2 * Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
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.
11 // Borrowed from Chromium's src/base/move.h.
13 #ifndef WEBRTC_SYSTEM_WRAPPERS_INTEFACE_MOVE_H_
14 #define WEBRTC_SYSTEM_WRAPPERS_INTEFACE_MOVE_H_
16 // Macro with the boilerplate that makes a type move-only in C++03.
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.
24 // A class using this macro must call .Pass() (or somehow be an r-value already)
27 // * Passed as a function argument
28 // * Used as the right-hand side of an assignment
29 // * Returned from a function
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:
35 // template <typename T>
37 // MOVE_ONLY_TYPE_FOR_CPP_03(scoped_ptr, RValue)
39 // scoped_ptr(RValue& other) : ptr_(other.release()) { }
40 // scoped_ptr& operator=(RValue& other) {
46 // Note that the constructor must NOT be marked explicit.
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.
56 // For a thorough explanation of this technique, see:
58 // http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Move_Constructor
60 // The summary is that we take advantage of 2 properties:
62 // 1) non-const references will not bind to r-values.
63 // 2) C++ can apply one user-defined conversion when initializing a
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.
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.
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
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.
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.
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=.
93 // Other ways to get r-values is to use the result of an expression like a
96 // Here's an example with comments explaining what gets triggered where:
99 // MOVE_ONLY_TYPE_FOR_CPP_03(Foo, RValue);
103 // Foo(RValue other); // Move constructor.
104 // Foo& operator=(RValue rhs); // Move operator=
107 // Foo MakeFoo(); // Function that returns a Foo.
110 // Foo f_copy(f); // ERROR: Foo(Foo&) is private in this context.
112 // f_assign = f; // ERROR: operator=(Foo&) is private in this context.
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.
120 // IMPLEMENTATION SUBTLETIES WITH RValue
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.
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
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.
143 // In optimized builds, both implementations generate the same assembly so we
144 // choose the one that adheres to the standard.
147 // WHY HAVE typedef void MoveOnlyTypeForCPP03
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
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().
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.
171 // COMPARED TO Boost.Move
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.
176 // In Boost.Move, RValue is the boost::rv<> template. This type can be used
177 // when writing APIs like:
179 // void MyFunc(boost::rv<Foo>& f)
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:
185 // MyFunc(f); // Uh oh, we probably just destroyed |f| w/o calling Pass().
187 // unless someone is very careful to also declare a parallel override like:
189 // void MyFunc(const Foo& f)
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.
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.
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.
208 // http://llvm.org/bugs/show_bug.cgi?id=11528
210 // The workaround is to explicitly declare your copy constructor.
212 #define WEBRTC_MOVE_ONLY_TYPE_FOR_CPP_03(type, rvalue_type) \
214 struct rvalue_type { \
215 explicit rvalue_type(type* object) : object(object) {} \
219 void operator=(type&); \
221 operator rvalue_type() { return rvalue_type(this); } \
222 type Pass() { return type(rvalue_type(this)); } \
223 typedef void MoveOnlyTypeForCPP03; \
226 #endif // WEBRTC_SYSTEM_WRAPPERS_INTEFACE_MOVE_H_