Variadic templates to solve C-style message callback problems

TemplateHacks.cpp

#include <stdio.h>
#include <stdarg.h>
#include <vector>

enum Message
{
    MsgAdd, //int(int, int)
    MsgNeg, //int(int)
    MsgRand, //int()
};

typedef int Result;
typedef int Param;
const Param DefaultParam = 0;

Result sendMessage(Message msg, int numArgs, ...)
{
    //get the arguments
    va_list vaList;
    va_start(vaList, numArgs);
    std::vector<Param> params;
    params.resize(numArgs);
    for (auto i = 0; i < numArgs;i++)
        params[i] = va_arg(vaList, Param);
    va_end(vaList);

    //TODO: nice implementation for this
    switch (msg)
    {
    case MsgAdd:
    {
        return params[0] + params[1];
    }

    case MsgNeg:
    {
        return -params[0];
    }

    case MsgRand:
    {
        static auto ran = 0;
        return ran++;
    }        

    default:
    {
        return DefaultParam;
    }
    }
}

template<Message M, typename R, typename... P>
class Msg
{
public:
    R operator()(P... params)
    {
        return R(sendMessage(M, sizeof...(P), params...));
    }
};

template<Message M, typename R, typename... P>
R MsgCall(P... params)
{
    return Msg<M, R, P...>()(params...);
}

typedef Msg<MsgAdd, int, int, int> AddMsg;
#define AddFunctionDef(a, b) AddMsg()(a, b)
static int AddFunctionInline(int a, int b) { return AddMsg()(a, b); }

typedef Msg<MsgNeg, int, int> NegMsg;
#define NegFunctionDef(x) NegMsg()(x)
static int NegFunctionInline(int x) { return NegMsg()(x); }

typedef Msg<MsgRand, int> RandMsg;
#define RandFunctionDef() RandMsg()()
static int RandFunctionInline() { return RandMsg()(); }

//C bindings
#define MsgC0(m) sendMessage(m, 0)
#define MsgC1(m, p1) sendMessage(m, 1, p1)
#define MsgC2(m, p1, p2) sendMessage(m, 2, p1, p2)

static int AddFunctionC(int a, int b)
{
    return MsgC2(MsgAdd, a, b);
}

static int NegFunctionC(int x)
{
    return MsgC1(MsgNeg, x);
}

static int RandFunctionC()
{
    return MsgC0(MsgRand);
}

int main()
{
    auto three = AddMsg()(1, 2); //shows (int, int) with autocompletion
    three = AddFunctionDef(1, 2); //shows (a, b) with autocompletion
    three = AddFunctionInline(1, 2); //shows (int a, int b) with autocompletion
    three = AddFunctionC(1, 2); //shows (int a, int b) with autocompletion
    auto one = NegMsg()(-1);
    one = NegFunctionDef(-1);
    one = NegFunctionInline(-1);
    one = NegFunctionC(-1);
    auto ran = RandMsg()();
    ran = RandFunctionDef();
    ran = RandFunctionInline();
    ran = RandFunctionC();
    printf("%d %d %d\n", three, one, ran);
    getchar();
    return 0;
}