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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* C++11-style, but C++98-usable, "move references" implementation. */
+
+#ifndef mozilla_Move_h
+#define mozilla_Move_h
+
+#include "mozilla/TypeTraits.h"
+
+namespace mozilla {
+
+/*
+ * "Move" References
+ *
+ * Some types can be copied much more efficiently if we know the original's
+ * value need not be preserved --- that is, if we are doing a "move", not a
+ * "copy". For example, if we have:
+ *
+ *   Vector<T> u;
+ *   Vector<T> v(u);
+ *
+ * the constructor for v must apply a copy constructor to each element of u ---
+ * taking time linear in the length of u. However, if we know we will not need u
+ * any more once v has been initialized, then we could initialize v very
+ * efficiently simply by stealing u's dynamically allocated buffer and giving it
+ * to v --- a constant-time operation, regardless of the size of u.
+ *
+ * Moves often appear in container implementations. For example, when we append
+ * to a vector, we may need to resize its buffer. This entails moving each of
+ * its extant elements from the old, smaller buffer to the new, larger buffer.
+ * But once the elements have been migrated, we're just going to throw away the
+ * old buffer; we don't care if they still have their values. So if the vector's
+ * element type can implement "move" more efficiently than "copy", the vector
+ * resizing should by all means use a "move" operation. Hash tables should also
+ * use moves when resizing their internal array as entries are added and
+ * removed.
+ *
+ * The details of the optimization, and whether it's worth applying, vary
+ * from one type to the next: copying an 'int' is as cheap as moving it, so
+ * there's no benefit in distinguishing 'int' moves from copies. And while
+ * some constructor calls for complex types are moves, many really have to
+ * be copies, and can't be optimized this way. So we need:
+ *
+ * 1) a way for a type (like Vector) to announce that it can be moved more
+ *    efficiently than it can be copied, and provide an implementation of that
+ *    move operation; and
+ *
+ * 2) a way for a particular invocation of a copy constructor to say that it's
+ *    really a move, not a copy, and that the value of the original isn't
+ *    important afterwards (although it must still be safe to destroy).
+ *
+ * If a constructor has a single argument of type 'T&&' (an 'rvalue reference
+ * to T'), that indicates that it is a 'move constructor'. That's 1). It should
+ * move, not copy, its argument into the object being constructed. It may leave
+ * the original in any safely-destructible state.
+ *
+ * If a constructor's argument is an rvalue, as in 'C(f(x))' or 'C(x + y)', as
+ * opposed to an lvalue, as in 'C(x)', then overload resolution will prefer the
+ * move constructor, if there is one. The 'mozilla::Move' function, defined in
+ * this file, is an identity function you can use in a constructor invocation to
+ * make any argument into an rvalue, like this: C(Move(x)). That's 2). (You
+ * could use any function that works, but 'Move' indicates your intention
+ * clearly.)
+ *
+ * Where we might define a copy constructor for a class C like this:
+ *
+ *   C(const C& rhs) { ... copy rhs to this ... }
+ *
+ * we would declare a move constructor like this:
+ *
+ *   C(C&& rhs) { .. move rhs to this ... }
+ *
+ * And where we might perform a copy like this:
+ *
+ *   C c2(c1);
+ *
+ * we would perform a move like this:
+ *
+ *   C c2(Move(c1));
+ *
+ * Note that 'T&&' implicitly converts to 'T&'. So you can pass a 'T&&' to an
+ * ordinary copy constructor for a type that doesn't support a special move
+ * constructor, and you'll just get a copy. This means that templates can use
+ * Move whenever they know they won't use the original value any more, even if
+ * they're not sure whether the type at hand has a specialized move constructor.
+ * If it doesn't, the 'T&&' will just convert to a 'T&', and the ordinary copy
+ * constructor will apply.
+ *
+ * A class with a move constructor can also provide a move assignment operator.
+ * A generic definition would run this's destructor, and then apply the move
+ * constructor to *this's memory. A typical definition:
+ *
+ *   C& operator=(C&& rhs) {
+ *     MOZ_ASSERT(&rhs != this, "self-moves are prohibited");
+ *     this->~C();
+ *     new(this) C(Move(rhs));
+ *     return *this;
+ *   }
+ *
+ * With that in place, one can write move assignments like this:
+ *
+ *   c2 = Move(c1);
+ *
+ * This destroys c2, moves c1's value to c2, and leaves c1 in an undefined but
+ * destructible state.
+ *
+ * As we say, a move must leave the original in a "destructible" state. The
+ * original's destructor will still be called, so if a move doesn't
+ * actually steal all its resources, that's fine. We require only that the
+ * move destination must take on the original's value; and that destructing
+ * the original must not break the move destination.
+ *
+ * (Opinions differ on whether move assignment operators should deal with move
+ * assignment of an object onto itself. It seems wise to either handle that
+ * case, or assert that it does not occur.)
+ *
+ * Forwarding:
+ *
+ * Sometimes we want copy construction or assignment if we're passed an ordinary
+ * value, but move construction if passed an rvalue reference. For example, if
+ * our constructor takes two arguments and either could usefully be a move, it
+ * seems silly to write out all four combinations:
+ *
+ *   C::C(X&  x, Y&  y) : x(x),       y(y)       { }
+ *   C::C(X&  x, Y&& y) : x(x),       y(Move(y)) { }
+ *   C::C(X&& x, Y&  y) : x(Move(x)), y(y)       { }
+ *   C::C(X&& x, Y&& y) : x(Move(x)), y(Move(y)) { }
+ *
+ * To avoid this, C++11 has tweaks to make it possible to write what you mean.
+ * The four constructor overloads above can be written as one constructor
+ * template like so[0]:
+ *
+ *   template <typename XArg, typename YArg>
+ *   C::C(XArg&& x, YArg&& y) : x(Forward<XArg>(x)), y(Forward<YArg>(y)) { }
+ *
+ * ("'Don't Repeat Yourself'? What's that?")
+ *
+ * This takes advantage of two new rules in C++11:
+ *
+ * - First, when a function template takes an argument that is an rvalue
+ *   reference to a template argument (like 'XArg&& x' and 'YArg&& y' above),
+ *   then when the argument is applied to an lvalue, the template argument
+ *   resolves to 'T&'; and when it is applied to an rvalue, the template
+ *   argument resolves to 'T'. Thus, in a call to C::C like:
+ *
+ *      X foo(int);
+ *      Y yy;
+ *
+ *      C(foo(5), yy)
+ *
+ *   XArg would resolve to 'X', and YArg would resolve to 'Y&'.
+ *
+ * - Second, Whereas C++ used to forbid references to references, C++11 defines
+ *   'collapsing rules': 'T& &', 'T&& &', and 'T& &&' (that is, any combination
+ *   involving an lvalue reference) now collapse to simply 'T&'; and 'T&& &&'
+ *   collapses to 'T&&'.
+ *
+ *   Thus, in the call above, 'XArg&&' is 'X&&'; and 'YArg&&' is 'Y& &&', which
+ *   collapses to 'Y&'. Because the arguments are declared as rvalue references
+ *   to template arguments, the lvalue-ness "shines through" where present.
+ *
+ * Then, the 'Forward<T>' function --- you must invoke 'Forward' with its type
+ * argument --- returns an lvalue reference or an rvalue reference to its
+ * argument, depending on what T is. In our unified constructor definition, that
+ * means that we'll invoke either the copy or move constructors for x and y,
+ * depending on what we gave C's constructor. In our call, we'll move 'foo()'
+ * into 'x', but copy 'yy' into 'y'.
+ *
+ * This header file defines Move and Forward in the mozilla namespace. It's up
+ * to individual containers to annotate moves as such, by calling Move; and it's
+ * up to individual types to define move constructors and assignment operators
+ * when valuable.
+ *
+ * (C++11 says that the <utility> header file should define 'std::move' and
+ * 'std::forward', which are just like our 'Move' and 'Forward'; but those
+ * definitions aren't available in that header on all our platforms, so we
+ * define them ourselves here.)
+ *
+ * 0. This pattern is known as "perfect forwarding".  Interestingly, it is not
+ *    actually perfect, and it can't forward all possible argument expressions!
+ *    There is a C++11 issue: you can't form a reference to a bit-field.  As a
+ *    workaround, assign the bit-field to a local variable and use that:
+ *
+ *      // C is as above
+ *      struct S { int x : 1; } s;
+ *      C(s.x, 0); // BAD: s.x is a reference to a bit-field, can't form those
+ *      int tmp = s.x;
+ *      C(tmp, 0); // OK: tmp not a bit-field
+ */
+
+/**
+ * Identical to std::Move(); this is necessary until our stlport supports
+ * std::move().
+ */
+template<typename T>
+inline typename RemoveReference<T>::Type&&
+Move(T&& aX)
+{
+  return static_cast<typename RemoveReference<T>::Type&&>(aX);
+}
+
+/**
+ * These two overloads are identical to std::forward(); they are necessary until
+ * our stlport supports std::forward().
+ */
+template<typename T>
+inline T&&
+Forward(typename RemoveReference<T>::Type& aX)
+{
+  return static_cast<T&&>(aX);
+}
+
+template<typename T>
+inline T&&
+Forward(typename RemoveReference<T>::Type&& aX)
+{
+  static_assert(!IsLvalueReference<T>::value,
+                "misuse of Forward detected!  try the other overload");
+  return static_cast<T&&>(aX);
+}
+
+/** Swap |aX| and |aY| using move-construction if possible. */
+template<typename T>
+inline void
+Swap(T& aX, T& aY)
+{
+  T tmp(Move(aX));
+  aX = Move(aY);
+  aY = Move(tmp);
+}
+
+} // namespace mozilla
+
+#endif /* mozilla_Move_h */