Prevent default-constructed variants from holding a type

Migration.md

@@ -32,6 +32,11 @@ release will remove the deprecated code.
     + Added `Scene::SetCameraPassCountPerGpuFlush`. Setting this value to 0 forces legacy behavior which eases porting.
     + Systems that rely on Graphics components like particle FXs and postprocessing are explicitly affected by Scene's Pre/PostRender. Once `Scene::PostRender` is called, the particle FXs' simulation is moved forward, as well as time values sent to postprocessing shaders. In previous ign-rendering versions each `Camera::Render` call would move the particle simulation forward, which could cause subtle bugs or inconsistencies when Cameras were rendering the same frame from different angles. Setting SetCameraPassCountPerGpuFlush to 0 will also cause these subtle bugs to reappear.
 
+1. **Visual.hh** and **Node.hh**
+    + `*UserData` methods and the `Variant` type alias have been moved from the `Visual` class to the `Node` class.
+    `Node::UserData` now returns no data for keys that don't exist (prior to Rendering 6.x, if
+    `Visual::UserData` was called with a key that doesn't exist, an `int` was returned by default).
+
 ## Ignition Rendering 4.0 to 4.1
 
 ## ABI break

include/ignition/rendering/Node.hh

@@ -35,8 +35,16 @@ namespace ignition
   {
     inline namespace IGNITION_RENDERING_VERSION_NAMESPACE {
     //
+    /// \brief Alias for a variant that can hold various types of data.
+    /// The first type of the variant is std::monostate in order to prevent
+    /// default-constructed variants from holding a type (a default-constructed
+    /// variant is returned when a user calls Node::UserData with a key that
+    /// doesn't exist for the node. In this case, since the key doesn't
+    /// exist, the variant that is returned shouldn't hold any types - an
+    /// "empty variant" should be returned for keys that don't exist)
     using Variant =
-      std::variant<int, float, double, std::string, bool, unsigned int>;
+      std::variant<std::monostate, int, float, double, std::string, bool,
+        unsigned int>;
 
     /// \class Node Node.hh ignition/rendering/Node.hh
     /// \brief Represents a single posable node in the scene graph
@@ -228,12 +236,12 @@ namespace ignition
       /// \param[in] _scale Scalars to alter the current scale
       public: virtual void Scale(const math::Vector3d &_scale) = 0;
 
-      /// \brief Determine if this visual inherits scale from this parent
-      /// \return True if this visual inherits scale from this parent
+      /// \brief Determine if this node inherits scale from this parent
+      /// \return True if this node inherits scale from this parent
       public: virtual bool InheritScale() const = 0;
 
-      /// \brief Specify if this visual inherits scale from its parent
-      /// \param[in] _inherit True if this visual inherits scale from its parent
+      /// \brief Specify if this node inherits scale from its parent
+      /// \param[in] _inherit True if this node inherits scale from its parent
       public: virtual void SetInheritScale(bool _inherit) = 0;
 
       /// \brief Get number of child nodes
@@ -309,15 +317,16 @@ namespace ignition
       /// This detaches all the child nodes but does not destroy them
       public: virtual void RemoveChildren() = 0;
 
-      /// \brief Store any custom data associated with this visual
+      /// \brief Store any custom data associated with this node
       /// \param[in] _key Unique key
       /// \param[in] _value Value in any type
       public: virtual void SetUserData(
         const std::string &_key, Variant _value) = 0;
 
-      /// \brief Get custom data stored in this visual
+      /// \brief Get custom data stored in this node
       /// \param[in] _key Unique key
-      /// \return Value in any type
+      /// \return Value in any type. If _key does not exist for the node, an
+      /// empty variant is returned (i.e., no data).
       public: virtual Variant UserData(const std::string &_key) const = 0;
     };
     }

src/Visual_TEST.cc

@@ -350,6 +350,29 @@ void VisualTest::UserData(const std::string &_renderEngine)
   value = visual->UserData(stringKey);
   EXPECT_EQ(stringValue, std::get<std::string>(value));
 
+  // bool
+  std::string boolKey = "bool";
+  bool boolValue = true;
+  visual->SetUserData(boolKey, boolValue);
+  value = visual->UserData(boolKey);
+  EXPECT_EQ(boolValue, std::get<bool>(value));
+
+  // unsigned int
+  std::string unsignedIntKey = "unsignedInt";
+  unsigned int unsignedIntValue = 5u;
+  visual->SetUserData(unsignedIntKey, unsignedIntValue);
+  value = visual->UserData(unsignedIntKey);
+  EXPECT_EQ(unsignedIntValue, std::get<unsigned int>(value));
+
+  // test a key that does not exist (should return no data)
+  value = visual->UserData("invalidKey");
+  EXPECT_FALSE(std::holds_alternative<int>(value));
+  EXPECT_FALSE(std::holds_alternative<float>(value));
+  EXPECT_FALSE(std::holds_alternative<double>(value));
+  EXPECT_FALSE(std::holds_alternative<std::string>(value));
+  EXPECT_FALSE(std::holds_alternative<bool>(value));
+  EXPECT_FALSE(std::holds_alternative<unsigned int>(value));
+
   // test invalid access
   EXPECT_THROW(
   {