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 <stddef.h> 12#include <map> 13#include <memory> 14#include <type_traits> 15#include <vector> 16 17#include "base/atomic_ref_count.h" 18#include "base/base_export.h" 19#include "base/macros.h" 20#include "base/memory/ref_counted.h" 21#include "base/memory/scoped_ptr.h" 22#include "base/template_util.h" 23 24namespace base { 25namespace internal { 26class CallbackBase; 27 28// BindStateBase is used to provide an opaque handle that the Callback 29// class can use to represent a function object with bound arguments. It 30// behaves as an existential type that is used by a corresponding 31// DoInvoke function to perform the function execution. This allows 32// us to shield the Callback class from the types of the bound argument via 33// "type erasure." 34// At the base level, the only task is to add reference counting data. Don't use 35// RefCountedThreadSafe since it requires the destructor to be a virtual method. 36// Creating a vtable for every BindState template instantiation results in a lot 37// of bloat. Its only task is to call the destructor which can be done with a 38// function pointer. 39class BindStateBase { 40 protected: 41 explicit BindStateBase(void (*destructor)(BindStateBase*)) 42 : ref_count_(0), destructor_(destructor) {} 43 ~BindStateBase() = default; 44 45 private: 46 friend class scoped_refptr<BindStateBase>; 47 friend class CallbackBase; 48 49 void AddRef(); 50 void Release(); 51 52 AtomicRefCount ref_count_; 53 54 // Pointer to a function that will properly destroy |this|. 55 void (*destructor_)(BindStateBase*); 56 57 DISALLOW_COPY_AND_ASSIGN(BindStateBase); 58}; 59 60// Holds the Callback methods that don't require specialization to reduce 61// template bloat. 62class BASE_EXPORT CallbackBase { 63 public: 64 CallbackBase(const CallbackBase& c); 65 CallbackBase& operator=(const CallbackBase& c); 66 67 // Returns true if Callback is null (doesn't refer to anything). 68 bool is_null() const { return bind_state_.get() == NULL; } 69 70 // Returns the Callback into an uninitialized state. 71 void Reset(); 72 73 protected: 74 // In C++, it is safe to cast function pointers to function pointers of 75 // another type. It is not okay to use void*. We create a InvokeFuncStorage 76 // that that can store our function pointer, and then cast it back to 77 // the original type on usage. 78 using InvokeFuncStorage = void(*)(); 79 80 // Returns true if this callback equals |other|. |other| may be null. 81 bool Equals(const CallbackBase& other) const; 82 83 // Allow initializing of |bind_state_| via the constructor to avoid default 84 // initialization of the scoped_refptr. We do not also initialize 85 // |polymorphic_invoke_| here because doing a normal assignment in the 86 // derived Callback templates makes for much nicer compiler errors. 87 explicit CallbackBase(BindStateBase* bind_state); 88 89 // Force the destructor to be instantiated inside this translation unit so 90 // that our subclasses will not get inlined versions. Avoids more template 91 // bloat. 92 ~CallbackBase(); 93 94 scoped_refptr<BindStateBase> bind_state_; 95 InvokeFuncStorage polymorphic_invoke_; 96}; 97 98// A helper template to determine if given type is non-const move-only-type, 99// i.e. if a value of the given type should be passed via std::move() in a 100// destructive way. Types are considered to be move-only if they have a 101// sentinel MoveOnlyTypeForCPP03 member: a class typically gets this from using 102// the DISALLOW_COPY_AND_ASSIGN_WITH_MOVE_FOR_BIND macro. 103// It would be easy to generalize this trait to all move-only types... but this 104// confuses template deduction in VS2013 with certain types such as 105// std::unique_ptr. 106// TODO(dcheng): Revisit this when Windows switches to VS2015 by default. 107template <typename T> struct IsMoveOnlyType { 108 template <typename U> 109 static YesType Test(const typename U::MoveOnlyTypeForCPP03*); 110 111 template <typename U> 112 static NoType Test(...); 113 114 static const bool value = sizeof((Test<T>(0))) == sizeof(YesType) && 115 !is_const<T>::value; 116}; 117 118// Specialization of IsMoveOnlyType so that std::unique_ptr is still considered 119// move-only, even without the sentinel member. 120template <typename T> 121struct IsMoveOnlyType<std::unique_ptr<T>> : std::true_type {}; 122 123template <typename> 124struct CallbackParamTraitsForMoveOnlyType; 125 126template <typename> 127struct CallbackParamTraitsForNonMoveOnlyType; 128 129// TODO(tzik): Use a default parameter once MSVS supports variadic templates 130// with default values. 131// http://connect.microsoft.com/VisualStudio/feedbackdetail/view/957801/compilation-error-with-variadic-templates 132// 133// This is a typetraits object that's used to take an argument type, and 134// extract a suitable type for storing and forwarding arguments. 135// 136// In particular, it strips off references, and converts arrays to 137// pointers for storage; and it avoids accidentally trying to create a 138// "reference of a reference" if the argument is a reference type. 139// 140// This array type becomes an issue for storage because we are passing bound 141// parameters by const reference. In this case, we end up passing an actual 142// array type in the initializer list which C++ does not allow. This will 143// break passing of C-string literals. 144template <typename T> 145struct CallbackParamTraits 146 : std::conditional<IsMoveOnlyType<T>::value, 147 CallbackParamTraitsForMoveOnlyType<T>, 148 CallbackParamTraitsForNonMoveOnlyType<T>>::type { 149}; 150 151template <typename T> 152struct CallbackParamTraitsForNonMoveOnlyType { 153 using ForwardType = const T&; 154 using StorageType = T; 155}; 156 157// The Storage should almost be impossible to trigger unless someone manually 158// specifies type of the bind parameters. However, in case they do, 159// this will guard against us accidentally storing a reference parameter. 160// 161// The ForwardType should only be used for unbound arguments. 162template <typename T> 163struct CallbackParamTraitsForNonMoveOnlyType<T&> { 164 using ForwardType = T&; 165 using StorageType = T; 166}; 167 168// Note that for array types, we implicitly add a const in the conversion. This 169// means that it is not possible to bind array arguments to functions that take 170// a non-const pointer. Trying to specialize the template based on a "const 171// T[n]" does not seem to match correctly, so we are stuck with this 172// restriction. 173template <typename T, size_t n> 174struct CallbackParamTraitsForNonMoveOnlyType<T[n]> { 175 using ForwardType = const T*; 176 using StorageType = const T*; 177}; 178 179// See comment for CallbackParamTraits<T[n]>. 180template <typename T> 181struct CallbackParamTraitsForNonMoveOnlyType<T[]> { 182 using ForwardType = const T*; 183 using StorageType = const T*; 184}; 185 186// Parameter traits for movable-but-not-copyable scopers. 187// 188// Callback<>/Bind() understands movable-but-not-copyable semantics where 189// the type cannot be copied but can still have its state destructively 190// transferred (aka. moved) to another instance of the same type by calling a 191// helper function. When used with Bind(), this signifies transferal of the 192// object's state to the target function. 193// 194// For these types, the ForwardType must not be a const reference, or a 195// reference. A const reference is inappropriate, and would break const 196// correctness, because we are implementing a destructive move. A non-const 197// reference cannot be used with temporaries which means the result of a 198// function or a cast would not be usable with Callback<> or Bind(). 199template <typename T> 200struct CallbackParamTraitsForMoveOnlyType { 201 using ForwardType = T; 202 using StorageType = T; 203}; 204 205// CallbackForward() is a very limited simulation of C++11's std::forward() 206// used by the Callback/Bind system for a set of movable-but-not-copyable 207// types. It is needed because forwarding a movable-but-not-copyable 208// argument to another function requires us to invoke the proper move 209// operator to create a rvalue version of the type. The supported types are 210// whitelisted below as overloads of the CallbackForward() function. The 211// default template compiles out to be a no-op. 212// 213// In C++11, std::forward would replace all uses of this function. However, it 214// is impossible to implement a general std::forward without C++11 due to a lack 215// of rvalue references. 216// 217// In addition to Callback/Bind, this is used by PostTaskAndReplyWithResult to 218// simulate std::forward() and forward the result of one Callback as a 219// parameter to another callback. This is to support Callbacks that return 220// the movable-but-not-copyable types whitelisted above. 221template <typename T> 222typename std::enable_if<!IsMoveOnlyType<T>::value, T>::type& CallbackForward( 223 T& t) { 224 return t; 225} 226 227template <typename T> 228typename std::enable_if<IsMoveOnlyType<T>::value, T>::type CallbackForward( 229 T& t) { 230 return std::move(t); 231} 232 233} // namespace internal 234} // namespace base 235 236#endif // BASE_CALLBACK_INTERNAL_H_ 237