How can I link to callback functions in Lua such that the callbacks will be updated when the scripts are reloaded?

Question

I'm implementing Lua scripting in my game using LuaBind, and one of the things I'm not clear on is the logistics of reloading the scripts live ingame.

Currently, using the LuaBind C++ class luabind::object, I save references to Lua callbacks directly in the classes that use them. Then I can use luabind::call_function using that object in order to call the Lua code from the C++ code.

I haven't tested this yet, but my assumption is that if I reload the scripts, then all the functions will be redefined, BUT the references to the OLD functions will still exist in the form of the luabind::object held by the C++ code. I would like to be able to swap out the old for the new without manually having to manage this for every script hook in the game.

How best to change this so the process works?

My first thought is to not save a reference to the function directly, but maybe save the function name instead, and grab the function by name every time we want to call it. I'm looking for better ideas!

Answer 1

My first thought is to not save a reference to the function directly, but maybe save the function name instead, and grab the function by name every time we want to call it.

If your classes are calling global functions with known names, then that pretty much solves your problem. No need to grab a reference in advance; it's not going to make a measurable performance difference. I think call_function supports passing the function name as a string anyway, right?

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You typically store reference to a function value when the Lua script is registering a callback. In that case, it's much better than storing a name, because it allows the Lua script to register functions which are local, anonymous, ect.

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If you really had to grab the value value in advance, as you're doing now (and there's really no reason to do that, but we'll pretend it's necessary), I would add a layer of indirection. You could have a LuaFunctionReference class which encapsulates a global name. During instantiation, it grabs a reference to the function the global contains. These objects could be acquired from a factory which maintains a list of all such references. When you reload a script, you could have the factory/manager/pool/etc. object iterate through the references and have them update themselves, so all the references tucked away in classes throughout the system would be updated.