Functions: provide dummy multi function

Sometimes it is convenient to be able to return a reference to some
dummy function.

source/blender/functions/CMakeLists.txt

@@ -29,6 +29,7 @@ set(INC_SYS
 set(SRC
   intern/attributes_ref.cc
   intern/cpp_types.cc
+  intern/multi_function.cc
   intern/multi_function_network.cc
   intern/multi_function_network_evaluation.cc
 

source/blender/functions/FN_multi_function.hh

@@ -88,9 +88,9 @@ class MultiFunction {
   }
 
  protected:
-  MFSignatureBuilder get_builder(StringRef function_name)
+  MFSignatureBuilder get_builder(std::string function_name)
   {
-    m_signature.function_name = function_name;
+    m_signature.function_name = std::move(function_name);
     return MFSignatureBuilder(m_signature);
   }
 };
@@ -100,6 +100,8 @@ inline MFParamsBuilder::MFParamsBuilder(const class MultiFunction &fn, uint min_
 {
 }
 
+extern const MultiFunction &dummy_multi_function;
+
 }  // namespace fn
 }  // namespace blender
 

source/blender/functions/intern/multi_function.cc

@@ -0,0 +1,40 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ */
+
+#include "FN_multi_function.hh"
+
+namespace blender {
+namespace fn {
+
+class DummyMultiFunction : public MultiFunction {
+ public:
+  DummyMultiFunction()
+  {
+    this->get_builder("Dummy");
+  }
+
+  void call(IndexMask UNUSED(mask),
+            MFParams UNUSED(params),
+            MFContext UNUSED(context)) const override
+  {
+  }
+};
+
+static DummyMultiFunction dummy_multi_function_;
+const MultiFunction &dummy_multi_function = dummy_multi_function_;
+
+}  // namespace fn
+}  // namespace blender