Re: Almost, but not quite: C++ STL in LilyPond

[Hans Åberg]

> On 5 May 2020, at 17:09, David Kastrup <address@hidden> wrote:
> 
> One reason is that we want to get rid of finalisers particularly in
> connection with the Boehm GC.  Finalisers are called when garbage is
> collected, the collection of garbage is typically indicative of the
> expected lifetime of our objects (there are a few that might be
> unambiguously dead before), and thus it will cause C++ destructors to be
> called.  And those trigger, among other things, the deallocation of
> memory resources, but we'd rather want the deallocation to be determined
> by Scheme mechanisms and not have any resources in need of destruction
> other than falling into oblivion.

For the GC objects, I use operator new/delete with an extra argument, the 
latter with empty body, wrapped in structure similar to std:shared_ptr. Then 
the memory resources of those objects are only handled by the GC, regardless of 
finalizers.

> I've tried to come up with a clever callback scheme where the first use
> of a structure causes fake nodes to be created with a "Pointer" type
> that actually calls back with its address to its caller that then
> constructs a layout of SCM elements within its governed class.  However,
> I have found no way to make this "parallel construction" where I could
> then swap out this specific "Pointer" behavior for regular SCM.

Similarly, it is necessary to call GC_INIT() on the first use on MacOS, which 
can happen before ‘main’ is called. So for that, I let operator new/delete call 
a function pointer which on the first call initializes, and also changes the 
pointer to the ordinary function pointer on future calls.