Extracting C Function's Parameters Using C++ Metaprogramming (Example from “Practical C++ Metaprogramming”)

Question

The following is an example from "Practical C++ Metaprogramming" (pages 16/17):

#include <tuple>
#include <typeinfo>

template <typename F>
struct make_tuple_of_params;

template <typename Ret, typename... Args>
struct make_tuple_of_params<Ret (Args...)>
{
   using type = std::tuple<Args...>;
};

template <typename F>
using make_tuple_of_params_t = typename make_tuple_of_params<F>::type;

template<typename F>
void some_magic_function(F f)
{
   // if F is in the form void(double*, double*)
   // make_tuple_of_params is std::tuple<double*, double*>
   make_tuple_of_params_t<F> params;

   // ...
}

void Foo(double* x, double* y) { }

int main()
{
   some_magic_function(Foo);
}

It's failing to compile:

$ clang++ -std=c++14 MakeTuple.cpp
MakeTuple.cpp:14:5: error: implicit instantiation of undefined template 'make_tuple_of_params<void (*)(double *, double*)>'

Is this because the non specialized version of make_tuple_of_params (lines 4 and 5 of the code above) is not defined?

Answer 1

You actually need to different overloading, depending if you extracting it from a pointer or from a signature template argument, see below.

template <typename F>
struct make_tuple_of_params;

template <typename Ret, typename... Args>
struct make_tuple_of_params<Ret (*)(Args...)> {
  using type = std::tuple<Args...>;
};

template <typename Ret, typename... Args>
struct make_tuple_of_params<Ret(Args...)> {
  using type = std::tuple<Args...>;
};

template <typename F>
using make_tuple_of_params_t = typename make_tuple_of_params<F>::type;

template <typename F>
bool some_magic_function(F f) {
  // if F is in the form void(double*, double*)
  // make_tuple_of_params is std::tuple<double*, double*>
  return std::is_same<std::tuple<double*, double*>, make_tuple_of_params_t<F>>::value;
}

void Foo(double* x, double* y) {}

int main() {
  cerr << some_magic_function(Foo) << endl;
  cerr
    << std::is_same<std::tuple<int, int>, make_tuple_of_params_t<void(int, int)>>::value
    << endl;
  // The latter one might be handy in some of template metaprogramming constructs
  return 0;
}

Sorry, have not look at the book pages, so do not know what the authro meant there.