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1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
4 
5 // This file contains utility functions and classes that help the
6 // implementation, and management of the Callback objects.
7 
8 #ifndef BASE_CALLBACK_INTERNAL_H_
9 #define BASE_CALLBACK_INTERNAL_H_
10 
11 #include "base/base_export.h"
12 #include "base/callback_forward.h"
13 #include "base/macros.h"
14 #include "base/memory/ref_counted.h"
15 
16 namespace base {
17 
18 struct FakeBindState;
19 
20 namespace internal {
21 
22 class CallbackBase;
23 class CallbackBaseCopyable;
24 
25 class BindStateBase;
26 
27 template <typename Functor, typename... BoundArgs>
28 struct BindState;
29 
30 struct BindStateBaseRefCountTraits {
31   static void Destruct(const BindStateBase*);
32 };
33 
34 template <typename T>
35 using PassingType = std::conditional_t<std::is_scalar<T>::value, T, T&&>;
36 
37 // BindStateBase is used to provide an opaque handle that the Callback
38 // class can use to represent a function object with bound arguments.  It
39 // behaves as an existential type that is used by a corresponding
40 // DoInvoke function to perform the function execution.  This allows
41 // us to shield the Callback class from the types of the bound argument via
42 // "type erasure."
43 // At the base level, the only task is to add reference counting data. Don't use
44 // RefCountedThreadSafe since it requires the destructor to be a virtual method.
45 // Creating a vtable for every BindState template instantiation results in a lot
46 // of bloat. Its only task is to call the destructor which can be done with a
47 // function pointer.
48 class BASE_EXPORT BindStateBase
49     : public RefCountedThreadSafe<BindStateBase, BindStateBaseRefCountTraits> {
50  public:
51   REQUIRE_ADOPTION_FOR_REFCOUNTED_TYPE();
52 
53   using InvokeFuncStorage = void(*)();
54 
55  private:
56   BindStateBase(InvokeFuncStorage polymorphic_invoke,
57                 void (*destructor)(const BindStateBase*));
58   BindStateBase(InvokeFuncStorage polymorphic_invoke,
59                 void (*destructor)(const BindStateBase*),
60                 bool (*is_cancelled)(const BindStateBase*));
61 
62   ~BindStateBase() = default;
63 
64   friend struct BindStateBaseRefCountTraits;
65   friend class RefCountedThreadSafe<BindStateBase, BindStateBaseRefCountTraits>;
66 
67   friend class CallbackBase;
68   friend class CallbackBaseCopyable;
69 
70   // Whitelist subclasses that access the destructor of BindStateBase.
71   template <typename Functor, typename... BoundArgs>
72   friend struct BindState;
73   friend struct ::base::FakeBindState;
74 
IsCancelled()75   bool IsCancelled() const {
76     return is_cancelled_(this);
77   }
78 
79   // In C++, it is safe to cast function pointers to function pointers of
80   // another type. It is not okay to use void*. We create a InvokeFuncStorage
81   // that that can store our function pointer, and then cast it back to
82   // the original type on usage.
83   InvokeFuncStorage polymorphic_invoke_;
84 
85   // Pointer to a function that will properly destroy |this|.
86   void (*destructor_)(const BindStateBase*);
87   bool (*is_cancelled_)(const BindStateBase*);
88 
89   DISALLOW_COPY_AND_ASSIGN(BindStateBase);
90 };
91 
92 // Holds the Callback methods that don't require specialization to reduce
93 // template bloat.
94 // CallbackBase<MoveOnly> is a direct base class of MoveOnly callbacks, and
95 // CallbackBase<Copyable> uses CallbackBase<MoveOnly> for its implementation.
96 class BASE_EXPORT CallbackBase {
97  public:
98   inline CallbackBase(CallbackBase&& c) noexcept;
99   CallbackBase& operator=(CallbackBase&& c) noexcept;
100 
101   explicit CallbackBase(const CallbackBaseCopyable& c);
102   CallbackBase& operator=(const CallbackBaseCopyable& c);
103 
104   explicit CallbackBase(CallbackBaseCopyable&& c) noexcept;
105   CallbackBase& operator=(CallbackBaseCopyable&& c) noexcept;
106 
107   // Returns true if Callback is null (doesn't refer to anything).
is_null()108   bool is_null() const { return !bind_state_; }
109   explicit operator bool() const { return !is_null(); }
110 
111   // Returns true if the callback invocation will be nop due to an cancellation.
112   // It's invalid to call this on uninitialized callback.
113   bool IsCancelled() const;
114 
115   // Returns the Callback into an uninitialized state.
116   void Reset();
117 
118  protected:
119   using InvokeFuncStorage = BindStateBase::InvokeFuncStorage;
120 
121   // Returns true if this callback equals |other|. |other| may be null.
122   bool EqualsInternal(const CallbackBase& other) const;
123 
124   constexpr inline CallbackBase();
125 
126   // Allow initializing of |bind_state_| via the constructor to avoid default
127   // initialization of the scoped_refptr.
128   explicit inline CallbackBase(BindStateBase* bind_state);
129 
polymorphic_invoke()130   InvokeFuncStorage polymorphic_invoke() const {
131     return bind_state_->polymorphic_invoke_;
132   }
133 
134   // Force the destructor to be instantiated inside this translation unit so
135   // that our subclasses will not get inlined versions.  Avoids more template
136   // bloat.
137   ~CallbackBase();
138 
139   scoped_refptr<BindStateBase> bind_state_;
140 };
141 
142 constexpr CallbackBase::CallbackBase() = default;
143 CallbackBase::CallbackBase(CallbackBase&&) noexcept = default;
CallbackBase(BindStateBase * bind_state)144 CallbackBase::CallbackBase(BindStateBase* bind_state)
145     : bind_state_(AdoptRef(bind_state)) {}
146 
147 // CallbackBase<Copyable> is a direct base class of Copyable Callbacks.
148 class BASE_EXPORT CallbackBaseCopyable : public CallbackBase {
149  public:
150   CallbackBaseCopyable(const CallbackBaseCopyable& c);
151   CallbackBaseCopyable(CallbackBaseCopyable&& c) noexcept = default;
152   CallbackBaseCopyable& operator=(const CallbackBaseCopyable& c);
153   CallbackBaseCopyable& operator=(CallbackBaseCopyable&& c) noexcept;
154 
155  protected:
156   constexpr CallbackBaseCopyable() = default;
CallbackBaseCopyable(BindStateBase * bind_state)157   explicit CallbackBaseCopyable(BindStateBase* bind_state)
158       : CallbackBase(bind_state) {}
159   ~CallbackBaseCopyable() = default;
160 };
161 
162 }  // namespace internal
163 }  // namespace base
164 
165 #endif  // BASE_CALLBACK_INTERNAL_H_
166