Re: Accessing Grob properties

[Urs Liska]

Am 22.01.2016 um 21:54 schrieb David Kastrup:
>> However, from there I don't get any further, and it seems I totally
>> > don't understand how the Scheme types are matched in C++.
>> > Any attempt to compare the content of beam_count_prop with a set of
>> > predefined values seems to fail, and even
>> >
>> >     if (scm_equal_p (ly_symbol2scm ("one"), ly_symbol2scm ("two")))
>> >
>> > returns true (using scm_equal_p equally as for the eq and eqv
>> > versions.
> It does not return true but SCM_BOOL_F.  Which you then convert to a
> true C++ boolean since it is non-zero.
>
> You want   if (scm_is_eq (... ))
> instead: predicates with name xxx_is_yyy return a C (or C++) boolean.
> Predicates with name xxx_p return an SCM boolean which you need to
> convert to a C++ boolean using, for example, scm_is_true.
>
> ...
>
> SCM_BOOL_F as a C condition is _true_ rather than false. 

Thank you for this clear and general explanation that will serve me much
further than simply solving the problem at hand. Of course this totally
makes sense - and works perfectly.

Best
Urs