On 3/22/23 16:01, Daniela Engert via Boost wrote:

Am 22.03.2023 um 13:34 schrieb Andrey Semashev via Boost:

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Am 22.03.2023 um 11:24 schrieb Andrey Semashev via Boost:

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[[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

On 3/22/23 14:23, Daniela Engert via Boost wrote: 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().

I did not mean to mark either of the functions as [[nodiscard]], I was considering marking scope_exit as [[nodiscard]], which, I think, is what you suggested. If it was marked as such, the code in bar() would emit a warning, which I'm not sure is a good thing. For now I'm leaning towards not marking the scope guards with [[nodiscard]] as I don't like limiting the use cases of these classes.