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Re: [std-proposals] Dummy names for dummy objects

From: Harald Achitz <harald_at_[hidden]>
Date: Thu, 29 Jun 2023 18:01:35 +0200
> Use a cast to (void) to suppress nodiscardness.

Will not pass a code review, for reason
[[maybe_unused]] deserves a comment, for reasons, to pass a code review.

If auto _ silences nodiscard warnings, I can guarantee you, it will land
on the list of features not to use in some projects, and it will be a
point of discussion if / when we move to new standard version.

This can / should not be the indent of that actually very nice feature/idea.

Well, I addressed my concerns about this now the second time, if I am
the only one seeing a problem in code maintainability and readability
when _ makes nodiscad pointless, than maybe it's not such a concern for
people here and not worth to follow up further

On 2023-06-29 17:02, Arthur O'Dwyer wrote:
> On Thu, Jun 29, 2023 at 10:20 AM Harald Achitz via Std-Proposals
> <std-proposals_at_[hidden]> wrote:
> my concern is that auto _ = f();
> * will be abused by lazy developers to not care about nodiscard
> return values, instead of a clear syntax that makes it obvious
> that for any reason a nodiscard return value is ignored
> Today, there is a more verbose syntax required, and that makes it
> more readable to see what is going on in code reviews and when
> reading the code.
> * makes it impossible to catch unhandled return values if for any
> reason nodiscard gets added to a function and _ is already in use
> I think you're misunderstanding the merged proposal and possibly also
> the status quo.
> (I don't particularly care for the merged proposal either, but only
> because I don't like making `_` into a magic identifier; it's one more
> thing to teach and I don't think its benefit outweighs that cost.
> Nonetheless...)
> Suppose in C++26 someone writes:
> auto _ = f();
> This unambiguously means they want to save `f()`'s value until the end
> of scope, but that they won't use it for anything otherwise.
> "But what if `f` was [[nodiscard]]?"
> That's fine. The programmer /*is*/ using `f`'s value; they're saving
> it until the end of scope.
> "But shouldn't this still be diagnosed? They didn't say [[maybe_unused]]!"
> No, that's fine. `_` is unused by definition. If the programmer wanted
> to get a -Wunused warning, they wouldn't have called the variable `_`.
> auto x = f(); // OK, now we get a -Wunused warning if `x` is
> never used
> "So people will use `auto _ = f()` to conveniently suppress
> nodiscardness?"
> No, people already use
> (void)f();
> if all they want to do is suppress nodiscardness. They'll keep doing that.
> The reason to use
> auto _ = f();
> is that you want to save `f()`'s value until the end of scope (i.e.
> /*not*/ discard and destroy it immediately). If you just want to
> discard f()'s value, you wouldn't do that.
> In fact, personally I might hope for a /compiler warning/ whenever
> someone writes
> int _ = f();
> (or any trivially destructible type like that), because in that case
> there's no point to capturing `_` at all. That line of code probably
> indicates that the programmer doesn't understand the point of
> capturing `_`, which is to prolong the lifetime of a value whose
> destruction /will have side effects/.
> My suggestion would simply be, if a function is nodiscard, the _
> to ignore it's returnvalue should not compile (keep the warning).
> That is the exact opposite of the point.
> auto lk = my::lock_guard(m); // my::lock_guard is [[nodiscard]]
> compiles today without warning, and has the right behavior, but
> requires choosing a unique name `lk`.
> auto _ = my::lock_guard(m); // my::lock_guard is [[nodiscard]]
> compiles in C++26 and does /not/ require choosing any unique name, so,
> it's better.
> If the compiler gave a diagnostic on the latter line, then we couldn't
> write it, and so the entire `_` feature would have no point at all.
> Remember:
> > Use a cast to (void) to suppress nodiscardness.
> > Use `_` to prolong a lifetime to the end of scope without choosing a
> unique name.
> HTH,
> Arthur

Received on 2023-06-29 16:01:33