Blink GC API reference #

This document is work in progress.

[TOC]

Header file #

Unless otherwise noted, any of the primitives explained in this page requires the following #include statement:

#include "platform/heap/Handle.h"

Base class templates #

GarbageCollected #

A class that wants the lifetime management of its instances to be managed by Blink GC (Oilpan), it must inherit from GarbageCollected<YourClass>.

class YourClass : public GarbageCollected<YourClass> {
    // ...
};

Instances of such classes are said to be on Oilpan heap, or on heap for short, while instances of other classes are called off heap. In the rest of this document, the terms "on heap" or "on-heap objects" are used to mean the objects on Oilpan heap instead of on normal (default) dynamic allocator's heap space.

You can create an instance of your class normally through new, while you may not free the object with delete, as the Blink GC system is responsible for deallocating the object once it determines the object is unreachable.

You may not allocate an on-heap object on stack.

Your class may need to have a tracing method. See Tracing for details.

If your class needs finalization (i.e. some work needs to be done on destruction), use GarbageCollectedFinalized instead.

GarbageCollected<T> or any class deriving from GarbageCollected<T>, directly or indirectly, must be the first element in its base class list (called "leftmost derivation rule"). This rule is needed to assure each on-heap object has its own canonical address.

class A : public GarbageCollected<A>, public P { // OK, GarbageCollected<A> is leftmost.
};

class B : public A, public Q { // OK, A is leftmost.
};

// class C : public R, public A { // BAD, A must be the first base class.
// };

If a non-leftmost base class needs to retain an on-heap object, that base class needs to inherit from GarbageCollectedMixin. It's generally recommended to make any non-leftmost base class inherit from GarbageCollectedMixin, because it's dangerous to save a pointer to a non-leftmost non-GarbageCollectedMixin subclass of an on-heap object.

void someFunction(P*);

class A : public GarbageCollected<A>, public P {
public:
    void someMemberFunction()
    {
        someFunction(this); // DANGEROUS, a raw pointer to an on-heap object.
    }
};

GarbageCollectedFinalized #

If you want to make your class garbage-collected and the class needs finalization, your class needs to inherit from GarbageCollectedFinalized<YourClass> instead of GarbageCollected<YourClass>.

A class is said to need finalization when it meets either of the following criteria:

• It has non-empty destructor; or
• It has a member that needs finalization.

class YourClass : public GarbageCollectedFinalized<YourClass> {
public:
    ~YourClass() { ... } // Non-empty destructor means finalization is needed.

private:
    RefPtr<Something> m_something; // RefPtr<> has non-empty destructor, so finalization is needed.
};

Note that finalization is done at an arbitrary time after the object becomes unreachable.

Any destructor executed within the finalization period must not touch any other on-heap object, because destructors can be executed in any order. If there is a need of having such destructor, consider using EAGERLY_FINALIZE.

Because GarbageCollectedFinalized<T> is a special case of GarbageCollected<T>, all the restrictions that apply to GarbageCollected<T> classes also apply to GarbageCollectedFinalized<T>.

GarbageCollectedMixin #

A non-leftmost base class of a garbage-collected class may derive from GarbageCollectedMixin. If a direct child class of GarbageCollected<T> or GarbageCollectedFinalized<T> has a non-leftmost base class deriving from GarbageCollectedMixin, the garbage-collected class must declare the USING_GARBAGE_COLLECTED_MIXIN(ClassName) macro in its class declaration.

A class deriving from GarbageCollectedMixin can be treated similarly as garbage-collected classes. Specifically, it can have Member<T>s and WeakMember<T>s, and a tracing method. A pointer to such a class must be retained in the same smart pointer wrappers as a pointer to a garbage-collected class, such as Member<T> or Persistent<T>. The tracing method of a garbage-collected class, if any, must contain a delegating call for each mixin base class.

class P : public GarbageCollectedMixin {
public:
    DEFINE_INLINE_VIRTUAL_TRACE() { visitor->trace(m_q); } // OK, needs to trace m_q.
private:
    Member<Q> m_q; // OK, allowed to have Member<T>.
};

class A : public GarbageCollected<A>, public P {
    USING_GARBAGE_COLLECTED_MIXIN(A);
public:
    DEFINE_INLINE_VIRTUAL_TRACE() { ...; P::trace(visitor); } // Delegating call for P is needed.
    ...
};

Internally, GarbageCollectedMixin defines pure virtual functions, and USING_GARBAGE_COLLECTED_MIXIN(ClassName) implements these virtual functions. Therefore, you cannot instantiate a class that is a descendant of GarbageCollectedMixin but not a descendant of GarbageCollected<T>. Two or more base classes inheritng from GarbageCollectedMixin can be resolved with a single USING_GARBAGE_COLLECTED_MIXIN(ClassName) declaration.

class P : public GarbageCollectedMixin { };
class Q : public GarbageCollectedMixin { };
class R : public Q { };

class A : public GarbageCollected<A>, public P, public R {
    USING_GARBAGE_COLLECTED_MIXIN(A); // OK, resolving pure virtual functions of P and R.
};

class B : public GarbageCollected<B>, public P {
    USING_GARBAGE_COLLECTED_MIXIN(B); // OK, different garbage-collected classes may inherit from the same mixin (P).
};

void someFunction()
{
    new A; // OK, A can be instantiated.
    // new R; // BAD, R has pure virtual functions.
}

Class properties #

USING_GARBAGE_COLLECTED_MIXIN #

USING_GARBAGE_COLLECTED_MIXIN(ClassName) is a macro that must be declared in a garbage-collected class, if any of its base classes is a descendant of GarbageCollectedMixin.

See GarbageCollectedMixin for the use of GarbageCollectedMixin and this macro.

USING_PRE_FINALIZER #

USING_PRE_FINALIZER(ClassName, functionName) in a class declaration declares the class has a pre-finalizer of name functionName.

A pre-finalizer is a user-defined member function of a garbage-collected class that is called when the object is going to be swept but before the garbage collector actually sweeps any objects. Therefore, it is allowed for a pre-finalizer to touch any other on-heap objects, while a destructor is not. It is useful for doing some cleanups that cannot be done with a destructor.

A pre-finalizer must have the following function signature: void preFinalizer(). You can change the function name.

A pre-finalizer must be registered in the constructor by using the following statement: "ThreadState::current()->registerPreFinalizer(preFinalizerName);".

class YourClass : public GarbageCollectedFinalized<YourClass> {
    USING_PRE_FINALIZER(YourClass, dispose);
public:
    YourClass()
    {
        ThreadState::current()->registerPreFinalizer(dispose);
    }
    void dispose()
    {
        m_other->dispose(); // OK; you can touch other on-heap objects in a pre-finalizer.
    }
    ~YourClass()
    {
        // m_other->dispose(); // BAD.
    }

private:
    Member<OtherClass> m_other;
};

Pre-finalizers have some implications on the garbage collector's performance: the garbage-collector needs to iterate all registered pre-finalizers at every GC. Therefore, a pre-finalizer should be avoided unless it is really necessary. Especially, avoid defining a pre-finalizer in a class that can be allocated a lot.

EAGERLY_FINALIZE #

DISALLOW_NEW_EXCEPT_PLACEMENT_NEW #

STACK_ALLOCATED #

Handles #

Class templates in this section are smart pointers, each carrying a pointer to an on-heap object (think of RefPtr<T> for RefCounted<T>). Collectively, they are called handles.

On-heap objects must be retained by any of these, depending on the situation.

Raw pointers #

On-stack references to on-heap objects must be raw pointers.

void someFunction()
{
    SomeGarbageCollectedClass* object = new SomeGarbageCollectedClass; // OK, retained by a pointer.
    ...
}
// OK to leave the object behind. The Blink GC system will free it up when it becomes unused.

** aside Transitional only*

RawPtr<T> is a simple wrapper of a raw pointer T* equipped with common member functions defined in other smart pointer templates, such as get() or clear(). RawPtr<T> is only meant to be used during the transition period; it is only used in the forms like OwnPtrWillBeRawPtr<T> or RefPtrWillBeRawPtr<T> so you can share as much code as possible in both pre- and post-Oilpan worlds.

RawPtr<T> is declared and defined in wtf/RawPtr.h.

---

Member, WeakMember #

In a garbage-collected class, on-heap objects must be retained by Member<T> or WeakMember<T>, depending on the desired semantics.

Member<T> represents a strong reference to an object of type T, which means that the referred object is kept alive as long as the owner class instance is alive. Unlike RefPtr<T>, it is okay to form a reference cycle with members (in on-heap objects) and raw pointers (on stack).

WeakMember<T> is a weak reference to an object of type T. Unlike Member<T>, WeakMember<T> does not keep the pointed object alive. The pointer in a WeakMember<T> can become nullptr when the object gets garbage-collected. It may take some time for the pointer in a WeakMember<T> to become nullptr after the object actually goes unused, because this rewrite is only done within Blink GC's garbage collection period.

class SomeGarbageCollectedClass : public GarbageCollected<GarbageCollectedSomething> {
    ...
private:
    Member<AnotherGarbageCollectedClass> m_another; // OK, retained by Member<T>.
    WeakMember<AnotherGarbageCollectedClass> m_anotherWeak; // OK, weak reference.
};

The use of WeakMember<T> incurs some overhead in garbage collector's performance. Use it sparingly. Usually, weak members are not necessary at all, because reference cycles with members are allowed.

More specifically, WeakMember<T> should be used only if the owner of a weak member can outlive the pointed object. Otherwise, Member<T> should be used.

You need to trace every Member<T> and WeakMember<T> in your class. See Tracing.

Persistent, WeakPersistent, CrossThreadPersistent, CrossThreadWeakPersistent #

In a non-garbage-collected class, on-heap objects must be retained by Persistent<T>, WeakPersistent<T>, CrossThreadPersistent<T>, or CrossThreadWeakPersistent<T>, depending on the situations and the desired semantics.

Persistent<T> is the most basic handle in the persistent family, which makes the referred object alive unconditionally, as long as the persistent handle is alive.

WeakPersistent<T> does not make the referred object alive, and becomes nullptr when the object gets garbage-collected, just like WeakMember<T>.

CrossThreadPersistent<T> and CrossThreadWeakPersistent<T> are cross-thread variants of Persistent<T> and WeakPersistent<T>, respectively, which can point to an object in a different thread.

class NonGarbageCollectedClass {
    ...
private:
    Persistent<SomeGarbageCollectedClass> m_something; // OK, the object will be alive while this persistent is alive.
};

* note Warning: Persistent<T> and CrossThreadPersistent<T> are vulnerable to reference cycles. If a reference cycle is formed with Persistents, Members, RefPtrs and OwnPtrs, all the objects in the cycle will leak**, since nobody in the cycle can be aware of whether they are ever referred from anyone.

When you are about to add a new persistent, be careful not to create a reference cycle. If a cycle is inevitable, make sure the cycle is eventually cut by someone outside the cycle.

---

Persistents have small overhead in itself, because they need to maintain the list of all persistents. Therefore, it's not a good idea to create or keep a lot of persistents at once.

Weak variants have overhead just like WeakMember<T>. Use them sparingly.

The need of cross-thread persistents may indicate a poor design in multi-thread object ownership. Think twice if they are really necessary.

Tracing #

A garbage-collected class may need to have a tracing method, which lists up all the on-heap objects it has. The tracing method is called when the garbage collector needs to determine (1) all the on-heap objects referred from a live object, and (2) all the weak handles that may be filled with nullptr later. These are done in the "marking" phase of the mark-and-sweep GC.

The basic form of tracing is illustrated below:

// In a header file:
class SomeGarbageCollectedClass : public GarbageCollected<SomeGarbageCollectedClass> {
public:
    DECLARE_TRACE();

private:
    Member<AnotherGarbageCollectedClass> m_another;
};

// In an implementation file:
DEFINE_TRACE(SomeGarbageCollectedClass)
{
    visitor->trace(m_another);
}

Specifically, if your class needs a tracing method, you need to:

• Declare a tracing method in your class declaration, using the DECLARE_TRACE() macro; and
• Define a tracing method in your implementation file, using the DEFINE_TRACE(ClassName) macro.

The function implementation must contain:

• For each on-heap object m_object in your class, a tracing call: "visitor->trace(m_object);".
• For each base class of your class BaseClass that is a descendant of GarbageCollected<T> or GarbageCollectedMixin, delegation call to base class: "BaseClass::trace(visitor);"

It is recommended that the delegation call, if any, is put at the end of a tracing method.

If the class does not contain any on-heap object, the tracing method is not needed.

If you want to define your tracing method inline or need to have your tracing method polymorphic, you can use the following variants of the tracing macros:

• "DECLARE_VIRTUAL_TRACE();" in a class declaration makes the method virtual. Use "DEFINE_TRACE(ClassName) { ... }" in the implementation file to define.
• "DEFINE_INLINE_TRACE() { ... }" in a class declaration lets you define the method inline. If you use this, you may not write "DEFINE_TRACE(ClassName) { ... }" in your implementation file.
• "DEFINE_INLINE_VIRTUAL_TRACE() { ... }" in a class declaration does both of the above.

The following example shows more involved usage:

class A : public GarbageCollected<A> {
public:
    DEFINE_INLINE_VIRTUAL_TRACE() { } // Nothing to trace here. Just to declare a virtual method.
};

class B : public A {
    // Nothing to trace here; exempted from having a tracing method.
};

class C : public B {
public:
    DECLARE_VIRTUAL_TRACE();

private:
    Member<X> m_x;
    WeakMember<Y> m_y;
    HeapVector<Member<Z>> m_z;
};

DEFINE_TRACE(C)
{
    visitor->trace(m_x);
    visitor->trace(m_y); // Weak member needs to be traced.
    visitor->trace(m_z); // Heap collection does, too.
    B::trace(visitor); // Delegate to the parent. In this case it's empty, but this is required.
}

Heap collections #