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

[Dan Eble]

On Apr 20, 2022, at 18:04, Jean Abou Samra <jean@abou-samra.fr> wrote:
> 
> Calls to scm_dynwind_begin () and scm_dynwind_end () must be
> paired correctly. Obviously, if '...' contains statements
> causing non-local control flow like return, break, continue,
> and such, it is easy to miss a scm_dynwind_end (). I read
> that you can avoid that using RAII:

Yes.

>   {
>     // Constructor does scm_dynwind_begin () and scm_dynwind_fluid ()
>     Fluid_setter setter (fluid, value);

Might you want to call scm_dynwind_fluid () more than once?  How about this?

    {
      // 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 ()
    }

> What disturbs me slightly with this approach is the variable 'setter',
> which will never be used, but must nevertheless be defined in order
> for the object not to be thrown away as temporary.

If I understand you, turning the main effect (scm_dynwind_begin) into a side 
effect of the constructor bothers you.  It wouldn't bother me as long as the 
class had a good name.  I can think of two alternatives, but they have their 
own problems.

> Does that sound about right? Would this be considered an idiomatic
> approach?

Yes.
— 
Dan