Octave - SWIG now marshalls a C/C++ NULL pointer into the null matrix, []

SWIG has always marshalled the null matrix into a NULL pointer; this remains
and now we have consistency in representing a NULL pointer.

This is a pre-requisite for a pending commit to fully support std::unique_ptr.

CHANGES.current

@@ -7,8 +7,13 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.1.0 (in progress)
 ===========================
 
+2022-08-28: wsfulton
+            [Octave] SWIG now marshalls a C/C++ NULL pointer into the null matrix, [].
+            SWIG has always marshalled the null matrix into a NULL pointer; this remains
+            and now we have consistency in representing a NULL pointer.
+
 2022-08-26: wsfulton
-            [Racket] SWIG now converts a C/C++ NULL pointer into a null value by calling
+            [Racket] SWIG now marshalls a C/C++ NULL pointer into a null value by calling
             scheme_make_null(), so that scheme's null? is true for a NULL C/C++ pointer value.
 
 2022-08-18: wsfulton

Doc/Manual/Octave.html

@@ -363,6 +363,10 @@ octave:2> f=swigexample.fopen("not there", "r");
 error: value on right hand side of assignment is undefined
 error: evaluating assignment expression near line 2, column 2 </pre></div>
 
+<p>
+NULL C/C++ pointers are represented by the Octave null matrix, <tt>[]</tt>.
+</p>
+
 <H3><a name="Octave_nn13">30.3.6 Structures and C++ classes</a></H3>
 
 

Examples/test-suite/octave/null_pointer_runme.m

@@ -6,3 +6,13 @@ endif
 null_pointer;
 
 assert(funk([]));
+null_ptr = getnull();
+assert(ismatrix(null_ptr))
+assert(size(null_ptr) == size([]))
+assert(isequal([], null_ptr))
+
+# Can't use isnull as a test due to null matrix not being copyable...
+#   n = []
+#   isnull(n) # ans = 0
+#   isnull([]) # ans = 1
+#   isnull(getnull()) # ans = 0

Lib/octave/octrun.swg

@@ -1547,12 +1547,15 @@ octave_value_typeinfo::register_binary_op(octave_value::op_##name,tid1,tid2,swig
 SWIGRUNTIME octave_value SWIG_Octave_NewPointerObj(void *ptr, swig_type_info *type, int flags) {
   int own = (flags &SWIG_POINTER_OWN) ? SWIG_POINTER_OWN : 0;
 
+  if (ptr) {
 #ifdef SWIG_DIRECTORS
-  Swig::Director *d = Swig::get_rtdir(ptr);
-  if (d && Swig::swig_director_get_self(d))
-    return Swig::swig_director_get_self(d)->as_value();
+    Swig::Director *d = Swig::get_rtdir(ptr);
+    if (d && Swig::swig_director_get_self(d))
+      return Swig::swig_director_get_self(d)->as_value();
 #endif
-  return Swig::swig_value_ref(new octave_swig_type(ptr, type, own));
+    return Swig::swig_value_ref(new octave_swig_type(ptr, type, own));
+  }
+  return octave_value(Matrix()); // null matrix
 }
 
 SWIGRUNTIME int SWIG_Octave_ConvertPtrAndOwn(octave_value ov, void **ptr, swig_type_info *type, int flags, int *own) {