example_dll.cpp

class MyTest
{
	public:
		int a,b,c;
		void test()
		{
			
		}
};

extern "C"
{
	int testfunc()
	{
		return 42;
	}
}

tester.c

#define WINDOWS_LEAN_AND_MEAN
#include <windows.h>

int main()
{
	HMODULE dll = LoadLibrary("example_dll.dll");
	if(dll == NULL)
	{
		abort();
	}
	FreeLibrary(dll);
	return 0;
}

tester.lua

local ffi = require "ffi"

local dll = ffi.load("example_dll")
ffi.cdef "int testfunc()"
print(dll.testfunc())
os.exit(0)