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Re: C++ question on wrapper API for setting Guile fluids

From: Luca Fascione
Subject: Re: C++ question on wrapper API for setting Guile fluids
Date: Thu, 21 Apr 2022 08:55:30 +0200

On Thu, Apr 21, 2022 at 8:12 AM Jean Abou Samra <> wrote:

> Le 21/04/2022 à 04:57, Dan Eble a écrit :
> >      {
> >        // dwc constructor calls scm_dynwind_begin ()
> >        Dynwind_context dwc;
> >        scm_dynwind_fluid (fluid1, value1);
> >        scm_dynwind_fluid (fluid2, value2);
> >        . . .
> >        // dwc destructor calls scm_dynwind_end ()
> >      }
> Why not. There is likely just one caller that will need to introduce
> several
> fluids at once, but it is probably clearer this way.

I'd think you can up this by one, and get a cleaner looking piece of code
if you implement scm_dynwind_fluid() as a forwarded method on your context:


  Dynwind_context dwc; // overloaded so you can call
  dwc.fluid (fluid1, value1);
  dwc.fluid (fluid2, value2);

  dwc.unwind_handler(handler, data, flags); // overloaded for the SCM vs
void* cases,
  dwc.rewind_handler(handler, data, flags); // overloaded for the SCM vs
void* cases,
                                            // maybe it ought to check if
the constructor was DYNWIND_REWINDABLE and complain
                                            // (only in debug builds?) if
things are not set up right;


However in all this, I must say I don't understand this passage in manual:

The context is ended either implicitly when a non-local exit happens, or
explicitly with scm_dynwind_end. You must make sure that a dynwind context
is indeed ended properly. If you fail to call scm_dynwind_end for each
scm_dynwind_begin, the behavior is undefined.

It seems to me the first phrase and the rest slightly contradict each
other: as I hear it, the first phrase says "either the C side calls
scm_dynwind_end(), OR a non-local exit happens", whereas the rest seems to
be saying "the C side _shall_ call scm_dynwind_end". This bothers me,
because in the second case our C++ is nice and lovely, but in the first
meaning the destructor of dwc has to somehow figure out whether a non-local
exit has happened, and avoid calling scm_dynwind_end(). And I don't think
scm library can cope on its own, because these things look to me like they
would nest, so making scm_dynwind_end() idempotent without some sort of
explicit marker on the scope seems... hard.

So yes, I'd think RAII is the idiomatic way to go, I would add the wrappers
because they make the pattern cleaner, but do figure out what's up with
this last question first, because it could bring it all crumbling down.


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