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// Copyright 2020 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef INCLUDE_CPPGC_GARBAGE_COLLECTED_H_
#define INCLUDE_CPPGC_GARBAGE_COLLECTED_H_
#include <type_traits>
#include "cppgc/internal/api-constants.h"
#include "cppgc/macros.h"
#include "cppgc/platform.h"
#include "cppgc/trace-trait.h"
#include "cppgc/type-traits.h"
namespace cppgc {
class Visitor;
namespace internal {
class GarbageCollectedBase {
public:
// Must use MakeGarbageCollected.
void* operator new(size_t) = delete;
void* operator new[](size_t) = delete;
// The garbage collector is taking care of reclaiming the object. Also,
// virtual destructor requires an unambiguous, accessible 'operator delete'.
void operator delete(void*) {
#ifdef V8_ENABLE_CHECKS
internal::Abort();
#endif // V8_ENABLE_CHECKS
}
void operator delete[](void*) = delete;
protected:
GarbageCollectedBase() = default;
};
} // 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*) const` 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) const {
* // Dispatch using visitor->Trace(...);
* }
* };
*
* // Example using non-final base class.
* class NonFinalBase : public GarbageCollected<NonFinalBase> {
* public:
* virtual void Trace(cppgc::Visitor*) const {}
* };
*
* class FinalChild final : public NonFinalBase {
* public:
* void Trace(cppgc::Visitor* visitor) const final {
* // Dispatch using visitor->Trace(...);
* NonFinalBase::Trace(visitor);
* }
* };
* \endcode
*/
template <typename>
class GarbageCollected : public internal::GarbageCollectedBase {
public:
using IsGarbageCollectedTypeMarker = void;
protected:
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*) const` that dispatchs all managed
* pointers to the visitor and delegates to base classes.
*
* \code
* class Mixin : public GarbageCollectedMixin {
* public:
* void Trace(cppgc::Visitor* visitor) const override {
* // Dispatch using visitor->Trace(...);
* }
* };
* \endcode
*/
class GarbageCollectedMixin : public internal::GarbageCollectedBase {
public:
using IsGarbageCollectedMixinTypeMarker = void;
// Sentinel used to mark not-fully-constructed mixins.
static constexpr void* kNotFullyConstructedObject = nullptr;
// 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*) const {}
};
/**
* 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) const override {
* // Dispatch using visitor->Trace(...);
* }
* };
*
* class Foo : public GarbageCollected<Foo>, public Mixin {
* USING_GARBAGE_COLLECTED_MIXIN();
* public:
* void Trace(cppgc::Visitor* visitor) const 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 { \
static_assert( \
internal::IsSubclassOfTemplate< \
std::remove_const_t<std::remove_pointer_t<decltype(this)>>, \
cppgc::GarbageCollected>::value, \
"Only garbage collected objects can have garbage collected mixins"); \
return {this, TraceTrait<std::remove_const_t< \
std::remove_pointer_t<decltype(this)>>>::Trace}; \
} \
\
private: \
friend class internal::__thisIsHereToForceASemicolonAfterThisMacro
/**
* 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}; \
} \
\
private: \
friend class internal::__thisIsHereToForceASemicolonAfterThisMacro
} // namespace cppgc
#endif // INCLUDE_CPPGC_GARBAGE_COLLECTED_H_
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