Is it possible to pass a C++ class as a Lua table from a Gem?

Question

I have a C++ class inside of my Gem that holds data that I need to pass to the Lua environment. I am aware that Lua tables cannot currently be passed *into* Gems (a limitation of AZstd::any mentioned in the documentation and on the forums), but I want to go the other way. I know normal Lua has certain [C methods](https://www.lua.org/manual/5.1/manual.html) (see section 3.7 – Functions and Types) to facilitate conversion from C/C++ to Lua, but I don't know how to access those from Lumberyard (or even if those methods are available in Lumberyard). My questions are as follows:

- Is it possible to pass a C++ class as a Lua table from a Gem?
  - If so, how? I didn't see any documentation on this, and I don't know where to look.
  - If not, what other method can I use to pass this data? A pointer (I have done this before, but I wanted to try to avoid managing my own memory)?
(As for why I need to do it this particular way: I have a collection of C/C++ libraries, both 3rd party and personally created, that facilitate my development. A preliminary attempt to convert one of the simplest libraries into pure Lua failed spectacularly (it took a massive performance hit).)

Accepted Answer

Hello @REDACTEDUSER

Yes you can. You can use the Behavior Context to reflect a C++ class into a Lua table. Take a look at [this](https://docs.aws.amazon.com/lumberyard/latest/developerguide/component-entity-system-reflection-behavior-context.html) page for more information about the Behavior Context and how you would go about achieving that.

Thanks!

Answer

After much fiddling, I ended up getting a series of classes successfully passed between Lua and C/C++. Doing so required some modification to my core classes (which think I can avoid in the future using wrappers). Additional things that helped me while struggling through this:

- The actual [documentation ](https://docs.aws.amazon.com/lumberyard/latest/apireference/class_a_z_1_1_behavior_context.html)for the behavior context class has a larger code snippet. It essentially has all of the code you linked, but as a single piece, which helped me see exactly where this code had to go.
- Some type of macro is needed (AZ_TYPE_INFO_SPECIALIZE or AZ_RTTI) in order to reflect a class or type. I believe AZ_RTTI has to be in the public part of a class header, and the AZ_TYPE_INFO_SPECIALIZE has to be part of the AZ namespace in order for them to work. If you attempt to reflect without adding these macros, you will get some assertion error.
  - NOTE: This also applies to any type that your class returns, including lists/vectors of those classes (i.e. any type not AZ_TYPE_INFO_SPECIALIZE-d in the dev\Code\Framework\AzCore\AzCore\RTTI\TypeInfo.h file)
- \dev\Gems\LmbrCentral\Code\Source\Rendering\MaterialHandle.cpp showed me that methods attached to the reflected classes didn't need to actually be part of the reflected class (MaterialHandle reflection).
- \dev\Code\Framework\AzCore\AzCore\Script\ScriptSystemComponent.cpp Has a full example of reflecting an enum through a class (PlatformID)
Thanks! My code is so much easier for me to understand and performs faster as a result of this!

Answer

Good Question -- Let me find some answers for ya on this topic :)

Apologies for the delays.

Is it possible to pass a C++ class as a Lua table from a Gem?

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