1 /* 2 * Copyright (c) 2013 The WebRTC project authors. All Rights Reserved. 3 * 4 * Use of this source code is governed by a BSD-style license 5 * that can be found in the LICENSE file in the root of the source 6 * tree. An additional intellectual property rights grant can be found 7 * in the file PATENTS. All contributing project authors may 8 * be found in the AUTHORS file in the root of the source tree. 9 */ 10 11 // Borrowed from Chromium's src/base/move.h. 12 13 #ifndef WEBRTC_SYSTEM_WRAPPERS_INTEFACE_MOVE_H_ 14 #define WEBRTC_SYSTEM_WRAPPERS_INTEFACE_MOVE_H_ 15 16 // Macro with the boilerplate that makes a type move-only in C++03. 17 // 18 // USAGE 19 // 20 // This macro should be used instead of DISALLOW_COPY_AND_ASSIGN to create 21 // a "move-only" type. Unlike DISALLOW_COPY_AND_ASSIGN, this macro should be 22 // the first line in a class declaration. 23 // 24 // A class using this macro must call .Pass() (or somehow be an r-value already) 25 // before it can be: 26 // 27 // * Passed as a function argument 28 // * Used as the right-hand side of an assignment 29 // * Returned from a function 30 // 31 // Each class will still need to define their own "move constructor" and "move 32 // operator=" to make this useful. Here's an example of the macro, the move 33 // constructor, and the move operator= from the scoped_ptr class: 34 // 35 // template <typename T> 36 // class scoped_ptr { 37 // MOVE_ONLY_TYPE_FOR_CPP_03(scoped_ptr, RValue) 38 // public: 39 // scoped_ptr(RValue& other) : ptr_(other.release()) { } 40 // scoped_ptr& operator=(RValue& other) { 41 // swap(other); 42 // return *this; 43 // } 44 // }; 45 // 46 // Note that the constructor must NOT be marked explicit. 47 // 48 // For consistency, the second parameter to the macro should always be RValue 49 // unless you have a strong reason to do otherwise. It is only exposed as a 50 // macro parameter so that the move constructor and move operator= don't look 51 // like they're using a phantom type. 52 // 53 // 54 // HOW THIS WORKS 55 // 56 // For a thorough explanation of this technique, see: 57 // 58 // http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Move_Constructor 59 // 60 // The summary is that we take advantage of 2 properties: 61 // 62 // 1) non-const references will not bind to r-values. 63 // 2) C++ can apply one user-defined conversion when initializing a 64 // variable. 65 // 66 // The first lets us disable the copy constructor and assignment operator 67 // by declaring private version of them with a non-const reference parameter. 68 // 69 // For l-values, direct initialization still fails like in 70 // DISALLOW_COPY_AND_ASSIGN because the copy constructor and assignment 71 // operators are private. 72 // 73 // For r-values, the situation is different. The copy constructor and 74 // assignment operator are not viable due to (1), so we are trying to call 75 // a non-existent constructor and non-existing operator= rather than a private 76 // one. Since we have not committed an error quite yet, we can provide an 77 // alternate conversion sequence and a constructor. We add 78 // 79 // * a private struct named "RValue" 80 // * a user-defined conversion "operator RValue()" 81 // * a "move constructor" and "move operator=" that take the RValue& as 82 // their sole parameter. 83 // 84 // Only r-values will trigger this sequence and execute our "move constructor" 85 // or "move operator=." L-values will match the private copy constructor and 86 // operator= first giving a "private in this context" error. This combination 87 // gives us a move-only type. 88 // 89 // For signaling a destructive transfer of data from an l-value, we provide a 90 // method named Pass() which creates an r-value for the current instance 91 // triggering the move constructor or move operator=. 92 // 93 // Other ways to get r-values is to use the result of an expression like a 94 // function call. 95 // 96 // Here's an example with comments explaining what gets triggered where: 97 // 98 // class Foo { 99 // MOVE_ONLY_TYPE_FOR_CPP_03(Foo, RValue); 100 // 101 // public: 102 // ... API ... 103 // Foo(RValue other); // Move constructor. 104 // Foo& operator=(RValue rhs); // Move operator= 105 // }; 106 // 107 // Foo MakeFoo(); // Function that returns a Foo. 108 // 109 // Foo f; 110 // Foo f_copy(f); // ERROR: Foo(Foo&) is private in this context. 111 // Foo f_assign; 112 // f_assign = f; // ERROR: operator=(Foo&) is private in this context. 113 // 114 // 115 // Foo f(MakeFoo()); // R-value so alternate conversion executed. 116 // Foo f_copy(f.Pass()); // R-value so alternate conversion executed. 117 // f = f_copy.Pass(); // R-value so alternate conversion executed. 118 // 119 // 120 // IMPLEMENTATION SUBTLETIES WITH RValue 121 // 122 // The RValue struct is just a container for a pointer back to the original 123 // object. It should only ever be created as a temporary, and no external 124 // class should ever declare it or use it in a parameter. 125 // 126 // It is tempting to want to use the RValue type in function parameters, but 127 // excluding the limited usage here for the move constructor and move 128 // operator=, doing so would mean that the function could take both r-values 129 // and l-values equially which is unexpected. See COMPARED To Boost.Move for 130 // more details. 131 // 132 // An alternate, and incorrect, implementation of the RValue class used by 133 // Boost.Move makes RValue a fieldless child of the move-only type. RValue& 134 // is then used in place of RValue in the various operators. The RValue& is 135 // "created" by doing *reinterpret_cast<RValue*>(this). This has the appeal 136 // of never creating a temporary RValue struct even with optimizations 137 // disabled. Also, by virtue of inheritance you can treat the RValue 138 // reference as if it were the move-only type itself. Unfortunately, 139 // using the result of this reinterpret_cast<> is actually undefined behavior 140 // due to C++98 5.2.10.7. In certain compilers (e.g., NaCl) the optimizer 141 // will generate non-working code. 142 // 143 // In optimized builds, both implementations generate the same assembly so we 144 // choose the one that adheres to the standard. 145 // 146 // 147 // WHY HAVE typedef void MoveOnlyTypeForCPP03 148 // 149 // Callback<>/Bind() needs to understand movable-but-not-copyable semantics 150 // to call .Pass() appropriately when it is expected to transfer the value. 151 // The cryptic typedef MoveOnlyTypeForCPP03 is added to make this check 152 // easy and automatic in helper templates for Callback<>/Bind(). 153 // See IsMoveOnlyType template and its usage in base/callback_internal.h 154 // for more details. 155 // 156 // 157 // COMPARED TO C++11 158 // 159 // In C++11, you would implement this functionality using an r-value reference 160 // and our .Pass() method would be replaced with a call to std::move(). 161 // 162 // This emulation also has a deficiency where it uses up the single 163 // user-defined conversion allowed by C++ during initialization. This can 164 // cause problems in some API edge cases. For instance, in scoped_ptr, it is 165 // impossible to make a function "void Foo(scoped_ptr<Parent> p)" accept a 166 // value of type scoped_ptr<Child> even if you add a constructor to 167 // scoped_ptr<> that would make it look like it should work. C++11 does not 168 // have this deficiency. 169 // 170 // 171 // COMPARED TO Boost.Move 172 // 173 // Our implementation similar to Boost.Move, but we keep the RValue struct 174 // private to the move-only type, and we don't use the reinterpret_cast<> hack. 175 // 176 // In Boost.Move, RValue is the boost::rv<> template. This type can be used 177 // when writing APIs like: 178 // 179 // void MyFunc(boost::rv<Foo>& f) 180 // 181 // that can take advantage of rv<> to avoid extra copies of a type. However you 182 // would still be able to call this version of MyFunc with an l-value: 183 // 184 // Foo f; 185 // MyFunc(f); // Uh oh, we probably just destroyed |f| w/o calling Pass(). 186 // 187 // unless someone is very careful to also declare a parallel override like: 188 // 189 // void MyFunc(const Foo& f) 190 // 191 // that would catch the l-values first. This was declared unsafe in C++11 and 192 // a C++11 compiler will explicitly fail MyFunc(f). Unfortunately, we cannot 193 // ensure this in C++03. 194 // 195 // Since we have no need for writing such APIs yet, our implementation keeps 196 // RValue private and uses a .Pass() method to do the conversion instead of 197 // trying to write a version of "std::move()." Writing an API like std::move() 198 // would require the RValue struct to be public. 199 // 200 // 201 // CAVEATS 202 // 203 // If you include a move-only type as a field inside a class that does not 204 // explicitly declare a copy constructor, the containing class's implicit 205 // copy constructor will change from Containing(const Containing&) to 206 // Containing(Containing&). This can cause some unexpected errors. 207 // 208 // http://llvm.org/bugs/show_bug.cgi?id=11528 209 // 210 // The workaround is to explicitly declare your copy constructor. 211 // 212 #define WEBRTC_MOVE_ONLY_TYPE_FOR_CPP_03(type, rvalue_type) \ 213 private: \ 214 struct rvalue_type { \ 215 explicit rvalue_type(type* object) : object(object) {} \ 216 type* object; \ 217 }; \ 218 type(type&); \ 219 void operator=(type&); \ 220 public: \ 221 operator rvalue_type() { return rvalue_type(this); } \ 222 type Pass() { return type(rvalue_type(this)); } \ 223 typedef void MoveOnlyTypeForCPP03; \ 224 private: 225 226 #endif // WEBRTC_SYSTEM_WRAPPERS_INTEFACE_MOVE_H_ 227
