Re: lua and C - creating tables and fields set/change event

[Peng Zhicheng <pengzhicheng1986@...>]

On 09/11/2014 12:39 AM, Flávio Alberto wrote:
> I did this code ando for this lua code :
>
> print(">> type(mastertable)="..type(mastertable))
> print(">> type(mastertable.table1)="..type(mastertable.table1))
> print(">> type(mastertable.table2)="..type(mastertable.table2))
> print(">> type(mastertable.table3)="..type(mastertable.table1))
>
> mastertable.table2.last = 2
> mastertable.table1.a = 7
>
> mastertable.table3.b = 8
>
> mastertable.table3.name = "Test"
>
> print(">> mastertable.table3.name="..mastertable.table3.name)
> print(">> mastertable.table3.b="..mastertable.table3.b)
> print(">> mastertable.table2.last="..mastertable.table2.last)
> print(">> mastertable.table1.a="..mastertable.table1.a)
>
> And get this :
>
> > > type(mastertable)=table
> > > type(mastertable.table1)=table
> > > type(mastertable.table2)=table
> > > type(mastertable.table3)=table
> IN : l_newindex
> > > mastertable.table3.name=Teste
> > > mastertable.table3.b=8
> > > mastertable.table2.last=2
> IN : l_index
> LuaCode.cpp : load : 825 - ERROR: /opt/trunk/nclua/script.lua:26:
> attempt to concatenate field 'a' (a nil value)"
>
>
> Apparently table1 was not created,
`mastertabletable1` is there, but `mastertable.table1.a` is not,
because you didn't do anything in the __newindex metamethod.
see the documentation for `lua_rawset`, `lua_rawget`

> and to me appears be
> counterintuitive (for a newbie point of view) this code
>
> luaL_newmetatable(L, "luasystem.mastertable");
> luaL_register(L, NULL, methods);
>
> after two lua_newtable(L)'s,
>
> for me appears that creating a metatable for mastertable.table1, but I
> try put this code after the 1st lua_newtable to try associate this
> metatable to main table but doesn't works...
this shoud work. but if you associate the metatable with `mastertable`,
the following code would not behave as you expected:
`lua_setfield` would trigger the `__newindex` metamethod, which did
nothing, thus resulting an empty `mastertable`.
when the Lua code access `mastertable.table1` (same for `table2` & `table3`) ,
the `__index` metamethod would be triggered, which did nothing either.
thus giving : ``>> type(mastertable1) = nil``.
also the following lines (firstly `mastertable.table2.last = 2`) throw errors.

> Thanks

I felt like you were not very familiar to Lua's semantic.
I suggest reading the manual and PiL first.
also there are plenty of useful resouces in the lua-users wiki.

best regards.