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

[Flávio Alberto <flavioalsoares@...>]

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, 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...

Thanks

Flávio Lopes

2014-09-09 23:00 GMT-03:00 Peng Zhicheng <pengzhicheng1986@gmail.com>:
> On 09/10/2014 04:53 AM, Flávio Alberto wrote:
>>
>> Hello,
>> thanks for all help at now !
>>
>> I try :
>>
>> <...>
>>      lua_newtable(L); // create 'mastertable'
>>
>>      lua_newtable(L); // create 'table1'
>>      luaL_getmetatable(L, "luasystem.mastertable"); // get metatable
>> from the registry
>>      lua_setmetatable(L, -2); // setmetatable(table1, __mt)
>>      luaL_register(L, NULL, meths);
>>      lua_setfield(L, -2, "table1");
>>
>>      lua_newtable(L);
>>      lua_setfield(L, -2, "table2");
>>
>>      lua_newtable(L);
>>      lua_setfield(L, -2, "table3");
>>
>>      lua_setglobal(L,"mastertable");
>> <...>
>>
>> But again l_index and l_newindex was never called...
>> Really I need to understand the lua stack concepts and how to "link"
>> these lua objects.
>>
>>
>> Thanks
>> Flávio Lopes
>>
>>
>>
>>
> you should not call luaL_register this way -- lua_setmetatable would pop the
> metatable
> from the stack.
>
> you can register the metamethods at the creation time of the metatable,
> once.
> then you just retrieve it from the registry when you need it.
>
> the complete code should be:
>
> ~~~~
>
> void luaopen_mastertable (lua_State* L)
> {
>     // create 'mastertable'
>     lua_newtable(L);
>
>     // create new table
>     lua_newtable(L);
>
>     // set the metatable for newly created table
>     // but firstly create the metatable and set the metamethods
>     luaL_newmetatable(L, "luasystem.mastertable");
>     luaL_register(L, NULL, methods);
>
>     lua_setmetatable(L, -2);
>
>     // mastertable.table1 = <newly created table>
>     lua_setfield(L, -2, "table1");
>
>     // mastertable.table2 = {}
>     lua_newtable(L);
>     lua_setfield(L, -2, "table2");
>
>     // mastertable.table3 = {}
>     lua_newtable(L);
>     lua_setfield(L, -2, "table3");
>
>     // set the global variable
>     lua_setglobal(L,"mastertable");
>
>     // now the stack is balanced as we entered this function.
> }
>
> ~~~~
>
> note that I intentionally changed the function signature -- this function is
> not called from
> Lua, so it need not be a `lua_CFunction` type. you should not annotate it
> with LUALIB_API
> for the same reason. (besides, a lua_CFunction shoud be called indirectly
> via lua_call,
> lua_pcall/lua_cpcall, in which case you leave the value to be returned to
> its caller on the
> stack and Lua would adjust the stack if needed.)
>
> the Lua C API is actually very intuitive imo. you just need some time to get
> used to it.
> in general, many operations need the RHS operand to be at the top and would
> pop the value
> after the operations. Think in Lua (not in C) and you'll feel comfortable
> for that.
>
>
>