Re: [scope] The comparison with Boost.ScopeExit is unfair

From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2023-12-01 18:49:14

On 12/1/23 20:25, Andrzej Krzemienski via Boost wrote:
> Hi All,
> This is not a review yet. I am just looking at the front page of the docs,
> and I think the comparison with Boost.ScopeGuard is unfair.
> 
> You are comparing C++03 Boost.ScopeGuard with C++11 Boost.Scope. Try
> comparing it with C++03 Boost.Scope.

Boost.Scope doesn't support C++03.

I'm comparing the libraries using the syntax they provide, regardless of
the C++ version, when they both provide equivalent functionality.

> Otherwise compare C++11 to C++11, and there Boost.ScopeGuard has a nicer
> syntax:
> 
>  int compute()
>     {
>         // Reset variables on return
>         BOOST_SCOPE_EXIT(this_)
>         {
>             this_->x = 0;
>             this_->y = 0;
>         };
> 
>         return x + y;
>     }

I don't see where the documentation makes BOOST_SCOPE_EXIT_END optional.
In every example I can see, BOOST_SCOPE_EXIT is accompanied by
BOOST_SCOPE_EXIT_END, C++11 or not. Am I missing something?

Perhaps, you are defining BOOST_SCOPE_EXIT_CONFIG_USE_LAMBDAS and that
allows to omit BOOST_SCOPE_EXIT_END? In that case, there is a discussion
at the end of this section:

https://lastique.github.io/scope/libs/scope/doc/html/scope/scope_guards.html#scope.scope_guards.comparison_with_boost_scope_exit

that pertains to your use case, and that makes Boost.ScopeExit very much
not equivalent to Boost.Scope.

> Regarding the example with exceptions, nobody would use
> Boost.ScopeGuard like this. A robust usage is in Boost.ScopeGuard's
> docs:
> 
> 
> void world::add_person(person const& a_person) {
>     bool commit = false;
> 
>     persons_.push_back(a_person);           // (1) direct action
>     // Following block is executed when the enclosing scope exits.
>     BOOST_SCOPE_EXIT(&commit, &persons_) {
>         if(!commit) persons_.pop_back();    // (2) rollback action
>     };
> 
>     // ...                                  // (3) other operations
> 
>     commit = true;                          // (4) disable rollback actions
> 
> }
> 
> 
> Which is longer, but usable in coroutines. Whereas Boost.Scope's
> solution is not usable in coroutines.

First, what example are you referring to?

Second, if you mean incompatibility of exception_checker with coroutines
then that is already addressed in a note in this section:

https://lastique.github.io/scope/libs/scope/doc/html/scope/scope_guards.html#scope.scope_guards.condition_functions

If you're using coroutines then just don't use it. Where it is used in
library examples, it is implied that coroutines are not used.