Add blank/monostate support

Author: jorisv

include/eigenpy/variant.hpp

@@ -41,6 +41,10 @@ struct VariantVisitorType<ResultType, std::variant<Alternatives...> > {
     return std::visit(std::forward<Visitor>(visitor),
                       std::forward<Visitable>(v));
   }
+
+  result_type operator()(std::monostate) const {
+    return boost::python::incref(boost::python::object().ptr());  // None
+  }
 };
 
 template <typename... Alternatives>
@@ -61,6 +65,10 @@ struct VariantVisitorType<ResultType, boost::variant<Alternatives...> >
   static result_type visit(Visitor&& visitor, Visitable&& visitable) {
     return std::forward<Visitable>(visitable).apply_visitor(visitor);
   }
+
+  result_type operator()(boost::blank) const {
+    return boost::python::incref(boost::python::object().ptr());  // None
+  }
 };
 
 template <typename... Alternatives>
@@ -84,6 +92,8 @@ struct VariantValueToObject : VariantVisitorType<PyObject*, Variant> {
   result_type operator()(T& t) const {
     return boost::python::incref(boost::python::object(t).ptr());
   }
+
+  using Base::operator();
 };
 
 /// Convert {boost,std}::variant<class...> alternative reference to a Python
@@ -104,6 +114,9 @@ struct VariantRefToObject : VariantVisitorType<PyObject*, Variant> {
   result_type operator()(T& t) const {
     return boost::python::detail::make_reference_holder::execute(&t);
   }
+
+  /// Copy the object when it's None
+  using Base::operator();
 };
 
 /// Converter used in \see ReturnInternalVariant.

unittest/python/test_variant.py.in

@@ -4,7 +4,9 @@ variant_module = importlib.import_module("@MODNAME@")
 V1 = variant_module.V1
 V2 = variant_module.V2
 VariantHolder = variant_module.VariantHolder
+VariantNoneHolder = variant_module.VariantNoneHolder
 make_variant = variant_module.make_variant
+make_variant_none = variant_module.make_variant_none
 
 variant = make_variant()
 assert isinstance(variant, V1)
@@ -40,3 +42,23 @@ assert v1.v == 1000
 variant_holder.variant = v2
 assert isinstance(variant_holder.variant, V2)
 assert variant_holder.variant.v == v2.v
+
+# Test variant that hold a None value
+v_none = make_variant_none()
+assert v_none is None
+v_none = 1
+assert v_none == 1
+v_none = None
+assert v_none is None
+
+variant_none_holder = VariantNoneHolder()
+v_none = variant_none_holder.variant
+assert v_none is None
+
+v1 = V1()
+v1.v = 1
+variant_none_holder.variant = v1
+assert variant_none_holder.variant.v == 1
+v1 = variant_none_holder.variant
+v1.v = 10
+assert variant_none_holder.variant.v == 10

unittest/variant.cpp.in

@@ -17,12 +17,35 @@ struct V2 {
 };
 typedef VARIANT<V1, V2> MyVariant;
 
+template <typename Variant>
+struct MyVariantNoneHelper {};
+
+template <typename... Alternatives>
+struct MyVariantNoneHelper<boost::variant<Alternatives...> > {
+  typedef VARIANT<boost::blank, Alternatives...> type;
+};
+
+#ifdef EIGENPY_WITH_CXX17_SUPPORT
+template <typename... Alternatives>
+struct MyVariantNoneHelper<std::variant<Alternatives...> > {
+  typedef VARIANT<std::monostate, Alternatives...> type;
+};
+#endif
+
+typedef typename MyVariantNoneHelper<VARIANT<V1> >::type MyVariantNone;
+
 MyVariant make_variant() { return V1(); }
 
+MyVariantNone make_variant_none() { return MyVariantNone(); }
+
 struct VariantHolder {
   MyVariant variant;
 };
 
+struct VariantNoneHolder {
+  MyVariantNone variant;
+};
+
 BOOST_PYTHON_MODULE(@MODNAME@) {
   using namespace eigenpy;
 
@@ -41,4 +64,14 @@ BOOST_PYTHON_MODULE(@MODNAME@) {
                     bp::make_getter(&VariantHolder::variant,
                                     Converter::return_internal_reference()),
                     bp::make_setter(&VariantHolder::variant));
+
+  typedef eigenpy::VariantConverter<MyVariantNone> ConverterNone;
+  ConverterNone::registration();
+  bp::def("make_variant_none", make_variant_none);
+
+  boost::python::class_<VariantNoneHolder>("VariantNoneHolder", bp::init<>())
+      .add_property("variant",
+                    bp::make_getter(&VariantNoneHolder::variant,
+                                    ConverterNone::return_internal_reference()),
+                    bp::make_setter(&VariantNoneHolder::variant));
 }