functions as template argument, plus variadic template argument - c++

Question

I'm writing a generalized function wrapper, that can wrap any function into a lua-style call, which has the form
int lua_function( lua_State *L)
And I wish the wrapper function is generated on-the-fly, so I'm thinking of passing the function as a template argument. This is trivial if you know the number (e.g, 2) of arguments:
template <typename R, typename Arg1, typename Arg2, R F(Arg1, Args)>
struct wrapper
However, I don't know the number, so I beg for variadic template argument for help
// This won't work
template <typename R, typename... Args, R F(Args...)>
struct wrapper
The above won't compile, since variadic argument has to be the last one. So I use two level template, the outer template captures types, the inner template captures the function:
template <typename R, typename... Args>
struct func_type<R(Args...)>
{
// Inner function wrapper take the function pointer as a template argument
template <R F(Args...)>
struct func
{
static int call( lua_State *L )
{
// extract arguments from L
F(/*arguments*/);
return 1;
}
};
};
That works, except that to wrap a function like
double sin(double d) {}
the user has to write
func_type<decltype(sin)>::func<sin>::apply
which is tedious.
The question is: is there any better, user-friendlier way to do it? (I can't use a function template to wrap the whole thing, coz a function parameter can't be used as a template argument.)

Answer 1

Things like std::function and std::result_of use the following technique to do what you want regarding variadic templates:
template<typename Signature>
struct wrapper; // no base template
template<typename Ret, typename... Args>
struct wrapper<Ret(Args...)> {
// instantiated for any function type
};
You could expand the above to add a non-type Ret(&P)(Args...) template parameter (pointers to function work just as well) but you'd still need a decltype at the user level, i.e. wrapper<decltype(sin), sin>::apply. Arguably it would be a legitimate use of the preprocessor if you decide to use a macro to remove the repetition.
template<typename Sig, Sig& S>
struct wrapper;
template<typename Ret, typename... Args, Ret(&P)(Args...)>
struct wrapper<Ret(Args...), P> {
int
static apply(lua_State*)
{
// pop arguments
// Ret result = P(args...);
// push result & return
return 1;
}
};
// &wrapper<decltype(sin), sin>::apply is your Lua-style wrapper function.
The above compiles with gcc-4.5 at ideone.
Good luck with implementing the apply that (variadically) pops the arguments (leave me a comment if you open a question about that). Have you considered using Luabind?

Answer 2

As #Juraj says in his comment, the function pointer can be a template argument, see the following simple example:
#include <iostream>
#include <boost/typeof/typeof.hpp>
void f(int b, double c, std::string const& g)
{
std::cout << "f(): " << g << std::endl;
}
template <typename F, F* addr>
struct wrapper
{
void operator()()
{
std::string bar("bar");
(*addr)(1, 10., bar);
}
};
int main(void)
{
wrapper<BOOST_TYPEOF(f), &f> w;
w();
return 0;
}
working version: http://www.ideone.com/LP0TO
I'm using BOOST_TYPEOF as normally I always provide examples in the current standard, but it does something similar to decltype. Is this what you were after?