Re: passing pointers using LuaBridge

[Andrew Starks <andrew.starks@...>]

On Monday, April 7, 2014, Moose <moose@posteo.de> wrote:
> Hi all,
>
> I am making moderate progress integrating Lua into my C++ environment. After trying many different ways of wrapping C or C++ code I ended up with two candidates that seemed to suit my needs best. LuaBridge (https://github.com/vinniefalco/LuaBridge) and LuaWrapper (https://bitbucket.org/alexames/luawrapper).
>
> I was able to create C++ wrappers for existing classes with both, and, inside my scripts, to create objects of those types and call methods upon them. So far so good. However, with both implementations I have failed to pass existing objects with C++ lifetime into the script.
>
> With LuaBridge for example I do this (simplified):
>
> int luaopen_mylib_1(lua_State *L) {
>
>         using namespace luabridge;
>         getGlobalNamespace(L)
>                 .beginNamespace("mylib")
>                         .addFunction ("testfree", testfree)
>                         .beginClass<MyClass>("MyClass")
>                                 .addFunction("foo",   &MyClass::foo   )
>                         .endClass()
>                 .endNamespace();
>
>         // quite like this interface btw.
>
>         return 0;
> }
>
> ... then ...
>
>    using namespace luabridge;
>    lua_State *L = luaL_newstate();
>    luaL_openlibs(L);
>
>    // and register the functions within that context
>    luaopen_mylib_1(L);
>
>    // create MyClass instance
>    MyClass *p = new MyClass();
>    setGlobal(L, p, "mc");     // a LuaBridge Method that pushes the pointer as
>                                          // global userdata on the stack
>
>    if (luaL_dofile(L, "luawrapper_reuse.lua")) {
>                 std::cerr << "Lua program returned an error: \n";
>                 std::cerr << lua_tostring(L, -1);
>                 std::cerr << std::endl;
>                 lua_close(L);
>    }
>
>    delete p;
>
>
>
> All very straight forward.
> Then in my script...
>
> [[
> print("Executing LuaBridge test")
> mylib.testfree()
> mc:foo()
> ]]
>
> The method testfree() is being called all right. At least I see the output. But then, when I try to access the instance "mc", I get:
>
> luawrapper_reuse.lua:6: attempt to index global 'mc' (a userdata value)
>
>
> Despite the fact that I do everything by the book, exactly the way it is described in LuaBridges manual. Can anyone tell me what I do wrong? How can I access the object?
> It seems like Lua does know it. A pointer by the name 'mc' has been pushed as userdata into the context, which is how I understand this is supposed to work. But why can I not de-reference it and call a method?
>
> So close to the goal and yet all my efforts fail on this....
>
> With that Test I use LuaJit 2.0 on Windows64 (MSVC9).
>
>
> Help is much appreciated.
>
> Thanks,
> Moose.

Smart guy question: have you tried "Lua the C API?" 

But that's an question from someone who only trusts his own abstractions... Nothing to see here...