Purpose


I tried to find a simple example that showed me:

  • How to call some Lua script from within C
  • How to call a C function from within the Lua script
  • How to pass an instance of a structure/class to the script and have it call functions on it

Being naive I thought finding something like this would be easy. We all know how stories starting with "Being naive..." end (in road-side ditches surrounded by police tape).
I used the latest Lua release (5.2) and stumbled across errors where functions were deprecated (lua_open no longer exists), examples where there was so much going on I simply couldn't understand the basics, examples where a script.lua file was simply executed, examples where in order to pass a C object over I had to create something called a MetaTable and and...

So here it is, a simple example that pretends it's a game. The idea would be that you have an enemy object and it's intelligence would be stored in a Lua script that is loaded at the start of the game, and the "onUpdate" function in that script is called every frame wherein it can change the position, state, colour, happiness, whatever of the object.

Note


This contains no error checking to keep things simple. The Lua documentation on how to do error checking is great, and you should check the result of every call you make, as well as checking to see if variables are actually the type you expect them to be before using them.
Also, I started using Lua for the first time today, so this is simply an example that works for me, your mileage and joy in ignorance may vary.

Code


#include <stdio.h>
#include <stdlib.h>
#include "lua.hpp"

/* This is our game object structure, specifying all the properties that may pertain to an
   object instance in our game. For this example, only the position is important. */
typedef struct GameObject
{
	int x;
	int y;
}
GameObject;

/* This function is called from Lua via GameObject.move to move our object instance.
   In Lua-land it passes 3 parameters: the instance; and x and y. */
static int GameObject_move(lua_State *Lua)
{
	GameObject *instance   = (GameObject*)lua_topointer(Lua, 1);
	const int   x          = lua_tointeger(Lua, 2);
	const int   y          = lua_tointeger(Lua, 3);

	/* adjust the position of the instance and print out that we've done something. */
	instance->x += x;
	instance->y += y;
	printf("GameObject_move(%d, %d)\n\tinstance->x = %d\n\tinstance->y = %d\n", x, y, instance->x, instance->y);

	return 0;
}

/* This is called from C-land and simply serves as a helper function to set up the update
   function and call it. */
static void scriptOnUpdate(lua_State *Lua, GameObject *Instance)
{
	/* Fetch the Lua function and make it the thing currently on the stack */
	lua_getglobal(Lua, "onUpdate");
	/* Push the first argument onto the stack, which is the instance we are currently updating */
	lua_pushlightuserdata(Lua, Instance);
	/* Call the function on the stack telling it that it has 1 parameter and no return values */
	lua_call(Lua, 1, 0);
}

/* This sets up the Lua context and registers our functions with it. */
static lua_State* createLuaContext()
{
	/* Build up an array of functions we want to expost for the "GameObject" namespace. */
	/* Add whatever functions you want to expose to this array. */
	const luaL_Reg gameObjectFunctions[] =
	{
		{ "move", GameObject_move }, /* Lua GameObject.move() maps to GameObject_move() in C-land */
		{ 0, 0 }                     /* NULL terminator to signal the end of the array */
	};

	/* This is the Lua script we plan on running, it would probably be in a resource file somewhere. */
	/* scriptOnUpdate() above calls this function, which in turn calls the GameObject_move() function above */
	const char *script = 
		"function onUpdate(instance)\n" \
		"    GameObject.move(instance, 4, 2)\n" \
		"end\n";

	lua_State *lua = luaL_newstate();      /* Creates a new context, lua_open replacement in versions > 5 */
	luaL_openlibs(lua);                    /* Adds the default Lua libraries to the context */
	luaL_newlib(lua, gameObjectFunctions); /* Adds our functions to the context */
	lua_setglobal(lua, "GameObject");      /* Assigns a namespace to our most recently added functions */
	luaL_loadstring(lua, script);          /* Loads our script into the context */
	lua_call(lua, 0, 0);                   /* This "primes" our context, without this, things will fail! */

	return lua;
}

int main(int argc, char *argv[])
{
	/* This is our game instance that we intend on passing to the Lua script for it to have its way with. */
	GameObject instance;
	instance.x = 6;
	instance.y = 8;

	lua_State *lua = createLuaContext(); /* Set up the lua context */
	scriptOnUpdate(lua, &instance);      /* Call the Lua script "onUpdate" function passing our current object instance */
	lua_close(lua);                      /* ... and we're done. clean up shop and go home! */

	printf("\nPress return to quit.\n");
	getchar();
	return 0;
}


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