| // 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 <typename T> |
| // 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 has subtle properties: |
| // |
| // 1) All its methods are declared, but intentionally not defined. |
| // 2) It is *never* instantiated. |
| // 3) It is a child of the move-only type. |
| // |
| // (1) is a guard against accidental violation of (2). If an instance of |
| // RValue were ever created, either as a temporary, or as a copy to some |
| // function parameter or field of a class, the binary will not link. |
| // |
| // This ensures that RValue can only exist as a temporary which is important |
| // to avoid accidental dangling references. |
| // |
| // (3) allows us to get around instantiations because our user-defined |
| // conversion can return a downcast of this pointer. |
| // |
| // operator RValue&() { return *reinterpret_cast<RValue*>(this); } |
| // |
| // Because RValue does not extend the object size or add any virtual methods, |
| // this type-pun is safe. |
| // |
| // An alternative implementation would be to make RValue into a concrete |
| // struct that holds a reference to the type. But in the non-optimized build, |
| // this causes unnecessary temporaries to be made bloating the object files. |
| // Also, it would then be possible to accidentally persist an RValue instance. |
| // |
| // |
| // 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 an function "void Foo(scoped_ptr<Parent> p)" accept a |
| // value of type scoped_ptr<Child> 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 is based on Boost.Move, but we keep the RValue struct |
| // private to the move-only type. |
| // |
| // In Boost.Move, RValue is the boost::rv<> template. This type can be used |
| // when writing APIs like: |
| // |
| // void MyFunc(boost::rv<Foo>& 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 structs 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 : public type { \ |
| rvalue_type(); \ |
| ~rvalue_type(); \ |
| rvalue_type(const rvalue_type&); \ |
| void operator=(const rvalue_type&); \ |
| }; \ |
| type(type&); \ |
| void operator=(type&); \ |
| public: \ |
| operator rvalue_type&() { return *reinterpret_cast<rvalue_type*>(this); } \ |
| type Pass() { return type(*reinterpret_cast<rvalue_type*>(this)); } \ |
| private: |
| |
| #endif // BASE_MOVE_H_ |