Transfer the C++03 move-only type emulation into base/move.h and also make ScopedVector move-only.

Also:
  * Add a lot of documentation explaining what this macro does.
  * Change the implementation of RValue so it cannot be instantiated.

The change to always use RValue& makes for more efficent code in debug builds.  Looking at the disassembly for a simple use case (calling a function with one parameter), it removes the creation of one temporary.

BUG=96118
TEST=new unittests. exist code still compiles.

Review URL: https://chromiumcodereview.appspot.com/9207021

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@118388 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 211 insertions, 0 deletions

diff --git a/base/move.h b/base/move.h
new file mode 100644
index 0000000..f9408f7
--- /dev/null
+++ b/base/move.h
@@ -0,0 +1,211 @@
+// 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
+//   * Return 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 danging 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_