What kind of syntax is this in C++?

Asked Apr 22 '23 at 06:21

Active Apr 22 '23 at 06:27

Viewed 71 times

I see this code in C++ STL, and I don't really know why is ((void)Pred, true)... valid in C++, I never have seen this kind of code.

template <bool... Pred>
struct all_helper {};

#include <type_traits>
template <bool... Pred>
using all = std::_IsSame<all_helper<Pred...>, all_helper<((void)Pred, true)...>>;

asked Apr 22 '23 at 06:21

LunarEclipse

It is called [parameter pack expansion](https://en.cppreference.com/w/cpp/language/parameter_pack) – Jason Apr 22 '23 at 06:24
1

And the cast to `(void)` is to prevent the `,` being a overloaded comma operator; as you can't overload operators on `void`. – Richard Critten Apr 22 '23 at 06:54
1

Bad coding style. Could simply use `std::conjunction` or the C++17 fold expressions. – Red.Wave Apr 22 '23 at 07:10

What kind of syntax is this in C++?

0 Answers0