Its seems that Microsoft is slow to support Fold Expressions and I'm struggling to find a way to emulate it. I have the following cutdown example. This fails to compile on Visual Studio 2017 because of no support for Fold Expressions. Any idea how to emulate it please?
template<typename... Args>
void get(weak_ptr<sqlite::database> awdb_, Args&&... args) {
(*awdb_.lock() << ... << args);
}
Before fold expressions, there was the expander trick:
template <class... Args>
void get(weak_ptr<sqlite::database> awdb_, Args&&... args) {
if (auto db = awdb_.lock()) {
using expander = int[];
(void)expander{0,
(void(*db << std::forward<Args>(args)), 0)...
};
}
}
As T.C. points out, this isn't necessarily strictly equivalent if << does something other than returning *this back, so we could write:
template <class F, class Arg>
decltype(auto) fold(F&& , Arg&& arg) {
return std::forward<Arg>(arg);
}
template <class F, class A0, class A1, class... Args>
decltype(auto) fold(F&& f, A0&& a0, A1&& a1, Args&&... args) {
return fold(f, f(a0, a1), std::forward<Args>(args)...);
}
template <class... Args>
void get(weak_ptr<sqlite::database> awdb_, Args&&... args) {
if (auto db = awdb_.lock()) {
auto lshift = [](auto&& a, auto&& b) -> decltype(auto) { return a << std::forward<decltype(b)>(b); };
fold(lshift, *db, std::forward<Args>(args)...);
}
}
