Pass pure lua object to C function and get value

Question

In Lua Code

Test = {}
function Test:new()
  local obj = {}
  setmetatable(obj, self)
  self.__index = self
  return obj
end
local a = Test:new()
a.ID = "abc123"
callCfunc(a)

In C Code

int callCfunc(lua_State * l)
{
   void* obj = lua_topointer(l, 1);            //I hope get lua's a variable
   lua_pushlightuserdata(l, obj);   
   lua_getfield(l, 1, "ID");
   std::string id = lua_tostring(l, 1);        //I hoe get the value "abc123"
   ...
   return 0;
}

But My C result is

id = null

Why? How to modify code to work fine ?
PS: I don't hope create C Test Class mapping to lua

==== update1 ====
In addition, I have added the test code to confirm correct incoming parameters.

int callCfunc(lua_State * l)
{
   std::string typeName = lua_typename(l, lua_type(l, 1));    // the typeName=="table"
   void* obj = lua_topointer(l, 1);            //I hope get lua's a variable
   lua_pushlightuserdata(l, obj);   
   lua_getfield(l, 1, "ID");
   std::string id = lua_tostring(l, 1);        //I hoe get the value "abc123"
   ...
   return 0;
}

the result

typeName == "table"

so incoming parameter type is Correct

Aren't objects indexed from 0? Should not all the 1s be zeroes instead? — user529758
– user529758, Commented Jan 8, 2013 at 5:55
You may like to accept some of your answers - you'll get more responses. 0% accept is less than acceptable :-) — daven11
– daven11, Commented Jan 8, 2013 at 7:23
In addition, I have added the test code to confirm correct incoming parameters. std::string typeName = lua_typename(l, lua_type(l, 1)); the result typeName == "table" so incoming parameter type is Correct. — Flash
– Flash, Commented Jan 8, 2013 at 7:34

Flash · Accepted Answer · 2013-01-08 08:21:45Z

3

I found the reason
Correct c code should is ...
In C Code

int callCfunc(lua_State * l)
{
   lua_getfield(l, 1, "ID");
   std::string id = lua_tostring(l, -1);        //-1
   ...
   return 0;
}

answered Jan 8, 2013 at 8:21

Flash

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2 Comments

Flash Over a year ago

How to close the question? I don't find the close menu.

daven11 Over a year ago

use lua_istable() to check the input parameter is a table - pgl.yoyo.org/luai/i/lua_istable. To close the question just accept an answer - if your's then you'll have to wait a while I think.

daven11 · Accepted Answer · 2013-01-08 07:07:24Z

0

Maybe this - haven't tested sorry - don't have a compiler handy

Input is the table from lua on top of the stack, so getfield(l,1, "ID") should get the field ID from the table at the top of the stack - which in this case is your input table. It then pushes the result to the top of the stack

int callCfunc(lua_State * l)
{
   lua_getfield(l, 1, "ID");
   std::string id = lua_tostring(l, 1);        //I hoe get the value "abc123"
   ...
   return 0;
}

answered Jan 8, 2013 at 7:07

daven11

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