doc: fix the example implementation of MemoryRetainer

We need to be careful not to include the size of non-pointer fields in the parent's self size if we want to track them separately as a different node. Refs: https://github.com/nodejs/node/pull/26161/files#r259170771 PR-URL: #26262 Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com> Reviewed-By: Anna Henningsen <anna@addaleax.net> Reviewed-By: James M Snell <jasnell@gmail.com>

Author: joyeecheung

src/memory_tracker.h

@@ -44,6 +44,13 @@ class NodeBIO;
  * // Node name and size comes from the MemoryInfoName and SelfSize of
  * // AnotherRetainerClass
  * tracker->TrackField("another_retainer", another_retainer_);
+ *
+ * // Add non_pointer_retainer as a separate node into the graph
+ * // and track its memory information recursively.
+ * // Note that we need to make sure its size is not accounted in
+ * // ExampleRetainer::SelfSize().
+ * tracker->TrackField("non_pointer_retainer", &non_pointer_retainer_);
+ *
  * // Specify node name and size explicitly
  * tracker->TrackFieldWithSize("internal_member",
  * internal_member_.size(),
@@ -60,9 +67,12 @@ class NodeBIO;
  * return "ExampleRetainer";
  * }
  *
- * // Or use SET_SELF_SIZE(ExampleRetainer)
+ * // Classes that only want to return its sizeof() value can use the
+ * // SET_SELF_SIZE(Class) macro instead.
  * size_t SelfSize() const override {
- * return sizeof(ExampleRetainer);
+ * // We need to exclude the size of non_pointer_retainer so that
+ * // we can track it separately in ExampleRetainer::MemoryInfo().
+ * return sizeof(ExampleRetainer) - sizeof(NonPointerRetainerClass);
  * }
  *
  * // Note: no need to implement these two methods when implementing
@@ -71,8 +81,10 @@ class NodeBIO;
  * v8::Local<v8::Object> WrappedObject() const override {
  * return node::PersistentToLocal(wrapped_);
  * }
+ *
  * private:
- * AnotherRetainerClass another_retainer_;
+ * AnotherRetainerClass* another_retainer_;
+ * NonPointerRetainerClass non_pointer_retainer;
  * InternalClass internal_member_;
  * std::vector<uv_async_t> vector_;
  * node::Persistent<Object> target_;