Release: bump Welcome versions.

[lilypond.git] / lily / include / smobs.hh

diff --git a/lily/include/smobs.hh b/lily/include/smobs.hh
index 889d86a8cab3a342b374593a7a6afa1879d98f9f..00217866dc07dc92eff400e23050d50b46f67a3b 100644 (file)
--- 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:
@@ -120,7 +144,7 @@
     debugging purposes.  If the class does not define this function,
     the output will be #<Classname> when printing.
 
-  - a static const type_p_name_[] string set to something like
+  - a static const * const type_p_name_ string set to something like
     "ly:grob?".  When provided, an accordingly named function for
     checking for the given smob type will be available in Scheme.
 
@@ -177,6 +201,7 @@ private:
   // Most default functions are do-nothings.  void init() will
   // recognize their address when not overriden and will then refrain
   // altogether from passing the the respective callbacks to GUILE.
+
   SCM mark_smob (void) const;
   static SCM mark_trampoline (SCM); // Used for calling mark_smob
   static size_t free_smob (SCM obj);
@@ -185,11 +210,10 @@ private:
   static int print_trampoline (SCM, SCM, scm_print_state *);
   static void smob_proc_init (scm_t_bits) { };
 
-  // type_p_name_ has to be defined in the Super class, either with a
-  // static const char [] string or as a null pointer of type const
-  // char *.  We used to provide a default here for convenience, but
-  // battling the various conflicting C++ standards was too much of a
-  // hassle.
+  // Define type_p_name_ in the Super class as a const char * const.
+  // Without such definition it defaults to 0, producing no predicate.
+
+  static const char * const type_p_name_; // = 0
 
   // LY_DECLARE_SMOB_PROC is used in the Super class definition for
   // making a smob callable like a function.  Its first argument is a