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 // 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 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<RValue*>(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<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 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<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 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_