Re: [boost] [scope] Scope review starts on November 26th

On 11/30/23 00:35, Janko Dedic via Boost wrote:

On Wed, Nov 29, 2023 at 4:56 PM Andrey Semashev via Boost < boost@lists.boost.org> wrote:

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I don't see the current definition of BOOST_SCOPE_FINAL as a deficiency of the interface compared to passing the capture list in the macro arguments. The latter doesn't offer any advantages compared to the current definition, and in fact is more limiting as it only allows for lambda functions for the scope guard action.

It offers better syntax.

Better in what way, other than your personal preference? Can you describe objective advantages of your proposed syntax and why the current BOOST_SCOPE_FINAL doesn't work for you?

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Besides, note that the lambda definition syntax has evolved over time and may now include additional elements such as template parameters, noexcept and mutable qualifiers and the trailing return type. There may be new syntax additions in the future, which could be incompatible with the macro definition. Although some of the syntax elements are useless for the purpose of scope guards, at least noexcept and mutable are meaningful and would look awkward if BOOST_SCOPE_FINAL accepted the capture list in its parameters.

I only suggested adding [&] to the macro, or adding a macro that includes the [&].

Sorry, but no. That would limit user's options for no reason, and binding variables by value is not uncommon. Users of Boost.Scope should have the full range of function object definition ways that is permitted by the language.

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scope_success isn't as widely useful as e.g. scope_fail, but you do occasionally want it to schedule some commit steps when the enclosing function succeeds, if there are multiple exit points and a lot of the function-local state that is difficult or inconvenient to pass to a separate function. You can sometimes work around it with a goto, and scope_success is just another, possibly cleaner way to do this.

Do you have a code example? I'm really curious about this use case.

I had something like this in mind: void foo() { auto private_data = do_something(); scope_success commit_guard{[&private_data] { commit_data(private_data); }}; if (condition1) { do_something_on_condition1(private_data); return; } if (condition2) { do_something_on_condition2(private_data); return; } bool succeeded = complete_processing(private_data); if (!succeeded) { commit_guard.set_active(false); return; } } In the above pseudocode, private_data may be any number of data items of various types, including those defined locally in foo(), and function calls like commit_data, do_something_on_condition1, etc. would be pieces of code working on that and other data.

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Also, and this pertains to some other questions you asked in your review, Boost.Scope tries to provide components that are defined in the Library Fundamentals TS with a compatible interface. scope_success is defined in the TS, so Boost.Scope also provides it.

I very much dislike this reasoning. Why should Boost copy std even when std is making a mistake? Boost should rely on its review process to judge libraries. "std does the same" is not a valid argument (especially in the light of recent discussions on the list about the quality of the WG21 "review process"). I can understand if it's the stated goal of the library to copy std, but (1) scope guards are not even standardized yet and (2) for copying std there is Boost.Compat.

At the end of the day, users are robbed of better libraries because "std has already done it this way."

If you read earlier commits in this review, you'll see that there are people who would prefer Boost.Scope to *exactly* reflect the TS with no extensions or deviations. Although I disagree with that point of view, I still think there is value in providing interface that is *compatible* with the TS, if possible, so that users, at least potentially, have the option to switch between Boost.Scope and std, should they need to for whatever reason. Another reason is that this compatibility reduces the element of surprise and makes learning the library (whether Boost.Scope or std) easier. If this review shows that no such compatibility is desired by the community, I can remove some elements of the library that are deemed unwanted or detrimental to its quality.

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If you want to enforce that your action never throws, you can declare its operator() as noexcept:

BOOST_SCOPE_FINAL []() noexcept { /* I guarantee I won't throw */ };

Does anyone really want to write code like this?

I see nothing wrong with this code. I actually wrote code like this, where the no-throw guarantee was important.

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Movability is necessary for the factory functions, at the very least.

I can see why. C++17 fixes this "defect". You can technically rely on reference lifetime extension, but that doesn't look as good.

auto&& guard = make_scope_exit(...);

Reference lifetime extension has nothing to do with it. Without RVO, in order to return the scope guard from the factory function by value, its copy or move constructor needs to be invoked. Besides, TS scope guards are move-constructible.

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And I do specifically prefer set_active() over a pair of methods like activate()/deactivate() because passing a bool is fundamentally more flexible than having two distinct methods. You can receive that bool from elsewhere and just forward to the method instead of testing it yourself. Like in the example above, you could have written it like this:

void add_object(std::shared_ptr< object > const& obj) { // Create a deactivated scope guard initially std::set< std::shared_ptr< object > >::iterator it; boost::scope::scope_fail rollback_guard{[&, this] { objects.erase(it); }, false};

bool inserted; std::tie(it, inserted) = objects.insert(obj);

// Activate rollback guard, if needed rollback_guard.set_active(inserted);

obj->on_added_to_collection(*this); }

which wouldn't be as simple with two different methods.

This looks much better honestly. Why isn't this variant used for the example?

I can update the example in the docs.

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I did not like the locked_write_string example. Reading this code locally:

// Lock the file while (flock(fd, LOCK_EX) < 0) { err = errno; if (err != EINTR) return; }

it's hard to get what's happening, why we're setting err and how big of an impact that has on function behavior. A better way to write this code would be to simply extract this piece into its own function:

err = errno; if (err != EINTR) return;

together with the ec = std::error_code(err, std::generic_category()), which doesn't require a scope guard.

Sorry, I don't understand your suggestion. How would this help with other return points of the function?

It helps with all the return points in the example function.

I don't see how. Can you provide a code example? Note that the checks for EINTR are made in different contexts. Some of them cause return, other - restart loop iteration, third - affect error_code initialization.

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I think a similar utility would be more useful in form of a class template, for example:

resource windows_handle; resource sdl_window_handle;

This would be very useful for concisely implementing RAII wrappers over C handles (you don't have to manually write destructors, move constructors and move assignment operators).

This would require C++17, if I'm not mistaken (or when were the auto non-type template parameters introduced?). I don't think it offers much advantage compared to the current unique_resource with resource traits.

The advantage is doing the same thing in 1 line instead of 20 lines of code, at which point writing the destructor and moves manually has a better ROI than implementing traits classes.

This would also prohibit resource types with non-constexpr constructors, as well as non-default-constructible resources. I don't like this tradeoff.