// Copyright (c) 2012 The Chromium Authors. All rights reserved. // Use of this source code is governed by a BSD-style license that can be // found in the LICENSE file. #ifndef BASE_MOVE_H_ #define BASE_MOVE_H_ // Macro with the boilerplate that makes a type move-only in C++03. // // USAGE // // This macro should be used instead of DISALLOW_COPY_AND_ASSIGN to create // a "move-only" type. Unlike DISALLOW_COPY_AND_ASSIGN, this macro should be // the first line in a class declaration. // // A class using this macro must call .Pass() (or somehow be an r-value already) // before it can be: // // * Passed as a function argument // * Used as the right-hand side of an assignment // * Returned from a function // // Each class will still need to define their own "move constructor" and "move // operator=" to make this useful. Here's an example of the macro, the move // constructor, and the move operator= from the scoped_ptr class: // // template // class scoped_ptr { // MOVE_ONLY_TYPE_FOR_CPP_03(scoped_ptr, RValue) // public: // scoped_ptr(RValue& other) : ptr_(other.release()) { } // scoped_ptr& operator=(RValue& other) { // swap(other); // return *this; // } // }; // // Note that the constructor must NOT be marked explicit. // // For consistency, the second parameter to the macro should always be RValue // unless you have a strong reason to do otherwise. It is only exposed as a // macro parameter so that the move constructor and move operator= don't look // like they're using a phantom type. // // // HOW THIS WORKS // // For a thorough explanation of this technique, see: // // http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Move_Constructor // // The summary is that we take advantage of 2 properties: // // 1) non-const references will not bind to r-values. // 2) C++ can apply one user-defined conversion when initializing a // variable. // // The first lets us disable the copy constructor and assignment operator // by declaring private version of them with a non-const reference parameter. // // For l-values, direct initialization still fails like in // DISALLOW_COPY_AND_ASSIGN because the copy constructor and assignment // operators are private. // // For r-values, the situation is different. The copy constructor and // assignment operator are not viable due to (1), so we are trying to call // a non-existent constructor and non-existing operator= rather than a private // one. Since we have not committed an error quite yet, we can provide an // alternate conversion sequence and a constructor. We add // // * a private struct named "RValue" // * a user-defined conversion "operator RValue()" // * a "move constructor" and "move operator=" that take the RValue& as // their sole parameter. // // Only r-values will trigger this sequence and execute our "move constructor" // or "move operator=." L-values will match the private copy constructor and // operator= first giving a "private in this context" error. This combination // gives us a move-only type. // // For signaling a destructive transfer of data from an l-value, we provide a // method named Pass() which creates an r-value for the current instance // triggering the move constructor or move operator=. // // Other ways to get r-values is to use the result of an expression like a // function call. // // Here's an example with comments explaining what gets triggered where: // // class Foo { // MOVE_ONLY_TYPE_FOR_CPP_03(Foo, RValue); // // public: // ... API ... // Foo(RValue other); // Move constructor. // Foo& operator=(RValue rhs); // Move operator= // }; // // Foo MakeFoo(); // Function that returns a Foo. // // Foo f; // Foo f_copy(f); // ERROR: Foo(Foo&) is private in this context. // Foo f_assign; // f_assign = f; // ERROR: operator=(Foo&) is private in this context. // // // Foo f(MakeFoo()); // R-value so alternate conversion executed. // Foo f_copy(f.Pass()); // R-value so alternate conversion executed. // f = f_copy.Pass(); // R-value so alternate conversion executed. // // // IMPLEMENTATION SUBTLETIES WITH RValue // // The RValue struct is just a container for a pointer back to the original // object. It should only ever be created as a temporary, and no external // class should ever declare it or use it in a parameter. // // It is tempting to want to use the RValue type in function parameters, but // excluding the limited usage here for the move constructor and move // operator=, doing so would mean that the function could take both r-values // and l-values equially which is unexpected. See COMPARED To Boost.Move for // more details. // // An alternate, and incorrect, implementation of the RValue class used by // Boost.Move makes RValue a fieldless child of the move-only type. RValue& // is then used in place of RValue in the various operators. The RValue& is // "created" by doing *reinterpret_cast(this). This has the appeal // of never creating a temporary RValue struct even with optimizations // disabled. Also, by virtue of inheritance you can treat the RValue // reference as if it were the move-only type itself. Unfortunately, // using the result of this reinterpret_cast<> is actually undefined behavior // due to C++98 5.2.10.7. In certain compilers (e.g., NaCl) the optimizer // will generate non-working code. // // In optimized builds, both implementations generate the same assembly so we // choose the one that adheres to the standard. // // // COMPARED TO C++11 // // In C++11, you would implement this functionality using an r-value reference // and our .Pass() method would be replaced with a call to std::move(). // // This emulation also has a deficiency where it uses up the single // user-defined conversion allowed by C++ during initialization. This can // cause problems in some API edge cases. For instance, in scoped_ptr, it is // impossible to make a function "void Foo(scoped_ptr p)" accept a // value of type scoped_ptr even if you add a constructor to // scoped_ptr<> that would make it look like it should work. C++11 does not // have this deficiency. // // // COMPARED TO Boost.Move // // Our implementation similar to Boost.Move, but we keep the RValue struct // private to the move-only type, and we don't use the reinterpret_cast<> hack. // // In Boost.Move, RValue is the boost::rv<> template. This type can be used // when writing APIs like: // // void MyFunc(boost::rv& f) // // that can take advantage of rv<> to avoid extra copies of a type. However you // would still be able to call this version of MyFunc with an l-value: // // Foo f; // MyFunc(f); // Uh oh, we probably just destroyed |f| w/o calling Pass(). // // unless someone is very careful to also declare a parallel override like: // // void MyFunc(const Foo& f) // // that would catch the l-values first. This was declared unsafe in C++11 and // a C++11 compiler will explicitly fail MyFunc(f). Unfortunately, we cannot // ensure this in C++03. // // Since we have no need for writing such APIs yet, our implementation keeps // RValue private and uses a .Pass() method to do the conversion instead of // trying to write a version of "std::move()." Writing an API like std::move() // would require the RValue struct to be public. // // // CAVEATS // // If you include a move-only type as a field inside a class that does not // explicitly declare a copy constructor, the containing class's implicit // copy constructor will change from Containing(const Containing&) to // Containing(Containing&). This can cause some unexpected errors. // // http://llvm.org/bugs/show_bug.cgi?id=11528 // // The workaround is to explicitly declare your copy constructor. // #define MOVE_ONLY_TYPE_FOR_CPP_03(type, rvalue_type) \ private: \ struct rvalue_type { \ explicit rvalue_type(type* object) : object(object) {} \ type* object; \ }; \ type(type&); \ void operator=(type&); \ public: \ operator rvalue_type() { return rvalue_type(this); } \ type Pass() { return type(rvalue_type(this)); } \ private: #endif // BASE_MOVE_H_