X-Git-Url: https://git.donarmstrong.com/?a=blobdiff_plain;f=lily%2Finclude%2Fsmobs.hh;h=00217866dc07dc92eff400e23050d50b46f67a3b;hb=cfc2fa64c372a7e2b2dbe0975a3f62274428b69c;hp=7ef46b23e41106eb444907d665cd1a6aecac80c8;hpb=ef9512388b15e1e5e2b3592299ffe489ac43abb4;p=lilypond.git

diff --git a/lily/include/smobs.hh b/lily/include/smobs.hh
index 7ef46b23e4..00217866dc 100644
--- a/lily/include/smobs.hh
+++ b/lily/include/smobs.hh
@@ -95,6 +95,30 @@
 
   Complex smobs are created by deriving from Smob<Classname>.
 
+  However, this is not sufficient when classes with their own
+  protectable elements are derived from the Complex base class.  This
+  is because initialization order is a tricky thing: once a base class
+  calls smobify_self () in its constructor, further allocations during
+  construction of base class and derived classes might lead to
+  mark_smob calls on the object under construction.  When those call a
+  virtual function like derived_mark, the virtual function
+  corresponding to the incompletely initialized object of derived
+  class type is likely to be called.
+
+  The order of initialization of an object consists in calling the
+  constructors of virtual base classes, then of non-virtual base
+  classes, then initializing all data members.
+
+  As a result, the constructor of a derived class comes too late for
+  initialization of data members that may be accessed in the
+  derived_mark kind of functions.
+
+  Such data members are consequently moved into Preinit_* classes
+  which come before the smobifying base class in derivation order and
+  construct the contained data members in a state suitable for
+  derived_mark calls.
+
+
   CALLING INTERFACE
 
   Common global functions for accessing C++ smob objects: