prapin · Accepted Answer · 2012-10-08T15:45:03

You have to provide a __tostring metamethod, in the similar way you wrote the __index.

// [...]
int luaw_point_index(lua_State* L)
{
    lua_pushfstring(L, "Point: %p", lua_topointer(L, 1));
    return 1;
}

int luaopen_wpoint(lua_State* L)
{
    luaL_newmetatable(L, "WEAVE.Point");
    lua_pushvalue(L, -1);
    lua_setfield(L, -2, "__index");
    lua_pushcfunction(L, luaw_point_index);
    lua_setfield(L, -2, "__tostring");
    luaL_register(L, NULL, luaw_point_m);
    return 1;
}

Lua: give custom userdata a tostring method

2 Answers