How to directly map a Lua variable to a C ++ variable?

Question

I am working on a C ++ game engine using Lua to define NPCs.

I can define an NPC prototype as follows:

orc = { name = "Generic Orc", health = 100 } function orc:onIdle() print("Orc idles...") end 

and then create an instance of "Orc" using entitySpawn(orc) . This is a C ++ function that reads values ​​such as health and name from a given table, creates an Entity object in C ++ with the given values ​​and, in addition, creates a Lua table for a specific NPC.

Now I would like to have a direct connection between the orc.health variable in Lua and the mHealth member mHealth corresponding Entity object in C ++, so I could assign a value in Lua and use it instantly in C ++ and vice versa.

Is it possible? Or do I need to use setter / getter functions? I took a look at the light user data and got to the point of storing a pointer to a C ++ variable in Lua, but couldn't assign a value.

Answer 1

It is possible. I assume entitySpawn returns a table created for a C ++ object. I also assume that you can expose a function from C ++ that takes an entity table and returns the current state of health, and similarly for customization. (To implement this, you can use the easy userdata pointer for a C ++ object as a member of this table.)

So the ugly way would look like this:

 local myOrc = entitySpawn(orc) local curHealth = getEntityHealth(myOrc) setEntityHealth(myOrc, curHealth + 10) 

To make it more beautiful, we can have fun with metatables. First, we will place accessors for all the properties we care about.

 entityGetters = { health = getEntityHealth, -- ... } entitySetters = { health = setEntityHealth, -- ... } 

Then we will create a metatable that uses these functions to handle property assignments.

 entityMetatable = {} function entityMetatable:__index(key) local getter = entityGetters[key] if getter then return getter(self) else return nil end end function entityMetable:__newindex(key, value) local setter = entitySetters[key] if setter then return setter(self, value) end end 

Now you need to make entitySpawn assign a metatable. You would do this with the lua_setmetatable function.

 int entitySpawn(lua_State* L) { // ... // assuming that the returned table is on top of the stack lua_getglobal(L, "entityMetatable"); lua_setmetatable(L, -2); return 1; } 

Now you can write it beautifully:

 local myOrc = entitySpawn(orc) myOrc.health = myOrc.health + 10 

Note that this requires that the table returned by entitySpawn does not have a set of health properties. If so, then metatable will never consult on this property.

You can create entityGetters , entitySetters and entityMetatable , as well as __index and __newindex in C ++ instead of Lua, if that seems cleaner to you, but the general idea is the same.

Answer 2

I don’t have time to give you any code, but keep in mind that orc.health same as orc["health"] ; that is, orc is a table, and "health" is an element.

With that in mind, you can change the index and newindex your table to have your own specific behavior. Save your “real” instance of orc as some private metadata, and then use it to update.