/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* 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 */