How can I generate as many parameters as is the arity of a function?

Question

Suppose I have the following function, that takes a function as a parameter.

template <typename F>
void test_func(F f)
{
    // typedef typename function_traits<F>::return_type T;
    typedef int T;

    std::mt19937 rng(std::time(0));
    std::uniform_int_distribution<T> uint_dist10(0, std::numeric_limits<T>::max());

    f(uint_dist10(rng), uint_dist10(rng)); // Problem!
}

Usage would be:

int foo(int, int) { return 0; }
int bar(int, int, int, int) { return 0; }

int main()
{
    test_func(foo);
    // test_func(bar);
}

Just like foo and bar, I have several functions that return T, and take some amount of parameters of type T. I would like test_func to generate as many calls to my RNG as the function f takes parameters. In other words, we can assume T is always an integer type, and that each parameter will be the same, i.e. a function call to an RNG.

Using function_traits (such as the ones in Boost), I can fetch the return type of F, and that helps a little. Roughly, my question is

How can I generate a needed amount of function calls so that it matches the arity of the function F?

Before C++11, I would have looked at Boost.Preprocessor, or maybe relied on template specialization. Is there a nicer way of doing it now?

Answer 1

First define a meta function called arity to compute arity of the function (it is just a simple implementation; can be improved to compute arity of functors also. See my answer here.):

template<typename F> 
struct arity;

template<typename R, typename ...Args> 
struct arity<R (*)(Args...)>
{
    static const std::size_t value = sizeof ... (Args);
};

then define another meta function called genseq to generate a compile time sequence of integral values:

template<int ... N>
struct seq
{
    using type = seq<N...>;

    template<int I>
    struct push_back : seq<N..., I> {};
};

template<int N>
struct genseq : genseq<N-1>::type::template push_back<N-1> {};

template<>
struct genseq<0> : seq<> {};

template<int N>
using genseq_t = typename genseq<N>::type;  //Just a friendly alias!

then a function invoker as:

template<typename F, typename ArgEvaluator, int ...N>
void invoke(seq<N...>, F f, ArgEvaluator arg_evaluator)
{
    using arg_type = decltype(arg_evaluator());

    constexpr std::size_t arity = sizeof ... (N);

    arg_type args[] { (N, arg_evaluator()) ... }; //enforce order of evaluation

    f( args[N] ... );
}

And then your code would become this:

template <typename F>
void test_func(F f)
{
    // typedef typename function_traits<F>::return_type T;
    typedef int T;

    std::mt19937 rng(std::time(0));
    std::uniform_int_distribution<T> uint_dist10(0, std::numeric_limits<T>::max());

    //f(uint_dist10(rng), uint_dist10(rng)); // Problem!

      auto arg_evaluator = [&]() mutable { return uint_dist10(rng); };
      invoke(genseq_t<arity<F>::value>(), f, arg_evaluator);
}

Here is a sample demo.

Hope that helps.

Answer 2

No need for complicated meta calculations.

template <typename Ret, typename ... T>
void test_func (Ret f (T...))
{
  std::mt19937 rng(std::time(0));
  f((std::uniform_int_distribution<T>(0, std::numeric_limits<T>::max())(rng))...);
}

int moo(int, int, int){ return 0; }

int main ()
{
  test_func(moo);
}

To support functors one needs a bit longer implementation, still not too complicated:

// separate arguments type from function/functor type
template <typename F, typename ... T> 
void test_func_impl (F f)
{
  std::mt19937 rng(std::time(0));
  f((std::uniform_int_distribution<T>(0, std::numeric_limits<T>::max())(rng))...);
}

// overload for a straight function
template <typename Ret, typename ... T>
void test_func (Ret f (T...))
{
  test_func_impl<decltype(f), T...>(f);
}

// forwarder for a functor with a normal operator()
template <typename F, typename Ret, typename... T>
void test_func_for_functor (F f, Ret (F::*)(T...))
{
  test_func_impl<F, T...>(f);
}

// forwarder for a functor with a const operator()
template <typename F, typename Ret, typename... T>
void test_func_for_functor (F f, Ret (F::*)(T...)const)
{
  test_func_impl<F, T...>(f);
}

// overload for anything that has operator()
template <typename F>
void test_func (F f)
{
  test_func_for_functor(f, &F::operator());
}