Are fold expressions fully supported in VS2017?

Question

I found an interesting article and tried its code with MSVS 2017:

#include <utility>
#include <tuple>

template <typename... Args, typename Func, std::size_t... Idx>
void for_each(const std::tuple<Args...>& t, Func&& f, std::index_sequence<Idx...>) {
    f(std::get<Idx>(t))...;
}

template <typename... Args, typename Func>
void for_each(const std::tuple<Args...>& t, Func&& f) {
    for_each(t, f, std::index_sequence_for<Args...>{});
}

template <typename T>
void Write(std::wostream & out, const T & t)
{
    out << t;
}

template<typename ...Args>
void WriteV(std::wostream & out, Args&... args)
{
    for_each(std::tuple<Args&...>(args...), [&out](auto& a) { Write(out, a); });
}

struct A
{
    int n;
    std::wstring s;
    double d;
};

    void main()
    {
        std::wostringstream out;

        A a{ 1, std::wstring(L"2"), 3.0 };
        WriteV(a.n, a.s, a.d);
    }

, but the code did not compile with errors:

error C2760: syntax error: unexpected token '...', expected ';'
error C3520: 'Idx': parameter pack must be expanded in this context

does it mean that VS2017 does not fully support fold expressions?

Doesn't work in GCC or Clang, either, even with the necessary includes. This is possibly a cautionary tale about copy/pasting code from random websites ;) — Lightness Races in Orbit, Jan 21 '19 at 12:48
This code lacks includes. Also it is not clear which VS version and compilation options you are using — user7860670, Jan 21 '19 at 12:51
"Keen readers might have noticed that I lied in the above section. `(std::get(t))...;` does not generate that sequence of calls to f." — Caleth, Jan 21 '19 at 13:14

user7860670 · Accepted Answer · 2019-01-21T13:11:07.870

6

This code needs just a couple of syntax fixes:

(f(std::get<Idx>(t)), ...);

and

WriteV(out, a.n, a.s, a.d);

Note that this code is unnecessary long for some reason. It can be replaced with just

 template<typename ... Args>
 void WriteV(std::wostream & out, Args const & ... args)
 {
     (out << ... << args);
 }

edited Jan 21 '19 at 13:11

answered Jan 21 '19 at 12:59

user7860670

35,849
4
58
84

2

… and `void main()` … – Konrad Rudolph Jan 21 '19 at 14:53

max66 · Answer 2 · 2019-01-21T13:14:16.633

This way is wrong

f(std::get<Idx>(t))...;

You have to choose.

(1) do you want to call f() only one time with all arguments? In this case you have to put the ellipsis ("...") inside the call

f(std::get<Idx>(t)...);

(2) or do you want (I suppose is the case, in your example) call f() with every argument (N argument, N calls)? In this case you can (starting from C++17) use template folding with comma operator adding a couple of parentheses

    (f(std::get<Idx>(t) , ...);
// .^...................^....^   <- comma and parentheses

The second way, pre C++17, can be simulated inside the inizialization of an (usually unused) array. Something as follows

using unused = int[];

(void) unused { 0, ((void)f(std::get<Idx>(t)), 0)... };

Are fold expressions fully supported in VS2017?

2 Answers2