Can some C++ guru translate in D the template below?

I gave up after reading a lot, but I didn't manage to understand the meaning "&& ..."

template <typename...Tables> static uint8_t composite_index_size(Tables const&... tables) { return (composite_index_size(tables.size(), impl::bits_needed(sizeof...(tables))) && ...) ? 2 : 4; }

Thanks.

On 2/5/21 1:10 PM, Rumbu wrote:

> I gave up after reading a lot, but I didn't manage to understand the meaning "&& ..."

I think it's the universal reference.

> template <typename...Tables> static uint8_t composite_index_size(Tables const&... tables) { return (composite_index_size(tables.size(), impl::bits_needed(sizeof...(tables))) && ...) ? 2 : 4; }

With some guesses, I made the following compile:

import std.meta : staticMap;
import std.algorithm : sum;

ubyte composite_index_size(tables...)() {
  return (composite_index_size(tables.length,
                               bits_needed(staticMap!(SizeOf, typeof(tables)))))
          ? 2
          : 4;
}

auto composite_index_size(size_t count, size_t total) {
  return 42;
}

enum SizeOf(T) = T.sizeof;

auto bits_needed(size_t[] args...) {
  return args.sum;
}

void main() {
  int[] i;
  double[] d;
  pragma(msg, composite_index_size!(i, d));
}

Ali

On Saturday, 6 February 2021 at 00:35:12 UTC, Ali Çehreli wrote:
> On 2/5/21 1:10 PM, Rumbu wrote:
>
>> I gave up after reading a lot, but I didn't manage to understand the meaning "&& ..."
>
> I think it's the universal reference.

Thank you Ali, but nope, it's "parameter pack folding". This allows starting with C++ 17 to have recursive templates in one liner. Basically, it means "&& template(the rest of arguments)"

https://github.com/microsoft/cppwin32/blob/8a6b2507734dd9e9892059b5794f35109688fc20/cppwin32/winmd/impl/winmd_reader/database.h#L490

On Friday, 5 February 2021 at 21:10:21 UTC, Rumbu wrote:
> Can some C++ guru translate in D the template below?
>
> I gave up after reading a lot, but I didn't manage to understand the meaning "&& ..."
>
> template <typename...Tables> static uint8_t composite_index_size(Tables const&... tables) { return (composite_index_size(tables.size(), impl::bits_needed(sizeof...(tables))) && ...) ? 2 : 4; }
>
> Thanks.

modern cpp looks like 10 lines of decl for 1 of actual code.