cppgc: First round of API docs

Bug: chromium:1056170
Change-Id: Idbbab7af430e3bc2791eba630b331a75a67e35a9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2142262Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67112}

include/cppgc/allocation.h

@@ -41,30 +41,53 @@ class V8_EXPORT MakeGarbageCollectedTraitInternal {
 
 }  // namespace internal
 
-// Users with custom allocation needs (e.g. overriding size) should override
-// MakeGarbageCollectedTrait (see below) and inherit their trait from
-// MakeGarbageCollectedTraitBase to get access to low-level primitives.
+/**
+ * Base trait that provides utilities for advancers users that have custom
+ * allocation needs (e.g., overriding size). It's expected that users override
+ * MakeGarbageCollectedTrait (see below) and inherit from
+ * MakeGarbageCollectedTraitBase and make use of the low-level primitives
+ * offered to allocate and construct an object.
+ */
 template <typename T>
 class MakeGarbageCollectedTraitBase
     : private internal::MakeGarbageCollectedTraitInternal {
  protected:
-  // Allocates an object of |size| bytes on |heap|.
-  //
-  // TODO(mlippautz): Allow specifying arena for specific embedder uses.
+  /**
+   * Allocates memory for an object of type T.
+   *
+   * \param heap The heap to allocate this object on.
+   * \param size The size that should be reserved for the object.
+   * \returns the memory to construct an object of type T on.
+   */
   static void* Allocate(Heap* heap, size_t size) {
+    // TODO(chromium:1056170): Allow specifying arena for specific embedder
+    // uses.
     return internal::MakeGarbageCollectedTraitInternal::Allocate(
         heap, size, internal::GCInfoTrait<T>::Index());
   }
 
-  // Marks an object as being fully constructed, resulting in precise handling
-  // by the garbage collector.
+  /**
+   * Marks an object as fully constructed, resulting in precise handling by the
+   * garbage collector.
+   *
+   * \param payload The base pointer the object is allocated at.
+   */
   static void MarkObjectAsFullyConstructed(const void* payload) {
-    // internal::MarkObjectAsFullyConstructed(payload);
     internal::MakeGarbageCollectedTraitInternal::MarkObjectAsFullyConstructed(
         payload);
   }
 };
 
+/**
+ * Default trait class that specifies how to construct an object of type T.
+ * Advanced users may override how an object is constructed using the utilities
+ * that are provided through MakeGarbageCollectedTraitBase.
+ *
+ * Any trait overriding construction must
+ * - allocate through MakeGarbageCollectedTraitBase<T>::Allocate;
+ * - mark the object as fully constructed using
+ *   MakeGarbageCollectedTraitBase<T>::MarkObjectAsFullyConstructed;
+ */
 template <typename T>
 class MakeGarbageCollectedTrait : public MakeGarbageCollectedTraitBase<T> {
  public:
@@ -83,8 +106,14 @@ class MakeGarbageCollectedTrait : public MakeGarbageCollectedTraitBase<T> {
   }
 };
 
-// Default MakeGarbageCollected: Constructs an instance of T, which is a garbage
-// collected type.
+/**
+ * Constructs a managed object of type T where T transitively inherits from
+ * GarbageCollected.
+ *
+ * \param args List of arguments with which an instance of T will be
+ *   constructed.
+ * \returns an instance of type T.
+ */
 template <typename T, typename... Args>
 T* MakeGarbageCollected(Heap* heap, Args&&... args) {
   return MakeGarbageCollectedTrait<T>::Call(heap, std::forward<Args>(args)...);

include/cppgc/garbage-collected.h

@@ -38,6 +38,41 @@ class GarbageCollectedBase {
 
 }  // namespace internal
 
+/**
+ * Base class for managed objects. Only descendent types of GarbageCollected can
+ * be constructed using MakeGarbageCollected. Must be inherited from as
+ * left-most base class.
+ *
+ * Types inheriting from GarbageCollected must provide a method of
+ * signature `void Trace(cppgc::Visitor*)` that dispatchs all managed pointers
+ * to the visitor and delegates to garbage-collected base classes. The method
+ * must be virtual if the type is not directly a child of GarbageCollected and
+ * marked as final.
+ *
+ * \code
+ * // Example using final class.
+ * class FinalType final : public GarbageCollected<FinalType> {
+ *  public:
+ *   void Trace(cppgc::Visitor* visitor) {
+ *     // Dispatch using visitor->Trace(...);
+ *   }
+ * };
+ *
+ * // Example using non-final base class.
+ * class NonFinalBase : public GarbageCollected<NonFinalBase> {
+ *  public:
+ *   virtual void Trace(cppgc::Visitor*) {}
+ * };
+ *
+ * class FinalChild final : public NonFinalBase {
+ *  public:
+ *   void Trace(cppgc::Visitor* visitor) final {
+ *     // Dispatch using visitor->Trace(...);
+ *     NonFinalBase::Trace(visitor);
+ *   }
+ * };
+ * \endcode
+ */
 template <typename>
 class GarbageCollected : public internal::GarbageCollectedBase {
  public:
@@ -47,6 +82,24 @@ class GarbageCollected : public internal::GarbageCollectedBase {
   GarbageCollected() = default;
 };
 
+/**
+ * Base class for managed mixin objects. Such objects cannot be constructed
+ * directly but must be mixed into the inheritance hierarchy of a
+ * GarbageCollected object.
+ *
+ * Types inheriting from GarbageCollectedMixin must override a virtual method of
+ * signature `void Trace(cppgc::Visitor*)` that dispatchs all managed pointers
+ * to the visitor and delegates to base classes.
+ *
+ * \code
+ * class Mixin : public GarbageCollectedMixin {
+ *  public:
+ *   void Trace(cppgc::Visitor* visitor) override {
+ *     // Dispatch using visitor->Trace(...);
+ *   }
+ * };
+ * \endcode
+ */
 class GarbageCollectedMixin : public internal::GarbageCollectedBase {
  public:
   using IsGarbageCollectedMixinTypeMarker = void;
@@ -54,14 +107,18 @@ class GarbageCollectedMixin : public internal::GarbageCollectedBase {
   // Sentinel used to mark not-fully-constructed mixins.
   static constexpr void* kNotFullyConstructedObject = nullptr;
 
-  virtual void Trace(cppgc::Visitor*) {}
-
   // Provide default implementation that indicate that the vtable is not yet
   // set up properly. This is used to to get GCInfo objects for mixins so that
   // these objects can be processed later on.
   virtual TraceDescriptor GetTraceDescriptor() const {
     return {kNotFullyConstructedObject, nullptr};
   }
+
+  /**
+   * This Trace method must be overriden by objects inheriting from
+   * GarbageCollectedMixin.
+   */
+  virtual void Trace(cppgc::Visitor*) {}
 };
 
 namespace internal {
@@ -70,12 +127,32 @@ class __thisIsHereToForceASemicolonAfterThisMacro {};
 
 }  // namespace internal
 
-// The USING_GARBAGE_COLLECTED_MIXIN macro defines all methods and markers
-// needed for handling mixins. HasUsingGarbageCollectedMixinMacro is used
-// by the clang GC plugin to check for proper usages of the
-// USING_GARBAGE_COLLECTED_MIXIN macro.
+/**
+ * Macro defines all methods and markers needed for handling mixins. Must be
+ * used on the type that is inheriting from GarbageCollected *and*
+ * GarbageCollectedMixin.
+ *
+ * \code
+ * class Mixin : public GarbageCollectedMixin {
+ *  public:
+ *   void Trace(cppgc::Visitor* visitor) override {
+ *     // Dispatch using visitor->Trace(...);
+ *   }
+ * };
+ *
+ * class Foo : public GarbageCollected<Foo>, public Mixin {
+ *   USING_GARBAGE_COLLECTED_MIXIN();
+ *  public:
+ *   void Trace(cppgc::Visitor* visitor) override {
+ *     // Dispatch using visitor->Trace(...);
+ *     Mixin::Trace(visitor);
+ *   }
+ * };
+ * \endcode
+ */
 #define USING_GARBAGE_COLLECTED_MIXIN()                                      \
  public:                                                                     \
+  /* Marker is used by clang to check for proper usages of the macro. */     \
   typedef int HasUsingGarbageCollectedMixinMacro;                            \
                                                                              \
   TraceDescriptor GetTraceDescriptor() const override {                      \
@@ -91,23 +168,23 @@ class __thisIsHereToForceASemicolonAfterThisMacro {};
  private:                                                                    \
   friend class internal::__thisIsHereToForceASemicolonAfterThisMacro
 
-// Merge two or more Mixins into one:
-//
-//  class A : public GarbageCollectedMixin {};
-//  class B : public GarbageCollectedMixin {};
-//  class C : public A, public B {
-//    // C::GetTraceDescriptor is now ambiguous because there are two
-//    // candidates: A::GetTraceDescriptor and B::GetTraceDescriptor.  Ditto for
-//    // other functions.
-//
-//    MERGE_GARBAGE_COLLECTED_MIXINS();
-//    // The macro defines C::GetTraceDescriptor, similar to
-//    // GarbageCollectedMixin, so that they are no longer ambiguous.
-//    // USING_GARBAGE_COLLECTED_MIXIN() overrides them later and provides the
-//    // implementations.
-//  };
+/**
+ * Merge two or more Mixins into one.
+ *
+ * \code
+ * class A : public GarbageCollectedMixin {};
+ * class B : public GarbageCollectedMixin {};
+ * class C : public A, public B {
+ *   MERGE_GARBAGE_COLLECTED_MIXINS();
+ *  public:
+ * };
+ * \endcode
+ */
 #define MERGE_GARBAGE_COLLECTED_MIXINS()                \
  public:                                                \
+  /* When using multiple mixins the methods become  */  \
+  /* ambigous. Providing additional implementations */  \
+  /* disambiguate them again.                       */  \
   TraceDescriptor GetTraceDescriptor() const override { \
     return {kNotFullyConstructedObject, nullptr};       \
   }                                                     \

include/cppgc/member.h

@@ -37,8 +37,9 @@ class BasicMember : private CheckingPolicy {
   template <typename U, typename OtherBarrierPolicy, typename OtherWeaknessTag,
             typename OtherCheckingPolicy,
             typename = std::enable_if_t<std::is_base_of<T, U>::value>>
-  BasicMember(const BasicMember<U, OtherWeaknessTag, OtherBarrierPolicy,
-                                OtherCheckingPolicy>& other)
+  BasicMember(  // NOLINT
+      const BasicMember<U, OtherWeaknessTag, OtherBarrierPolicy,
+                        OtherCheckingPolicy>& other)
       : BasicMember(other.Get()) {}
 
   BasicMember& operator=(const BasicMember& other) {
@@ -78,7 +79,7 @@ class BasicMember : private CheckingPolicy {
   }
 
   explicit operator bool() const { return Get(); }
-  operator T*() const { return Get(); }
+  operator T*() const { return Get(); }  // NOLINT
   T* operator->() const { return Get(); }
   T& operator*() const { return *Get(); }
 
@@ -146,27 +147,33 @@ struct IsWeak<
 
 }  // namespace internal
 
-// Members are used in classes to contain strong pointers to other garbage
-// collected objects. All Member fields of a class must be traced in the class'
-// trace method.
+/**
+ * Members are used in classes to contain strong pointers to other garbage
+ * collected objects. All Member fields of a class must be traced in the class'
+ * trace method.
+ */
 template <typename T>
 using Member = internal::BasicMember<T, internal::StrongMemberTag,
                                      internal::DijkstraWriteBarrierPolicy>;
 
-// WeakMember is similar to Member in that it is used to point to other garbage
-// collected objects. However instead of creating a strong pointer to the
-// object, the WeakMember creates a weak pointer, which does not keep the
-// pointee alive. Hence if all pointers to to a heap allocated object are weak
-// the object will be garbage collected. At the time of GC the weak pointers
-// will automatically be set to null.
+/**
+ * WeakMember is similar to Member in that it is used to point to other garbage
+ * collected objects. However instead of creating a strong pointer to the
+ * object, the WeakMember creates a weak pointer, which does not keep the
+ * pointee alive. Hence if all pointers to to a heap allocated object are weak
+ * the object will be garbage collected. At the time of GC the weak pointers
+ * will automatically be set to null.
+ */
 template <typename T>
 using WeakMember = internal::BasicMember<T, internal::WeakMemberTag,
                                          internal::DijkstraWriteBarrierPolicy>;
 
-// UntracedMember is a pointer to an on-heap object that is not traced for some
-// reason. Do not use this unless you know what you are doing. Keeping raw
-// pointers to on-heap objects is prohibited unless used from stack. Pointee
-// must be kept alive through other means.
+/**
+ * UntracedMember is a pointer to an on-heap object that is not traced for some
+ * reason. Do not use this unless you know what you are doing. Keeping raw
+ * pointers to on-heap objects is prohibited unless used from stack. Pointee
+ * must be kept alive through other means.
+ */
 template <typename T>
 using UntracedMember = internal::BasicMember<T, internal::UntracedMemberTag,
                                              internal::NoWriteBarrierPolicy>;