Re: [GD-General] variadic functions

[Thatcher U. <tu...@tu...>]

On Nov 27, 2003 at 10:22 +0800, Brett Bibby wrote:
> 
> I'm trying to bind our scripting engine to our game engine and having
> trouble with variadic functions

I'm surprised that you have variadic functions on the C/C++ side of
your engine.  What are they used for?

Anyway, one possible approach is to do the wrapping in reverse.
I.e. rewrite those functions avoiding the C/C++ implementation of
variadic parameters, and instead use your scripting language's
parameter-passing mechanism.  Bind the functions directly to your
scripting language.  Provide wrappers for calling them from C/C++.

E.g. let's say you're using Lua, and you have an add() function that
takes an arbitrary number of doubles and returns their sum:

double add(int arg_count, ...) { /* ... */ }

You can implement that using the va_ macros, but instead of doing
that, you could implement it using the Lua API:

int add_lua(lua_State* L)
{
	int arg_count = (int) luaL_checknumber(1);
	double sum = 0;
	for (int i = 0; i < arg_count; i++)
	{
		sum += luaL_checknumber(2 + i);
	}
	lua_pushnumber(L, sum);
	return 1;
}

// Wrapper for calling from C++.
double add(int arg_count, ...)
{
	// Seat of the pants code, probably incorrect!
	lua_pushnumber(L, arg_count);
	va_list argp;
	va_start(argp, arg_count);
	for (int i = 0; i < arg_count; i++)
	{
		lua_pushnumber(L, va_arg(argp, double));
	}
	va_end(argp)
	add_lua(L);
	double result = luaL_tonumber(L, -1);
	lua_pop(arg_count + 2);
	return result;
}
// etc.

-- 
Thatcher Ulrich
http://tulrich.com