C++ Variable number of variadic template parameters

Question

I have the following type alias:

using VoidFunc = void (*)();

I would like to define a class like so:

class SomeClass {
public:

 template<template<typename Return, typename... Args>... Funcs>
 static constexpr typename std::vector<VoidFunc> registerFunctions(Funcs<Return, Args...>... funcs) {
  return { ((VoidFunc)(funcs), ...) };
 }
};

I am aware that this is invalid syntax, as Return and Args are unresolvable outside of their template group. The goal is to allow the function registerFunction to accept a variable number of functions all with different prototypes. In the actual implementation, it is important to preserve the type information of each function for use with other constexpr functions. Is there a way to achieve this in C++17 or later?

Answer 1

You don't actually care about any of those underlying types (at least not in the code presented here). So just don't use it. All you care about is that these things are all function pointers:

typename <typename... F>
static std::vector<VoidFunc> registerFunctions(F*... funcs)
{
    static_assert((std::is_function_v<F> && ...));
    return { reinterpret_cast<VoidFunc>(funcs)... };
}

If you do need the signatures for something else, you can just pass each element of funcs into a different function template and just re-deduce the actually signature there.

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Note that the typename is unnecessary there, and the constexpr in C++17 makes this ill-formed (since you cannot in C++17 have a constexpr std::vector ).