-. * Spacing_request for manually adjusting spacing
-. * caching breakpoints
-. * used fixedpoint arithmetic for min. energy.
-. * move towards incremental algorithms.
-. * versioning stuff (cvt mudela, mudela, etc.)
-. * dynamic loading of engravers?
-.* SMOBS
-Han-Wen Nienhuys <hanwen@cs.uu.nl> writes:
-
-> mcmanus@IDT.NET writes:
-> > > I want different C++ objects (with scheme embedded) to be linked by
-> > > C++ pointers. I want to access these other objects from the Scheme
-> > > code, and I want access to these pointers from C++.
-> >
-> > You can do this with a combination of smob's and primitive functions,
-> > though it may not be be exactly what you wanted. You would have one
-> > smob for each class in your application, and then you would write a
-> > primitive function that would return the objects that are linked to a
-> > base object.
->
-> And the smob and the C++ class are interlinked? Like
->
-> class Foo_class {
-> Foo_smob *smob_ptr_;
-> }
->
-> struct Foo_smob {
-> Foo_class *class_ptr_
-> }
-
-Usually you can get away without having this interlinked structure.
-One piece of information you need when exporting objects to scheme is
-the smob tag for your class. You might store this value (a long) into
-a class static variable, or a file static variable. I'll use a class
-static variable in this example.
-
-I typically use code that works like this (untested code ahead):
-
-class Foo_class {
- static long *Foo_smob_tag;
- SCM obj; // representation as a scheme object, description comes later
-}
-
-// call this once on startup
-void init_Foo_class {
- static scm_smobfuns type_rec;
-
- type_rec.mark = foo_mark;
- type_rec.free = foo_free;
- type_rec.print = foo_display;
- type_rec.equalp = 0;
- Foo_class::Foo_smob_tag = new int(scm_new_smob(type_rec));
-}
-
-When you want to export an object to scheme, you put a pointer to the
-object itself in the cdr of the cell, and the tag in the car of the
-cell. The code looks like this:
-
-// Is this a Foo?
-static int
-foo_p(SCM obj)
-{
- return(SCM_NIMP(obj) && SCM_CAR(obj) == Foo_class::Foo_smob_tag);
-}
-
-// given a Scheme representation, return, a C++ representation
-static Foo_class *
-foo_unbox(SCM obj)
-{
- return((Foo_class*)SCM_CDR(obj));
-}
-
-// the SCM representation of the object is stored inside itself
-// this will become useful when destructor gets called
-static SCM
-foo_box(Foo_class *foo)
-{
- SCM_DEFER_INTS;
- SCM_NEWCELL(foo->obj);
- SCM_SETCAR(foo->obj, Foo_class::Foo_smob_tag);
- SCM_SETCDR(foo->obj, (SCM)foo);
- SCM_ALLOW_INTS;
- return foo->obj;
-}
-
-> C++ determines life time, so what is the strategy?
-
-What happens now when the destructor gets called? Lets set the cdr to
-NULL, and then check for that in all of the primitive functions that
-use your smob. We'll call this notion 'live'; a scheme object is
-'live' if the C++ representation still exists, it's dead if the C++
-object is gone. You can still have references to dead objects in
-scheme code; it's just an error to use them. This idea is stolen from
-Emacs's handling of buffers ('buffer-live-p' and friends).
-
-Add another function, foo_live_p:
-
-static int
-foo_live_p(SCM obj)
-{
- return(foo_p(obj) && SCM_CDR(obj) != NULL);
-}
-
-In you destructor, you need to do:
-
-~Foo_class()
-{
- SCM_CDR(this->obj, NULL);
-}
-
-When writing new primitives, now just check foo_live_p().
-
-I hope the above helps,
-
--russ
-
-
---
-"Even if you are on the right track, you'll get run over
-if you just sit there."
- --Will Rogers (1879-1935)