Is it possible to mark one element of an unpacked tuple as unused

Asked Feb 15 '19 at 08:48

Active Feb 15 '19 at 08:48

Viewed 59 times

This question is relevant to c++17 and above, which introduces the [[maybe_unused]] attribute.

I can apply maybe_unused to an entire unpacked tuple like so:

for ([[maybe_unused]] auto &[key, weak] : connection_cache_)
{
  if (auto locked = lock(weak); locked)
  {
    locked->cancel();
  }
}

However, marking the unused key does not work (at least on gcc-8):

for (auto &[ [[maybe_unused]] key, weak] : connection_cache_)
{
  if (auto locked = lock(weak); locked)
  {
    locked->cancel();
  }
}

This leaves me the choice of either not using the attribute or using a no-op to satisfy ensure no warnings when compiling with -Wall -Wextra -pedantic which is the norm for my codebase:

for (auto &[key, weak] : connection_cache_)
{
  boost::ignore_unused(key);
  if (auto locked = lock(weak); locked)
  {
    locked->cancel();
  }
}

Am I out of options?

asked Feb 15 '19 at 08:48

Richard Hodges

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1

Have you seen [this answer](https://stackoverflow.com/a/41404350/9593596)? I guess not a dupe, but relevant. – lubgr Feb 15 '19 at 08:50
2

[Upgrade your compiler](https://godbolt.org/z/kvdm3T). – Passer By Feb 15 '19 at 08:51
@PasserBy: your link doesn't use same flag as OP, and doesn't have attribute neither. – Jarod42 Feb 15 '19 at 09:05
1

@Jarod42 Oops, but the warning is in -Wall. I'm demonstrating newer gcc only issues unused warnings if none of the bindings were used. – Passer By Feb 15 '19 at 09:08
@PasserBy: Even gcc-8.1 don't issue warning. but warning might be legit... – Jarod42 Feb 15 '19 at 09:11
Thanks everyone for comments. I have accepted that the question as a dupe. Short answer is "yes, you're out of options". – Richard Hodges Feb 15 '19 at 09:12
Makes you want to switch to Go and have `_` huh? – Passer By Feb 15 '19 at 09:15
1

@PasserBy there's a lot about c++ that I like. I'd prefer if the standard release cycle was quarterly so we could have more nice things more often... – Richard Hodges Feb 15 '19 at 09:23

Is it possible to mark one element of an unpacked tuple as unused

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