1 /* 2 * Copyright 2012 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 #ifndef THIRD_PARTY_WEBRTC_FILES_WEBRTC_BASE_MOVE_H_ 12 #define THIRD_PARTY_WEBRTC_FILES_WEBRTC_BASE_MOVE_H_ 13 14 // Macro with the boilerplate that makes a type move-only in C++03. 15 // 16 // USAGE 17 // 18 // This macro should be used instead of DISALLOW_COPY_AND_ASSIGN to create 19 // a "move-only" type. Unlike DISALLOW_COPY_AND_ASSIGN, this macro should be 20 // the first line in a class declaration. 21 // 22 // A class using this macro must call .Pass() (or somehow be an r-value already) 23 // before it can be: 24 // 25 // * Passed as a function argument 26 // * Used as the right-hand side of an assignment 27 // * Returned from a function 28 // 29 // Each class will still need to define their own "move constructor" and "move 30 // operator=" to make this useful. Here's an example of the macro, the move 31 // constructor, and the move operator= from the scoped_ptr class: 32 // 33 // template <typename T> 34 // class scoped_ptr { 35 // TALK_MOVE_ONLY_TYPE_FOR_CPP_03(scoped_ptr, RValue) 36 // public: 37 // scoped_ptr(RValue& other) : ptr_(other.release()) { } 38 // scoped_ptr& operator=(RValue& other) { 39 // swap(other); 40 // return *this; 41 // } 42 // }; 43 // 44 // Note that the constructor must NOT be marked explicit. 45 // 46 // For consistency, the second parameter to the macro should always be RValue 47 // unless you have a strong reason to do otherwise. It is only exposed as a 48 // macro parameter so that the move constructor and move operator= don't look 49 // like they're using a phantom type. 50 // 51 // 52 // HOW THIS WORKS 53 // 54 // For a thorough explanation of this technique, see: 55 // 56 // http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Move_Constructor 57 // 58 // The summary is that we take advantage of 2 properties: 59 // 60 // 1) non-const references will not bind to r-values. 61 // 2) C++ can apply one user-defined conversion when initializing a 62 // variable. 63 // 64 // The first lets us disable the copy constructor and assignment operator 65 // by declaring private version of them with a non-const reference parameter. 66 // 67 // For l-values, direct initialization still fails like in 68 // DISALLOW_COPY_AND_ASSIGN because the copy constructor and assignment 69 // operators are private. 70 // 71 // For r-values, the situation is different. The copy constructor and 72 // assignment operator are not viable due to (1), so we are trying to call 73 // a non-existent constructor and non-existing operator= rather than a private 74 // one. Since we have not committed an error quite yet, we can provide an 75 // alternate conversion sequence and a constructor. We add 76 // 77 // * a private struct named "RValue" 78 // * a user-defined conversion "operator RValue()" 79 // * a "move constructor" and "move operator=" that take the RValue& as 80 // their sole parameter. 81 // 82 // Only r-values will trigger this sequence and execute our "move constructor" 83 // or "move operator=." L-values will match the private copy constructor and 84 // operator= first giving a "private in this context" error. This combination 85 // gives us a move-only type. 86 // 87 // For signaling a destructive transfer of data from an l-value, we provide a 88 // method named Pass() which creates an r-value for the current instance 89 // triggering the move constructor or move operator=. 90 // 91 // Other ways to get r-values is to use the result of an expression like a 92 // function call. 93 // 94 // Here's an example with comments explaining what gets triggered where: 95 // 96 // class Foo { 97 // TALK_MOVE_ONLY_TYPE_FOR_CPP_03(Foo, RValue); 98 // 99 // public: 100 // ... API ... 101 // Foo(RValue other); // Move constructor. 102 // Foo& operator=(RValue rhs); // Move operator= 103 // }; 104 // 105 // Foo MakeFoo(); // Function that returns a Foo. 106 // 107 // Foo f; 108 // Foo f_copy(f); // ERROR: Foo(Foo&) is private in this context. 109 // Foo f_assign; 110 // f_assign = f; // ERROR: operator=(Foo&) is private in this context. 111 // 112 // 113 // Foo f(MakeFoo()); // R-value so alternate conversion executed. 114 // Foo f_copy(f.Pass()); // R-value so alternate conversion executed. 115 // f = f_copy.Pass(); // R-value so alternate conversion executed. 116 // 117 // 118 // IMPLEMENTATION SUBTLETIES WITH RValue 119 // 120 // The RValue struct is just a container for a pointer back to the original 121 // object. It should only ever be created as a temporary, and no external 122 // class should ever declare it or use it in a parameter. 123 // 124 // It is tempting to want to use the RValue type in function parameters, but 125 // excluding the limited usage here for the move constructor and move 126 // operator=, doing so would mean that the function could take both r-values 127 // and l-values equially which is unexpected. See COMPARED To Boost.Move for 128 // more details. 129 // 130 // An alternate, and incorrect, implementation of the RValue class used by 131 // Boost.Move makes RValue a fieldless child of the move-only type. RValue& 132 // is then used in place of RValue in the various operators. The RValue& is 133 // "created" by doing *reinterpret_cast<RValue*>(this). This has the appeal 134 // of never creating a temporary RValue struct even with optimizations 135 // disabled. Also, by virtue of inheritance you can treat the RValue 136 // reference as if it were the move-only type itself. Unfortunately, 137 // using the result of this reinterpret_cast<> is actually undefined behavior 138 // due to C++98 5.2.10.7. In certain compilers (e.g., NaCl) the optimizer 139 // will generate non-working code. 140 // 141 // In optimized builds, both implementations generate the same assembly so we 142 // choose the one that adheres to the standard. 143 // 144 // 145 // COMPARED TO C++11 146 // 147 // In C++11, you would implement this functionality using an r-value reference 148 // and our .Pass() method would be replaced with a call to std::move(). 149 // 150 // This emulation also has a deficiency where it uses up the single 151 // user-defined conversion allowed by C++ during initialization. This can 152 // cause problems in some API edge cases. For instance, in scoped_ptr, it is 153 // impossible to make a function "void Foo(scoped_ptr<Parent> p)" accept a 154 // value of type scoped_ptr<Child> even if you add a constructor to 155 // scoped_ptr<> that would make it look like it should work. C++11 does not 156 // have this deficiency. 157 // 158 // 159 // COMPARED TO Boost.Move 160 // 161 // Our implementation similar to Boost.Move, but we keep the RValue struct 162 // private to the move-only type, and we don't use the reinterpret_cast<> hack. 163 // 164 // In Boost.Move, RValue is the boost::rv<> template. This type can be used 165 // when writing APIs like: 166 // 167 // void MyFunc(boost::rv<Foo>& f) 168 // 169 // that can take advantage of rv<> to avoid extra copies of a type. However you 170 // would still be able to call this version of MyFunc with an l-value: 171 // 172 // Foo f; 173 // MyFunc(f); // Uh oh, we probably just destroyed |f| w/o calling Pass(). 174 // 175 // unless someone is very careful to also declare a parallel override like: 176 // 177 // void MyFunc(const Foo& f) 178 // 179 // that would catch the l-values first. This was declared unsafe in C++11 and 180 // a C++11 compiler will explicitly fail MyFunc(f). Unfortunately, we cannot 181 // ensure this in C++03. 182 // 183 // Since we have no need for writing such APIs yet, our implementation keeps 184 // RValue private and uses a .Pass() method to do the conversion instead of 185 // trying to write a version of "std::move()." Writing an API like std::move() 186 // would require the RValue struct to be public. 187 // 188 // 189 // CAVEATS 190 // 191 // If you include a move-only type as a field inside a class that does not 192 // explicitly declare a copy constructor, the containing class's implicit 193 // copy constructor will change from Containing(const Containing&) to 194 // Containing(Containing&). This can cause some unexpected errors. 195 // 196 // http://llvm.org/bugs/show_bug.cgi?id=11528 197 // 198 // The workaround is to explicitly declare your copy constructor. 199 // 200 #define TALK_MOVE_ONLY_TYPE_FOR_CPP_03(type, rvalue_type) \ 201 private: \ 202 struct rvalue_type { \ 203 explicit rvalue_type(type* object) : object(object) {} \ 204 type* object; \ 205 }; \ 206 type(type&); \ 207 void operator=(type&); \ 208 public: \ 209 operator rvalue_type() { return rvalue_type(this); } \ 210 type Pass() { return type(rvalue_type(this)); } \ 211 private: 212 213 #endif // THIRD_PARTY_WEBRTC_FILES_WEBRTC_BASE_MOVE_H_ 214
