Re: [scope] Proposing Boost.Scope

From: Daniela Engert (dani_at_[hidden])
Date: 2023-03-22 13:01:18

Am 22.03.2023 um 13:34 schrieb Andrey Semashev via Boost:
> On 3/22/23 14:23, Daniela Engert via Boost wrote:
>> Am 22.03.2023 um 11:24 schrieb Andrey Semashev via Boost:
>>> [[nodiscard]] is not usable here as there's nothing to apply it to, as
>>> we're talking about the scope guard constructor.
>> [[nodiscard]] is highly desirable here. You can apply the attribute to
>> class types (and thus their constructors) and avoid the (typically
>> unwanted) immediate destruction of pr-values. Manager classes should be
>> marked [[nodiscard]] by habit.
> I didn't know you can apply [[nodiscard]] to classes, but apparently you
> can. I suppose, I could mark scope guards with it, but I'm not sure if
> limiting the use cases would be desirable. For example, should we
> consider the following a wrong use of the scope guard?
>
>    auto foo()
>    {
>      // do something useful and return the epilogue
>      return scope_exit([] { whatever });
>    }
>
>    void bar()
>    {
>      foo(); // do what foo() does, plus the epilogue
>      // ...
>    }
>
>    void zoo()
>    {
>      auto epilogue = foo(); // do what foo() does, and delay the epilogue
>      // ...
>    }

Suppose you mark class scope_exit with [[nodiscard], then the return 
statement in foo() will construct a temporary scope_exit in its return 
expression and pass it out of function foo. This by itself is a use of 
the temporary and doesn't trigger a compiler warning. If you want to 
prevent the immediate destruction of the returned scope_exit in bar(), 
then you need to mark foo() as [[nodiscard]]. But if you consider this a 
valid use case (albeit a strange one) then you might want to mark foo() 
as [[maybe_unused]] to communicate this intent. Remember: the return 
object of foo() is conceptually different from the object created in the 
return expression of foo().