RE: a luaL_mapref with a map semantic ?

[sr reddy <mcsreddy2003@...>]

Hai, 

    i am new to Lua.

    How to compile a c++ which simply calls lua 

    Work paltform VC++ 2005, Lua 5.0.2 or  new version

Thank you

Jerome Vuarand <jerome.vuarand@ubisoft.com> wrote:
>  If I understand well you have a C++ object that own a table. To keep a reference to the table, you put in the registry and keep the numerical index in the C++ object. And now from the table you want to find the C++ object that own it.
>  
> The way you do it is ok; but you only retrieve the numerical index (ref), not the C++ object. Instead you could put in the registry a pointer to your C++ object as a light userdata:
>
> class Foo
> {
> public:
>    Foo() : ref(0) { }
>    int SetTable(lua_State* L, int index);
>    int GetTable(lua_State* L);
>    static
>  Foo* GetObject(lua_State* L);
> private:
>    int ref;
> };
>
> // Take associated table from stack
> int Foo::SetTable(lua_State* L)
> {
>    // Put table in registry
>    lua_pushvalue(L, 1); // Duplicate table ref, use it as value
>    this->ref = luaL_ref(L, LUA_REGISTRYINDEX);
>    // Associate the table to this
>    lua_pushvalue(L, 1); // Duplicate table ref, use it as key
>    lua_pushlightuserdata(L, (void*)this); // Push this, us it as value
>    lua_settable(L, LUA_REGISTRYINDEX);
>    return 0;
> }
>
> // Push associated table on stack
> int Foo::GetTable(lua_State* L)
> {
>    if (this->ref!=0)
>    {
>       // Get table from registry
>       lua_pushvalue(L, this->ref);
>       lua_gettable(L, LUA_REGISTRYINDEX);
>       return 1;
>    }
>    else
>    {
>       return 0;
>    }
> }
>
> // Find which object is associated to table on stack
> Foo* Foo::GetObject(lua_State* L)
> {
>    // Get object from
>  registry
>    lua_pushvalue(L, 1); // Duplicate table ref, use it as key
>    lua_gettable(L, LUA_REGISTRYINDEX);
>    // gettable returned the light userdata corresponding to the C++ object
>    return (Foo*)lua_touserdata(L, -1);
> }
>
>  
> ________________________________
>
> De : lua-bounces@bazar2.conectiva.com.br [mailto:lua-bounces@bazar2.conectiva.com.br] De la part de Grellier, Thierry
> Envoyé : 19 septembre 2006 06:31
> À : Lua list
> Objet : a luaL_mapref with a map semantic ?
>
>
>
> Hello,
>
>  
>
> I've been using lua_ref as it is recommended to keep a reference to a table.
>
> But my problem is that I'd like to find the C++ holder of the ref from another call. That is find which ref matches the table. I've been using this code: 
>
>  
>
> // find table
>
>   lua_pushvalue(L, 1);
>
>   lua_gettable(L, LUA_REGISTRYINDEX);
>
>   int ref;
>
> if lua_isnil(L, -1) {
>
> // not found, create ref
>
>    
>  lua_pop(L, 1);
>
>     lua_pushvalue(L, 1);
>
>     ref = luaL_ref(L, LUA_REGISTRYINDEX);
>
> // LUA_REGISTRYINDEX.table = ref
>
>     lua_pushvalue(L, 1);
>
>     lua_pushnumber(L, ref);
>
>     lua_settable(L, LUA_REGISTRYINDEX);
>
> } else {
>
> // found : retrieve ref
>
>     ref = (int) lua_tonumber(L, -1);
>
>     lua_pop(L, 1);
>
> }
>
>  
>
> But could it some better mechanisms ? And by the way does LUA_REGISTRYINDEX behaves as a weak table?
>
>  
>
>  



Srikanth Reddy Mudireddy

Master Student in Autonomous Systems        
Department of Computer Science,               
University of applied sciences Bonn-Rhein-Sieg, Sankt Augustin, Germany
AR, Fraunhofer-IAIS, Sankt Augustin, Germany.

Alternative e-mail:srikanthreddyrobotics@hotmail.com
Phone: +49-176-21718514

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