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// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
// This file contains utility functions and classes that help the
// implementation, and management of the Callback objects.
#include "base/atomic_ref_count.h"
#include "base/base_export.h"
#include "base/callback_forward.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"
namespace base {
namespace internal {
template <CopyMode copy_mode>
class CallbackBase;
// BindStateBase is used to provide an opaque handle that the Callback
// class can use to represent a function object with bound arguments. It
// behaves as an existential type that is used by a corresponding
// DoInvoke function to perform the function execution. This allows
// us to shield the Callback class from the types of the bound argument via
// "type erasure."
// At the base level, the only task is to add reference counting data. Don't use
// RefCountedThreadSafe since it requires the destructor to be a virtual method.
// Creating a vtable for every BindState template instantiation results in a lot
// of bloat. Its only task is to call the destructor which can be done with a
// function pointer.
class BASE_EXPORT BindStateBase {
using InvokeFuncStorage = void(*)();
BindStateBase(InvokeFuncStorage polymorphic_invoke,
void (*destructor)(BindStateBase*),
bool (*is_cancelled)(const BindStateBase*));
~BindStateBase() = default;
friend class scoped_refptr<BindStateBase>;
template <CopyMode copy_mode>
friend class CallbackBase;
bool IsCancelled() const {
return is_cancelled_(this);
void AddRef();
void Release();
// In C++, it is safe to cast function pointers to function pointers of
// another type. It is not okay to use void*. We create a InvokeFuncStorage
// that that can store our function pointer, and then cast it back to
// the original type on usage.
InvokeFuncStorage polymorphic_invoke_;
AtomicRefCount ref_count_;
// Pointer to a function that will properly destroy |this|.
void (*destructor_)(BindStateBase*);
bool (*is_cancelled_)(const BindStateBase*);
// Holds the Callback methods that don't require specialization to reduce
// template bloat.
// CallbackBase<MoveOnly> is a direct base class of MoveOnly callbacks, and
// CallbackBase<Copyable> uses CallbackBase<MoveOnly> for its implementation.
template <>
class BASE_EXPORT CallbackBase<CopyMode::MoveOnly> {
CallbackBase(CallbackBase&& c);
CallbackBase& operator=(CallbackBase&& c);
// Returns true if Callback is null (doesn't refer to anything).
bool is_null() const { return bind_state_.get() == NULL; }
explicit operator bool() const { return !is_null(); }
// Returns true if the callback invocation will be nop due to an cancellation.
// It's invalid to call this on uninitialized callback.
bool IsCancelled() const;
// Returns the Callback into an uninitialized state.
void Reset();
using InvokeFuncStorage = BindStateBase::InvokeFuncStorage;
// Returns true if this callback equals |other|. |other| may be null.
bool EqualsInternal(const CallbackBase& other) const;
// Allow initializing of |bind_state_| via the constructor to avoid default
// initialization of the scoped_refptr.
explicit CallbackBase(BindStateBase* bind_state);
InvokeFuncStorage polymorphic_invoke() const {
return bind_state_->polymorphic_invoke_;
// Force the destructor to be instantiated inside this translation unit so
// that our subclasses will not get inlined versions. Avoids more template
// bloat.
scoped_refptr<BindStateBase> bind_state_;
// CallbackBase<Copyable> is a direct base class of Copyable Callbacks.
template <>
class BASE_EXPORT CallbackBase<CopyMode::Copyable>
: public CallbackBase<CopyMode::MoveOnly> {
CallbackBase(const CallbackBase& c);
CallbackBase(CallbackBase&& c);
CallbackBase& operator=(const CallbackBase& c);
CallbackBase& operator=(CallbackBase&& c);
explicit CallbackBase(BindStateBase* bind_state)
: CallbackBase<CopyMode::MoveOnly>(bind_state) {}
~CallbackBase() {}
extern template class CallbackBase<CopyMode::MoveOnly>;
extern template class CallbackBase<CopyMode::Copyable>;
} // namespace internal
} // namespace base