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