折叠 std::initializer_list 的构造函数的参数列表与“正常”折叠

Question

I learn C++17 from C++17 STL Cookbook by Jacek Galowicz and there is such an example about lambdas :

template <typename... Ts> static auto multicall(Ts... functions)
{
    return [=](auto x) { (void)std::initializer_list<int>{((void)functions(x), 0)...}; };
}

template <typename F, typename... Ts> static auto for_each(F f, Ts... xs)
{
    (void)std::initializer_list<int>{((void)f(xs), 0)...};
}

static auto brace_print(char a, char b)
{
    return [=](auto x) { std::cout << a << x << b << ", "; };
}

int main()
{
    auto f(brace_print('(', ')'));
    auto g(brace_print('[', ']'));
    auto h(brace_print('{', '}'));
    auto nl([](auto) { std::cout << '\n'; });

    auto call_fgh(multicall(f, g, h, nl));

    for_each(call_fgh, 1, 2, 3, 4, 5);
}

Why is the std::initializer_list used here and why is this void casting used (The author writes that there should be reinterpret_cast used instead of C-like casting, but the question is about why is this casting used)?

When I change the multicall and for_each functions to such:

template <typename... Ts> static auto multicall(Ts... functions)
{
    return [=](auto x) { (functions(x), ...); };
}

template <typename F, typename... Ts> static auto for_each(F f, Ts... xs)
{
    (f(xs), ...);
}

Everything works as expected, I get the same results.

Answer 1

It looks like for some reasons this part of the book operates C++14-way. Trick with std::initializer_list was needed before fold expressions were introduced in C++17 to call emulate variadic call. In C++17 fold with comma operator is absolutely legit